Deflection Figures for a GT40 Chassis
- Thread starter James
- Start date
This is interesting. Anyone know anyone with a chassis in Sweden? He should be very wellcome to visit me for a torque test.
In my site http://hem.passagen.se/hemipanter/
uder "Chassis and components" I have a few words about twisting chassies, together with som Tq numbers for different cars.
Goran Malmberg
In my site http://hem.passagen.se/hemipanter/
uder "Chassis and components" I have a few words about twisting chassies, together with som Tq numbers for different cars.
Goran Malmberg
A search of the phrase "torsional ridgidity" brings up:
this thread called "Chassis Stiffness", and
this thread called "chassis rigidity".
I have considered measuring the torsional ridgidity of my RF and may yet do so.
this thread called "Chassis Stiffness", and
this thread called "chassis rigidity".
I have considered measuring the torsional ridgidity of my RF and may yet do so.
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This is interesting. Anyone know anyone with a chassis in Sweden? He should be very wellcome to visit me for a torque test.
In my site http://hem.passagen.se/hemipanter/
uder "Chassis and components" I have a few words about twisting chassies, together with som Tq numbers for different cars.
Goran Malmberg
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Goran, your web site contains much useful information. I like your car, too! I looked up your chassis stiffness information, and I'll quote it here:
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As for references. Lamborghini Countach 1900 fp/degree. Ferrari 360 spider 6250 fp/degree. Viper gts has a "tube space frame" and 9000 fp/degree. Viper gts-R (Le Mans 24 hr) is reinforced to 13600 fp/degree. Lamborghini Murcielago also uses a high strength tube frame supported with honeycomb carbon fibre to 15000 fp/degree. It clearly shows that the Ferrari has no roof. Here we have cars with cromolly tube frames, carbon fibre, etc. Exotic material, loudly advertised as great stuff that makes those sport scars outstanding. Let me mention that the new SAAB 9-3 Sport Sedan, steel monocoque has a torsional stability of 16000 fp/degree. Showing that good engineering is more important than the use of fancy materials. Embarrassing for the SUPER cars? The Panoz racing car tub carbonfibre monocoque has a stiffness of 45000 fp /degree, but due to the front motor installation the axle to axle ratio is 30000 fp/d, at a weight of 110 pound. A street car that uses a tub monocoque is Koenigsegg . Also made of carbon fibre. This tub is said to have strength of 20500 fp/degree. As this, like the Panoz, is a tub number, the axle to axle ratio should be less. With the same reduction as the Panoz, we should land at 13600 fp/degree. This show that a monocoque is the way to go, even if made in sheet metal. The reason for using steel tube frames is the ease of production in a small numbers. A steel monocoque takes a tremendous investment in tools and engineering.
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Could you explain the axle-to-axle ratio? My understanding is that chassis stiffness should be measured by securing the chassis at the rear suspension mounting points and one of the front suspension mounting points while putting a load on a moment arm outboard of the other front suspension mounting point and measuring the applied torque (moment length x mass) and the resulting deflection. I imagine if you measure from points in front of the front suspension or in back of the rear suspension then the torsional ridgidity value will be less, but in my opinion the only thing that matters is the chassis stiffness between the suspension pickup points (at least for a mid-engine car).
This is interesting. Anyone know anyone with a chassis in Sweden? He should be very wellcome to visit me for a torque test.
In my site http://hem.passagen.se/hemipanter/
uder "Chassis and components" I have a few words about twisting chassies, together with som Tq numbers for different cars.
Goran Malmberg
[/ QUOTE ]
Goran, your web site contains much useful information. I like your car, too! I looked up your chassis stiffness information, and I'll quote it here:
[ QUOTE ]
As for references. Lamborghini Countach 1900 fp/degree. Ferrari 360 spider 6250 fp/degree. Viper gts has a "tube space frame" and 9000 fp/degree. Viper gts-R (Le Mans 24 hr) is reinforced to 13600 fp/degree. Lamborghini Murcielago also uses a high strength tube frame supported with honeycomb carbon fibre to 15000 fp/degree. It clearly shows that the Ferrari has no roof. Here we have cars with cromolly tube frames, carbon fibre, etc. Exotic material, loudly advertised as great stuff that makes those sport scars outstanding. Let me mention that the new SAAB 9-3 Sport Sedan, steel monocoque has a torsional stability of 16000 fp/degree. Showing that good engineering is more important than the use of fancy materials. Embarrassing for the SUPER cars? The Panoz racing car tub carbonfibre monocoque has a stiffness of 45000 fp /degree, but due to the front motor installation the axle to axle ratio is 30000 fp/d, at a weight of 110 pound. A street car that uses a tub monocoque is Koenigsegg . Also made of carbon fibre. This tub is said to have strength of 20500 fp/degree. As this, like the Panoz, is a tub number, the axle to axle ratio should be less. With the same reduction as the Panoz, we should land at 13600 fp/degree. This show that a monocoque is the way to go, even if made in sheet metal. The reason for using steel tube frames is the ease of production in a small numbers. A steel monocoque takes a tremendous investment in tools and engineering.
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Could you explain the axle-to-axle ratio? My understanding is that chassis stiffness should be measured by securing the chassis at the rear suspension mounting points and one of the front suspension mounting points while putting a load on a moment arm outboard of the other front suspension mounting point and measuring the applied torque (moment length x mass) and the resulting deflection. I imagine if you measure from points in front of the front suspension or in back of the rear suspension then the torsional ridgidity value will be less, but in my opinion the only thing that matters is the chassis stiffness between the suspension pickup points (at least for a mid-engine car).
Interestingly an Audi A8 has a torsiional rigidity of 36,000nm. One could argue that it needs the stifness because of its weight and size. However one could also argue that a GT40 should have more torsional rigidity because of the power plants used and the handling importance.
I would hope that a GTD has more than 2,600 ft/lb per degree. That seems low in comparison to an orginal.
Regards,
J.P
I would hope that a GTD has more than 2,600 ft/lb per degree. That seems low in comparison to an orginal.
Regards,
J.P
Mark
What matters is the attaching points of the coilovers. In my own test I replaced the coilovers by solid bars, attaching the twisting arms to the under A-arms. Simulating what happen on the road.
However, a "tub" monocoque is a body that usually only has the FRONT A-arms attached to it. The rear is mounted together with the motor and transaxle. Therfore the correct number must include these parts. But tub manufacturers supply numbers for the tub only.
Goran Malmberg
What matters is the attaching points of the coilovers. In my own test I replaced the coilovers by solid bars, attaching the twisting arms to the under A-arms. Simulating what happen on the road.
However, a "tub" monocoque is a body that usually only has the FRONT A-arms attached to it. The rear is mounted together with the motor and transaxle. Therfore the correct number must include these parts. But tub manufacturers supply numbers for the tub only.
Goran Malmberg
Nowdays car manufacturer put a lot effort in impact safety. This also makes the car stiff at the same time. Ok, they also got softer crusch zones.
For a sportscar, we should look at the torsionally stability number as a "ratio" number, put in relation to the exposed load.
A car that is lighter and has lower CGH might then show a better value than a by number stiffer car.
It must also be put in relation to the total spring stiffness of the car. The stiffer the springs bars and schocks, the greater the torsional load on the chassies.
The picture is a bit complicated, so let us just make the car as stiff as possible.
Goran Malmberg
For a sportscar, we should look at the torsionally stability number as a "ratio" number, put in relation to the exposed load.
A car that is lighter and has lower CGH might then show a better value than a by number stiffer car.
It must also be put in relation to the total spring stiffness of the car. The stiffer the springs bars and schocks, the greater the torsional load on the chassies.
The picture is a bit complicated, so let us just make the car as stiff as possible.
Goran Malmberg
Interesting.......At the University of Auckland In 1999, under the supervision of Dr. R Halkyard, students performed a "Torsional Analysis of GT-40 Replica Chassis." Although the study is listed on the U of Auckland site, I could not find the paper, however it may have not been published.
Brian
Brian
In the DRB build manual there is a copy of a page from the ADR filing that shows the torsional rigidity number, If I can remember that long (work to home - the brain usually refreshes on the commute home /ubbthreads/images/graemlins/tongue.gif!) I'll look it up and post it.
Monocoques are the way to go. I once built one for an SAE project. Our club didn't know how much carbon to use, so we decided to play it safe and use eight layers with a one inch foam core. The monocoque weighed about 35 lbs. We never tested the structure, but it was like a tank. We did crash it a few times with only superficial dammage, it had no crush zones, the driver truly was spam in a can.
Aluminum monocoques work well also. CNC punches, CNC benders, laser and water jet cutters make it possible to design a complete monocoque on a computer and make all of the pieces line up the first time. Then it's just a matter of riveting it together. It's definitely a good way to go when you need to build a fair number of chassis.
Space frames are good because they are easy to build in low quantity, they are easy to repair if dammaged, and you can build a chassis with reasonable stiffness with out too much weight penalty compared to a monocoque. This is especially true for race cars because the rules usually call for a roll bar or cage. In a space frame, those bars are a functional part of the chassis, in a monocoque, they are just extra weight, and hard to attach to the monocoque because monocoques rely on the strength of their skin and don't have very good hard points.
The important thing to remember about chassis stiffnes is that you don't need very much. More is always better, but there are race cars that work very, very well and only have about twice as much chassis stiffness in ft lbs compared to their vehicle weight.
Aluminum monocoques work well also. CNC punches, CNC benders, laser and water jet cutters make it possible to design a complete monocoque on a computer and make all of the pieces line up the first time. Then it's just a matter of riveting it together. It's definitely a good way to go when you need to build a fair number of chassis.
Space frames are good because they are easy to build in low quantity, they are easy to repair if dammaged, and you can build a chassis with reasonable stiffness with out too much weight penalty compared to a monocoque. This is especially true for race cars because the rules usually call for a roll bar or cage. In a space frame, those bars are a functional part of the chassis, in a monocoque, they are just extra weight, and hard to attach to the monocoque because monocoques rely on the strength of their skin and don't have very good hard points.
The important thing to remember about chassis stiffnes is that you don't need very much. More is always better, but there are race cars that work very, very well and only have about twice as much chassis stiffness in ft lbs compared to their vehicle weight.
Torsional rigidity (TR) is not correctly expressed as a function of the weight of the car. There are just to many factors to be considerd ie 2 seat open car, single seat, front engined, rear engined, 2 door , 4 door, the list goes on. You can compare like for like but you need to compare bare chassis weights Vs TR. A higher TR for same weight is an indication of the effective use of the material. On a tubular frame chassis the placement of tubes can make a significant difference. Bonding or close rivetting of sheet panels onto a tube frame can be of enormous benefit. In a replica tube frame GT40 7000Nm would be about the right number. But I have have tested 600kg (total)cars with a TR of 4900Nm. Reputable manufacturers of rear engined Mono tubs will apply a correction factor if tested without engine etc. A car with a high TR will handle better, can run softer springs for the same result but it is more sensitive to spring rate change. The addition of a roll bar/cage can reduce the TR as it may change the effective centre of rotation causing a higher strain energy in critical parts. If you are concerned about the TR of your chassis ask the manufacturer for the test report.
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Torsional rigidity (TR) is not correctly expressed as a function of the weight of the car.
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I have to disagree a bit. The relation ship between chassis loads and vehicle weight is pretty straight forward. The heavier the car, the higher the load on the chassis. So one chassis will behave similarly to another with half the stiffness in a car that weighs half as much.
Of course the ratio of sprung to unsprung mass will change things a bit, wheel base and track are certainly factors, but in general it's pretty close to a linear relationship.
My point is that no one needs a 25,000 ft lb per degree chassis, 2,000 ft lbs per degree would work (drivable), and if you have more than 5,000 ft lb per degree, you won't gain very much by stiffening it further.
American road cars in the 60's had around 2-3,000 ft lbs per degree. The average car was a little bit stiffer in the 80's. Computers and crash test standards made big improvements in the 90's, now all of the manufacturers know how to make a unit body chassis with very high stiffness and crash safety.
Mure stiffness is always better, but just because a new Audi has a huge number does not mean that you car won't handle if it has less.
Torsional rigidity (TR) is not correctly expressed as a function of the weight of the car.
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I have to disagree a bit. The relation ship between chassis loads and vehicle weight is pretty straight forward. The heavier the car, the higher the load on the chassis. So one chassis will behave similarly to another with half the stiffness in a car that weighs half as much.
Of course the ratio of sprung to unsprung mass will change things a bit, wheel base and track are certainly factors, but in general it's pretty close to a linear relationship.
My point is that no one needs a 25,000 ft lb per degree chassis, 2,000 ft lbs per degree would work (drivable), and if you have more than 5,000 ft lb per degree, you won't gain very much by stiffening it further.
American road cars in the 60's had around 2-3,000 ft lbs per degree. The average car was a little bit stiffer in the 80's. Computers and crash test standards made big improvements in the 90's, now all of the manufacturers know how to make a unit body chassis with very high stiffness and crash safety.
Mure stiffness is always better, but just because a new Audi has a huge number does not mean that you car won't handle if it has less.
Hmmm, the torsional stability is a number in Nm per degree of deflection. Just like pound per inch of a spring.
Todays shock absorbers is very sofisticated, and may load the chassies by fare higher force than the springs. If the chassies deflect it will kill the function of the shock absorbers by the same amount as the deflection of the cahassie.
To say anything about the stiffness needed, we must know qoite a bit about the car in question.
From my own experience I can say that 2-3000 fp per degree can not withstand anything more than 1,7 hz of spring stiffness on a 3000 pound car.
Goran Malmberg
Todays shock absorbers is very sofisticated, and may load the chassies by fare higher force than the springs. If the chassies deflect it will kill the function of the shock absorbers by the same amount as the deflection of the cahassie.
To say anything about the stiffness needed, we must know qoite a bit about the car in question.
From my own experience I can say that 2-3000 fp per degree can not withstand anything more than 1,7 hz of spring stiffness on a 3000 pound car.
Goran Malmberg
I agree but you also have to take into account the durometer rating of the rubber isolation dampers mounted between subframes,front and rear, and the chassis on current vehicles ,also the deflection of the bushings in the shock mounts themselves......none of this is cut and dried and all subjective until all the relavent data is acquired.
We all know what we want but getting it sometimes takes a while...eh.... JP. /ubbthreads/images/graemlins/wink.gif
I would expect that most people with any inclination towards racing would buy a chassis that combines both light weight and good structural stability...namely good useful triangulation..
I have not seen an MDA chassis but would be interested in the upgrades that have been done chassis wise..any pics JP?
I would expect that most people with any inclination towards racing would buy a chassis that combines both light weight and good structural stability...namely good useful triangulation..
I have not seen an MDA chassis but would be interested in the upgrades that have been done chassis wise..any pics JP?
May be I did not explain it correctly. I am talking about the relationship between chassis (and or vehicle weight) and TR, not induced loads. It was described as a "ratio number" by Goran. my reply was intended as a general reply but the new "system" directs replies to someone, intended or not. However with all that said. Your Quote "The heavier the car, the higher the load on the chassis,So one chassis will behave similarly to another with half the stiffness in a car that weighs half as much." IMHO Not necessarily so. many factors influence the behaviour of a chassis subjected to torsional conditions, the roll centre , the cg height, and the cornering loads imposed. A heavier car has a higher static load but it is the induced load that puts it into a torsional condition. Generally a heavier car will have a higher induced load but only because it would generally have a higher cg and perhaps a higher roll axis. A GT40 type vehicle with 2000 ftlb would not handle as well as one with 5000 and I would suggest that the 2000 vehicle would be unpredictable. You dont need 25000 but 5000 is a whole lot better than 2000 and 10000 is better still. You would be amazed at the difference. Gorans post re 1.7 Hz springs in a 2-3000 (TR) car is on the money, much stiffer than that and the chassis winds up during cornering, gets to a point and lets go with "flick" (for want of a better word) and sends a pulse through the chassis which goes all the way back to the wheels. What you need is a chassis TR that will resist the induced loads for the particular layout of the vehicle. I am one of those "nuts" who does this shit on computers and truly you would be amazed at the difference between a "jelly" and a "block of cheese" (relatively speaking)
Hope this helps,
Trevor
Hope this helps,
Trevor
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I have not seen an MDA chassis but would be interested in the upgrades that have been done chassis wise..any pics JP?
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No pics yet. Or none that would clearly show the benefits.
However, I have spoken Derek Bell about the MDA chassis. He has had a very good look at it. He believes it to be a superior product.
As has been said before. A strong chassis doesn't necessarily make it a rigid chassis. Good use of triangulation does. Thats what the MDA is all about.
Light, rigid etc.
There are no figures for rigidity yet. They will come soon.
Regards,
J.P
I have not seen an MDA chassis but would be interested in the upgrades that have been done chassis wise..any pics JP?
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No pics yet. Or none that would clearly show the benefits.
However, I have spoken Derek Bell about the MDA chassis. He has had a very good look at it. He believes it to be a superior product.
As has been said before. A strong chassis doesn't necessarily make it a rigid chassis. Good use of triangulation does. Thats what the MDA is all about.
Light, rigid etc.
There are no figures for rigidity yet. They will come soon.
Regards,
J.P
Chris Duncan
Supporter
Thanks for this topic, I didn't want to bring it up myself but wanted to wait to see if anyone else cared.
To me handling is more important than horsepower and torsional rigidity is the foundation of handling.
A monocoque chassis is going to be more rigid than a tube chassis on a weight to rigidity basis. This doesn't mean a tube frame can't be adequate, just that it will be somewhat heavier.
Many people have done dyno HP tests on their GT40's both flywheel and chassis. How many have done a torsional rigidity test? It's easier to perform and can be setup and performed in your own garage, in a couple or three days, without the expense of a HP dyno.
You can assemble the pieces necessary for a chassis torsion test for less than the price of one flywheel dyno session. After that you can perform as many torsion tests as you want for no additional cost.
Just like you can improve on HP, you can also improve your chassis rigidity.
I think just like the HP claims of engine builders, the chassis rigidity claims of the manufactures aren't always accurate.
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CHASSIS TORSIONAL RIGIDITY TEST.
PARTS
One 14' piece of 2"x4"x1/8"wall rectangular steel tubing. (May be cheaper to buy a 20' piece and cut) $50.00. This will be the test beam.
One dial gauge with magnetic holder. 1 inch range minimum. $50.00- $100.00
Two 1/2" concrete anchors (request strongest available at Home Depot) Get the type that is a threaded sleeve insert that accepts a stud or bolt so that you can remove the stud so it's not sticking up from the floor when not in use.
Suitable steel to build a chain anchor that utilizes the 2 concrete anchor bolts.
If you have a 6" reinforced slab you might get away with only one (larger) anchor bolt. You could then bolt the chain directly to the anchor bolt and make the system much simpler.
Suitable clamps to anchor the test beam to the chassis. Depends on the chassis design, but most likely 2 beefy clamps like 12" C clamps.
4 beefy jack stands,1 floor jack
(anyone building a GT40 probably already has these)
4' of anchor chain, proof coil with minimum 5/16" links more if you think your chassis is going to be really rigid.
Three or four 5 gallon buckets filled with sand or gravel. Maybe some old batteries and barbell weights also if your chassis is really rigid.
Bathroom or chassis scale to weigh the buckets. (My bathroom scale was within 5 percent of the chassis scales I recently purchased)
synopsis
steel test beam
dial gauge with holder
2 concrete anchor bolts
chain anchor bracket
chain
2 clamps
4 jackstands and jack
weighted 5 gallon buckets
scale
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SETUP
Situate the chassis so that when one end of the 14' test beam is anchored on one side of the front of the chassis there is enough room for the weight bearing end to hang off 11' or so to the other side.
Fabricate the chain anchor bracket to fit the 2 concrete anchor bolts. The simplest would be a 15" piece of 4"x1/4" angle iron with 2 holes about 12" apart in the ends for the anchor bolts and one hole in the center of the other leg for the chain attachment point.
Using the chain anchor bracket as a guide mark and drill 2 holes in your concrete slab right where the rear corner of the chassis will be on the same side of the chassis as the beam anchor. This should be centered directly under the upper shock mount on that corner.
You will need a hammer drill and masonry bit to drill concrete slab. You may want to borrow or rent these items.
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TESTING
Set the chassis in position on the jackstands with the jackstands at the lowest setting.
For the sake of convention I'm going to indicate right or left according to the setup I used (from driver seat). You may swap this if your setup dictates otherwise.
The right front jackstand should be directly under where the beam is clamped and on the top of that jackstand you need a pivot point. I used a round piece of hardwood but you could sand a peak in the flat surface of a hardwood 2x4 block. Rest the chassis on the pivot point. You may even want to use metal because the wood will deform a little and make measurement inaccurate, but again another piece of scrap metal as padding to distribute the load so the chassis isn't damaged.
Remove the left rear shock and bolt one end of the chain to the upper shock mount typically using the existing shock bolt. If the shock mount has a spacer at this bolt you should include it or an equivalent to maintain the rigidity of the mount during testing. Attach the chain to the chain anchor bracket taking up as much slack as possible. It's nice to be able to take up slack in the chain later. I actually used 2 pieces of chain bolted together in the center with a long bolt. Then the chain was tightened up by tightening the bolt.
Position the test beam with the 4" dimension in the vertical position and clamp one end of it to the left front of the chassis as near to the front upper A-arm mount as possible. Look for a flat strong place to clamp it to. You may have to use scrap steel pieces to distribute the load and/or space the beam upwards for clearance of objects. You may also have to remove parts, like the wheels/tires, for the beam to clear. Clamp the beam at both the end and at the other side of the chassis where the beam crosses. You may also want to put a piece of scrap metal under the clamp pads to distribute load and keep from crushing the chassis.
Now that the beam is weighting the chassis a little bit you can tighten up the chain as previously mentioned. You want to try and get as much slack out of the entire system as possible.
Now using the floor jack remove the left front jackstand. The chassis should sit there after you remove the floor jack. With the back of the chassis anchored and the beam supplying counter weight the chassis should be resting on the 2 rear jackstands and the front pivot jackstand.
Now take something like a cinder block (heavy block) and set a piece of scrap metal on top to attach the magnetic dial gauge holder. Set up the dial gauge at the left front of the chassis directly opposite of the jackstand pivot point. This setup should be rigid, no wobbling or movement, in order for the dial gauge to be accurate. This setup should also be easily moveable as discussed later on.
Weigh the full 5 gallon buckets and mark them. Mine were about 75 pounds each.
The standard of measurement in this case is one degree of twist. This is nowhere near what it would take to permanently bend a chassis so that makes this a safe test.
Now you have to calculate what one degree is in a linear measurement so it can be measured with the dial gauge.
Think of the distance between the pivot and the measure points as the radius of a circle. The right front pivot point will be the center of the circle with the dial gauge measuring at the circumference of the circle.
Measure the distance between these points, mine is 24", or a 48" diameter circle. Pi x Diameter = Circumference. (Pi = 3.14)
3.14 x 48" = 150-3/4"
Now convert circumference to degrees
150-3/4" circumference divided by 360 degrees in a circle
150.75 / 360 = .41875 which rounds to .419 (accuracy of the system)
so .419" is one degree measured linearly at the dial gauge.
You also need to calculate how much weight is being applied at the measurement point due to the leverage of the beam. If your distance between measure and pivot point is 2' and your weight is 10' from the pivot point the weight would be multiplied by 10/2 or a factor of 5. So 75 lbs. at 10' on the test beam is loading the chassis at the measure point with 375 lbs.
Now what you want to do is add weight until your dial gauge moves .419" or one degree.
Position the floor jack under the beam as far out as possible without interfering with where the weight buckets will hang on the end. Now measure the height of the beam at the weight end with no weight on it. This should be a fairly accurate and repeatable measurement. You do this so that you can jack the beam back to zero with the weight on it so you don't have to take the weight on and off the beam each time you measure.
Start with just one bucket and work your way up. You want to test the systems integrity before fully loading it to avoid damage to the chassis.
It's nice to have a helper at this point to lower the jack as your watching the chain, chain mount point, the jackstands, and the beam clamps for movement or other problems. You can even have them bounce on the end of the beam while you look at everything to make sure all is OK.
Now take a measurement, the chassis should move upwards at the dial gauge when weight is applied.
Add one bucket of weight and record the measurement at the dial gauge. Keep adding weight until you get to .419" or thereabouts at the dial gauge. NOW, MOST IMPORTANTLY, realize that there is some inherent play in this system, so you have to measure 3 other points to obtain complete accuracy.
ONE. Move your dial gauge setup to the right front corner and measure as close to the front jackstand pivot point as possible to see if the weight is compressing it or the pivot block. Even a strong jackstand can compress enough to affect an accurate measurement. There can also be slack between the chassis, pivot block, jackstand, and floor. Take the weight on and off the chassis by raising a lowering the floor jack and measure the difference and record it. The chassis should move downward at this point when weight is applied.
TWO. Move the dial gauge setup to the chain anchor corner (left rear) and measure the difference with weight on and off. The chassis should move upwards when weight is applied.
THREE. Move the dial gauge to the right rear and measure the difference with weight on and off. Again measuring as close to the jackstand as possible. The chassis should move downwards when weight is applied.
Now take all these measurements of play in the system, 1,2,3. Add them all together and subtract this total from the initial measurement at the clamp end of the test beam. With 6,000 lbs. of weight I had about .35" total play from the 3 points.
This is your final accurate measurement for a given weight. You will probably have to add more weight to get the calculated .419" at the main measure point. You will then have to go back and measure the three other play points because they will move even more with the added weight.
After you have achieved one degree of calculated twist with a given weight repeat the measurement a couple times and average the results for more accuracy.
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Now you have to establish what is adequate for your application.
This could be argued endlessly but I would say anything between 5,000 and 10,000 lbs/degree is good. With the typical spring rate and shock travel of a street vehicle too much rigidity may even be detrimental.
According to what I've read the current Nascars are about 8,500 lbs/degree. They were up to 15,000 at one point but this was found to be undesirable due to the differences introduced when changing tires. A little flex accommodates these differences.
After you have established whether your chassis needs strengthening or not you can measure where the movement is while taking weight on and off the test beam.
Typically this is measured diagonally across open areas and in all 3 dimensions, like the top of the engine bay or the passenger compartment from one upper door hinge to opposite door latch. You are typically looking to correct small measurements here of 1/32" to 1/8".
Three ways of adding rigidity would be.
ONE. Using a structural rivet pattern, structural rivets, and structural adhesive to fasten sheet metal panels. Although my chassis is designed to acquire rigidity from the paneling it more than doubled the rigidity after the panels were fastened. This would especially apply anywhere the tubing is designed in a square pattern with no diagonals. Securely fastening sheet metal in this case essential adds a diagonal.
TWO. Add a structurally engineered roll cage at least 6, preferably 8 point.
THREE. Add tubing diagonals. Goran's Pantera would be an example. This might be harder to apply with most GT40 kits, but is still a possibility.
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It would be nice to see a couple more categories added to the "dyno results" category in the performance section. Namely chassis torsion test, skid pad test, and cone speed test (transitional handling).
If anyone wants to read about chassis look at two books
"Race Car Chassis, design and construction" by Forbes Aird
(very specific details in layman's terms)
and
"Chassis Engineering" by Herb Adams
(describes a chassis torsion test with photos but not in detail and makes a mistake in his calculations)
This topic has also been discussed at length on this forum.
These are some of the more extensive threads on the topic.
HERE
HERE
HERE
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Please excuse me for going long here on my day off but I spent about 3 weeks dialing in my chassis torsion test method and thought someone might want to benefit.
To me handling is more important than horsepower and torsional rigidity is the foundation of handling.
A monocoque chassis is going to be more rigid than a tube chassis on a weight to rigidity basis. This doesn't mean a tube frame can't be adequate, just that it will be somewhat heavier.
Many people have done dyno HP tests on their GT40's both flywheel and chassis. How many have done a torsional rigidity test? It's easier to perform and can be setup and performed in your own garage, in a couple or three days, without the expense of a HP dyno.
You can assemble the pieces necessary for a chassis torsion test for less than the price of one flywheel dyno session. After that you can perform as many torsion tests as you want for no additional cost.
Just like you can improve on HP, you can also improve your chassis rigidity.
I think just like the HP claims of engine builders, the chassis rigidity claims of the manufactures aren't always accurate.
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CHASSIS TORSIONAL RIGIDITY TEST.
PARTS
One 14' piece of 2"x4"x1/8"wall rectangular steel tubing. (May be cheaper to buy a 20' piece and cut) $50.00. This will be the test beam.
One dial gauge with magnetic holder. 1 inch range minimum. $50.00- $100.00
Two 1/2" concrete anchors (request strongest available at Home Depot) Get the type that is a threaded sleeve insert that accepts a stud or bolt so that you can remove the stud so it's not sticking up from the floor when not in use.
Suitable steel to build a chain anchor that utilizes the 2 concrete anchor bolts.
If you have a 6" reinforced slab you might get away with only one (larger) anchor bolt. You could then bolt the chain directly to the anchor bolt and make the system much simpler.
Suitable clamps to anchor the test beam to the chassis. Depends on the chassis design, but most likely 2 beefy clamps like 12" C clamps.
4 beefy jack stands,1 floor jack
(anyone building a GT40 probably already has these)
4' of anchor chain, proof coil with minimum 5/16" links more if you think your chassis is going to be really rigid.
Three or four 5 gallon buckets filled with sand or gravel. Maybe some old batteries and barbell weights also if your chassis is really rigid.
Bathroom or chassis scale to weigh the buckets. (My bathroom scale was within 5 percent of the chassis scales I recently purchased)
synopsis
steel test beam
dial gauge with holder
2 concrete anchor bolts
chain anchor bracket
chain
2 clamps
4 jackstands and jack
weighted 5 gallon buckets
scale
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SETUP
Situate the chassis so that when one end of the 14' test beam is anchored on one side of the front of the chassis there is enough room for the weight bearing end to hang off 11' or so to the other side.
Fabricate the chain anchor bracket to fit the 2 concrete anchor bolts. The simplest would be a 15" piece of 4"x1/4" angle iron with 2 holes about 12" apart in the ends for the anchor bolts and one hole in the center of the other leg for the chain attachment point.
Using the chain anchor bracket as a guide mark and drill 2 holes in your concrete slab right where the rear corner of the chassis will be on the same side of the chassis as the beam anchor. This should be centered directly under the upper shock mount on that corner.
You will need a hammer drill and masonry bit to drill concrete slab. You may want to borrow or rent these items.
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TESTING
Set the chassis in position on the jackstands with the jackstands at the lowest setting.
For the sake of convention I'm going to indicate right or left according to the setup I used (from driver seat). You may swap this if your setup dictates otherwise.
The right front jackstand should be directly under where the beam is clamped and on the top of that jackstand you need a pivot point. I used a round piece of hardwood but you could sand a peak in the flat surface of a hardwood 2x4 block. Rest the chassis on the pivot point. You may even want to use metal because the wood will deform a little and make measurement inaccurate, but again another piece of scrap metal as padding to distribute the load so the chassis isn't damaged.
Remove the left rear shock and bolt one end of the chain to the upper shock mount typically using the existing shock bolt. If the shock mount has a spacer at this bolt you should include it or an equivalent to maintain the rigidity of the mount during testing. Attach the chain to the chain anchor bracket taking up as much slack as possible. It's nice to be able to take up slack in the chain later. I actually used 2 pieces of chain bolted together in the center with a long bolt. Then the chain was tightened up by tightening the bolt.
Position the test beam with the 4" dimension in the vertical position and clamp one end of it to the left front of the chassis as near to the front upper A-arm mount as possible. Look for a flat strong place to clamp it to. You may have to use scrap steel pieces to distribute the load and/or space the beam upwards for clearance of objects. You may also have to remove parts, like the wheels/tires, for the beam to clear. Clamp the beam at both the end and at the other side of the chassis where the beam crosses. You may also want to put a piece of scrap metal under the clamp pads to distribute load and keep from crushing the chassis.
Now that the beam is weighting the chassis a little bit you can tighten up the chain as previously mentioned. You want to try and get as much slack out of the entire system as possible.
Now using the floor jack remove the left front jackstand. The chassis should sit there after you remove the floor jack. With the back of the chassis anchored and the beam supplying counter weight the chassis should be resting on the 2 rear jackstands and the front pivot jackstand.
Now take something like a cinder block (heavy block) and set a piece of scrap metal on top to attach the magnetic dial gauge holder. Set up the dial gauge at the left front of the chassis directly opposite of the jackstand pivot point. This setup should be rigid, no wobbling or movement, in order for the dial gauge to be accurate. This setup should also be easily moveable as discussed later on.
Weigh the full 5 gallon buckets and mark them. Mine were about 75 pounds each.
The standard of measurement in this case is one degree of twist. This is nowhere near what it would take to permanently bend a chassis so that makes this a safe test.
Now you have to calculate what one degree is in a linear measurement so it can be measured with the dial gauge.
Think of the distance between the pivot and the measure points as the radius of a circle. The right front pivot point will be the center of the circle with the dial gauge measuring at the circumference of the circle.
Measure the distance between these points, mine is 24", or a 48" diameter circle. Pi x Diameter = Circumference. (Pi = 3.14)
3.14 x 48" = 150-3/4"
Now convert circumference to degrees
150-3/4" circumference divided by 360 degrees in a circle
150.75 / 360 = .41875 which rounds to .419 (accuracy of the system)
so .419" is one degree measured linearly at the dial gauge.
You also need to calculate how much weight is being applied at the measurement point due to the leverage of the beam. If your distance between measure and pivot point is 2' and your weight is 10' from the pivot point the weight would be multiplied by 10/2 or a factor of 5. So 75 lbs. at 10' on the test beam is loading the chassis at the measure point with 375 lbs.
Now what you want to do is add weight until your dial gauge moves .419" or one degree.
Position the floor jack under the beam as far out as possible without interfering with where the weight buckets will hang on the end. Now measure the height of the beam at the weight end with no weight on it. This should be a fairly accurate and repeatable measurement. You do this so that you can jack the beam back to zero with the weight on it so you don't have to take the weight on and off the beam each time you measure.
Start with just one bucket and work your way up. You want to test the systems integrity before fully loading it to avoid damage to the chassis.
It's nice to have a helper at this point to lower the jack as your watching the chain, chain mount point, the jackstands, and the beam clamps for movement or other problems. You can even have them bounce on the end of the beam while you look at everything to make sure all is OK.
Now take a measurement, the chassis should move upwards at the dial gauge when weight is applied.
Add one bucket of weight and record the measurement at the dial gauge. Keep adding weight until you get to .419" or thereabouts at the dial gauge. NOW, MOST IMPORTANTLY, realize that there is some inherent play in this system, so you have to measure 3 other points to obtain complete accuracy.
ONE. Move your dial gauge setup to the right front corner and measure as close to the front jackstand pivot point as possible to see if the weight is compressing it or the pivot block. Even a strong jackstand can compress enough to affect an accurate measurement. There can also be slack between the chassis, pivot block, jackstand, and floor. Take the weight on and off the chassis by raising a lowering the floor jack and measure the difference and record it. The chassis should move downward at this point when weight is applied.
TWO. Move the dial gauge setup to the chain anchor corner (left rear) and measure the difference with weight on and off. The chassis should move upwards when weight is applied.
THREE. Move the dial gauge to the right rear and measure the difference with weight on and off. Again measuring as close to the jackstand as possible. The chassis should move downwards when weight is applied.
Now take all these measurements of play in the system, 1,2,3. Add them all together and subtract this total from the initial measurement at the clamp end of the test beam. With 6,000 lbs. of weight I had about .35" total play from the 3 points.
This is your final accurate measurement for a given weight. You will probably have to add more weight to get the calculated .419" at the main measure point. You will then have to go back and measure the three other play points because they will move even more with the added weight.
After you have achieved one degree of calculated twist with a given weight repeat the measurement a couple times and average the results for more accuracy.
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Now you have to establish what is adequate for your application.
This could be argued endlessly but I would say anything between 5,000 and 10,000 lbs/degree is good. With the typical spring rate and shock travel of a street vehicle too much rigidity may even be detrimental.
According to what I've read the current Nascars are about 8,500 lbs/degree. They were up to 15,000 at one point but this was found to be undesirable due to the differences introduced when changing tires. A little flex accommodates these differences.
After you have established whether your chassis needs strengthening or not you can measure where the movement is while taking weight on and off the test beam.
Typically this is measured diagonally across open areas and in all 3 dimensions, like the top of the engine bay or the passenger compartment from one upper door hinge to opposite door latch. You are typically looking to correct small measurements here of 1/32" to 1/8".
Three ways of adding rigidity would be.
ONE. Using a structural rivet pattern, structural rivets, and structural adhesive to fasten sheet metal panels. Although my chassis is designed to acquire rigidity from the paneling it more than doubled the rigidity after the panels were fastened. This would especially apply anywhere the tubing is designed in a square pattern with no diagonals. Securely fastening sheet metal in this case essential adds a diagonal.
TWO. Add a structurally engineered roll cage at least 6, preferably 8 point.
THREE. Add tubing diagonals. Goran's Pantera would be an example. This might be harder to apply with most GT40 kits, but is still a possibility.
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It would be nice to see a couple more categories added to the "dyno results" category in the performance section. Namely chassis torsion test, skid pad test, and cone speed test (transitional handling).
If anyone wants to read about chassis look at two books
"Race Car Chassis, design and construction" by Forbes Aird
(very specific details in layman's terms)
and
"Chassis Engineering" by Herb Adams
(describes a chassis torsion test with photos but not in detail and makes a mistake in his calculations)
This topic has also been discussed at length on this forum.
These are some of the more extensive threads on the topic.
HERE
HERE
HERE
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Please excuse me for going long here on my day off but I spent about 3 weeks dialing in my chassis torsion test method and thought someone might want to benefit.
