Chassis Torsional Stiffness

[Randy V]

Randy,
I think you will find if you do the research, that generally speaking in car design the stiffer the better. The chassis is not meant to take the place of the suspension.


Ron, none the less the way its measured, the stiffer the better.

But we are starting to drift...
Regards,
Scott

Scott - It is you that needs to do this research. I've done mine. As I said before, all chassis need to have some degree of compliance (read flex) in them to work properly.

The wing tips on a 747 flex up/down up to ~4 feet.
Buildings are engineered and built to sway a certain amount in the wind.

I've been building chassis for racecars for a lot of years. Many of my own design. From Karts to Top-Fuel Dragsters and most everything in between.
One of the types of racecars we built and fielded were Late Model Stock cars for both Dirt and Asphalt.
Before you automatically discount these cars as garbage like so many of the road-race crowd do, understand this - We were putting down 800+ HP to the ground with a car that weighed 2300# with driver on a track that's rougher than the south-40 of the Ponderosa ranch.
These chassis, when new, were rockets and handled quite well. As the chassis' aged, they would work-harden and get stiffer overall. They handling of these chassis were exceptionally difficult to get hold of. Part and parcel to the hardening of various parts of the chassis at different rates. We finally got a handle on how long it would take Mild Steel to stiffen and how long it would take CM to stiffen up. We could get 20 races from the mild steel and half again more from the Moly.
We would sell these used chassis to "Budget" racers that would run them until they broke.. Some used our old chassis to clone them and make their own. Pity that few of them really understood how to properly weld and anneal.

Guys would come out there with very stiff chassis' from time to time - Sometimed they worked pretty well - most of the time they would not do well at all..

Regardless of the venue, if you build it too stiff, something will break.

Your challenge is as it was before - finding out where to draw the line.

or

To pay someone else to do it for you....

 

[andys]

Ditto Jim and Russ. Comparing chassis stiffness' ultimate values leaves out a crucial standard unit of measure between chassis types/weight/application, and so on. So I believe Ron to be correct in his assesement of a lack of standard. The values, unless you are comparing like models, are of shall I say...little value. Throw in variations of test set up's, and I suspect the values will be all over the map. IMHO of course.

Andy

 

[Mark Worthington]

I think there's some questionable tech in this thread. First, Russ, from the perspective of NVH and vehicle dynamics, there is no such thing as a chassis that's too stiff. Period. Of course, the compromise is in trading more stiffness for more weight. Further, results from a a properly performed torsional rigidity test are anything but meaningless. As long as the test restrains the chassis at the same locations where the suspension loads are fed in, the results are the results. Yeah, OK, a car with a cage will be more stiff - of course. But it is what it is. More is better.

I think Neal measured torsional rigidity of his GT40NZ, but I'm too lazy to search for the thread now.

 

[Eric]

If a vehicle suspends the chassis/body system on springs, the chassis must simply be stiff enough to resist bending in torsion AND beam in order for the suspension to function as intended. More precision from the suspension requires more chassis stiffness. Simple as that. the degree of which the chassis must resist flex is a function of the purpose of the car.

The reason it is not possible to compare modern downforce loaded racing cars to road cars is simply that with high downforce loads, the suspension springs are ridiculously high rate, and therefore the chassis must match. It was getting harder and harder to make this work and stay lightweight enough with metal chassis, hence the change to ultra-stiff carbon chassis.

The SAE does not specify anything for chasis stiffness. Every manufacturer has its own bechmarks for this. lets also be aware that production cars are broken down into several structures, and each requires different propereties, also understand that a primary drive for modern cars is how theey fold up in impact rather than torsion stiffness.

The higher the performance required from the suspension, the stiffer the chassis must be to allow it to function properly in geometric changes. This is a given, however as has been said before, if your car is on rubber bushes, it is most likely that they deflect far more and long before the chassis does. If you are on solid suspension mounts (rose joints) - any flex in components will become quickly apparent.

I will also say that in chassis design, it is not so simple as designing for torsional or beam rigidity....they are simply related measurements to the more important design factor...LOAD PATHS. How load is distributed by the chassis from the 4 input points is critical. It is much more important to be able to see a chassis and know how it is distributing loads than to just have some simplified torsional number that is not truly comparable.

Okay, enough, its starting to look like a book

cheers
Eric

 

[BruceA]

I still believe that the chassis only needs to be strong enough torsionally to resist the maximum load from the wheel, generally in bounce. The second thing to that is the torque from a front engine/rear drive car that wants to twist the vehicle as well. (but most of us here have midmount engines anyway).
As the Ultima and Lotus Elan for want of perhaps other examples, are touted as being very good handling cars, why then would you need to double the torsional strength of their chassis?
So why then if the maximum loading into a shock mount does not twist a chassis even by 0.5 of a degree, why would you want to double or triple its strength? Seems a waste of material, engineering and weight penalties.
But are you going to really notice 1 degree of chassis twist on the limit? After all, your wheel rims, tyres, suspension arms and bushes all have flex and play, so you could then argue that all your suspension setup is really just a good compromise.

 

[Russ Noble]

I think there's some questionable tech in this thread. First, Russ, from the perspective of NVH and vehicle dynamics, there is no such thing as a chassis that's too stiff. Period. Yeah, I did refer to the perspective of the regulatory bodies! Of course, the compromise is in trading more stiffness for more weight. Mark, that's exactly what I said. Further, results from a a properly performed torsional rigidity test are anything but meaningless. As long as the test restrains the chassis at the same locations where the suspension loads are fed in, the results are the results. I agree with you, but it seems that between kit manufacturers there is no standard approach to the test. Therefore a comparison is meaningless. If a motor/trans is used as a stressed unit this should be included as should doors if they are designed to add rigidity, stressed skins, or any other element if it comes to that.The only meaningful test is on a complete car, in a laboratory using the same methods and equipment. Results from a bare chassis/mono/shell, do not tell anywhere near the whole story. Yeah, OK, a car with a cage will be more stiff - of course. But it is what it is. More is better.

I think Neal measured torsional rigidity of his GT40NZ, but I'm too lazy to search for the thread now.

BTW, Mark, do you think my internet skills are improving?epper:

Cheers

 

[Graeme Stebbing]

Guys this article is well worth a read,especially the parts covering testing with or without panels.
http://locost7.info/files/chassis/kitcaranalysis.doc
hope it helps.
Regards
Lambo

 

[Russ Noble]

Scott - It is you that needs to do this research. I've done mine. As I said before, all chassis need to have some degree of compliance (read flex) in them to work properly.

The wing tips on a 747 flex up/down up to ~4 feet.
Buildings are engineered and built to sway a certain amount in the wind.

I've been building chassis for racecars for a lot of years. Many of my own design. From Karts to Top-Fuel Dragsters and most everything in between.
One of the types of racecars we built and fielded were Late Model Stock cars for both Dirt and Asphalt. Before you automatically discount these cars as garbage like so many of the road-race crowd do, understand this - We were putting down 800+ HP to the ground with a car that weighed 2300# with driver on a track that's rougher than the south-40 of the Ponderosa ranch. Hi Randy, is this what we Down Under call Sprint Cars? Years ago the NZ Sprint Car Champion at the time used to work for me, so I am a bit familiar with those cars and used to help him in the pits from time to time, so I have a lot of respect for the specialised engineering and setup involved. He used to run a Gambler chassis when they were the hot thing out here.
These chassis, when new, were rockets and handled quite well. As the chassis' aged, they would work-harden and get stiffer overall. They handling of these chassis were exceptionally difficult to get hold of. Part and parcel to the hardening of various parts of the chassis at different rates. We finally got a handle on how long it would take Mild Steel to stiffen and how long it would take CM to stiffen up. Work hardening is caused by chassis flex? You are building them like a gokart so the chassis is part of the suspension? We could only! get 20 races from the mild steel No wonder! and half again more from the Moly.
We would sell these used chassis to "Budget" racers that would run them until they broke.. Some used our old chassis to clone them and make their own. Pity that few of them really understood how to properly weld and anneal.

Guys would come out there with very stiff chassis' from time to time - Sometimed they worked pretty well - most of the time they would not do well at all..

Regardless of the venue, if you build it too stiff, something will break. Sounds like a design flaw in the component that breaks or the one that acts on it! Maybe beefing things up so they don't break carries too much of a weight and performance penalty. So you build them light, tune them to suit and throw them away after 20 races?

Your challenge is as it was before - finding out where to draw the line.

or

To pay someone else to do it for you....

IMHO.

Cheers,

 

[Randy V]

Hi Russ,

The Sprint cars I'm familiar with were between 83 and 90 inch wheelbase open wheel cars that usually used a pair of airfoils. The Gambler chassis design was exceptionally rigid and the suspension exceptionally compliant. While not having built sprint cars from scratch - I've worked on them.

The Latemodels used 104 inch wheelbase closed wheel and usually did not use airfoils.

Yes the late models were built very lightweight and they had to be. Yes the chassis were designed to be an extension of the suspension of the car and needed to be in order to maintain control.

Were we doing it all wrong? Maybe - but we helped win a number of championships doing it that way.

Still there is more to be learned.

Examine the failure points of existing chassis as they hold many secrets..

 

[Russ Noble]

Randy, it's about winning races and championships. If you do that you've done it right. Whatever it takes to get across the line first. If you've got to build light and replace frequently to do that so be it.

Stirling Moss, I think, once said something to the effect that if a car didn't fall to bits as it crossed the finish line then it was overengineered and could have been built lighter to go faster! Or maybe that was Colin Chapman?

Not quite the intention of Scotts thread perhaps......

Good pics BTW. A Sprint Car is obviously a Sprint Car where ever you are. Your Late Model Stock Cars appear to be the approx equivalent of our Super Saloons

Cheers

 

[Ron Earp]

Further, results from a a properly performed torsional rigidity test are anything but meaningless. As long as the test restrains the chassis at the same locations where the suspension loads are fed in, the results are the results.

The issue is that "properly performed" depends upon who you ask. Once a number of different people/manufactuers perform the test, using different methods, then they cannot be compared to one another and for that purpose are essentially not useful. They are still useful to that company for looking at improvements or changes, but they aren't useful to say XYZ > ABC.

Ron

 

[Scott Calabro]

Holy Crap !

You guy's were busy last night !!!

Mark ... I completely agree.

Randy ... On a kart or a top fuel dragster, the frame IS the suspention. I would never discount anybody's ride as "Garbage". Dirt track cars are brutally fast and fun to watch, but we are speaking about Road Race cars and their particulars on this thread. I don't need aviation trivia, I live it every day.

Lambo ... I will definately read that article, thank you.

Russ ... A gentleman as always.

Ron ... I always enjoy when you participate.

I did not start this thread for it to end up with everybody pissed.

After re-reading this thread from the begining though it does seem that it is a topic as Ron said won't boring.

I am convinced now more than ever that the figures can be accurately measured, standardized and compared and posted, if mfg's wanted to, but they don't.

Except Bailey Edwards.

Thats a damn shame because some chassis are probably Very light and stiff and others are probably not. Some guy's probably don't care about the numbers, they just wan't the cool factor. I KNOW the cars are Bitchin', but I want the science too. Too bad because transparency with the customer should be paramount.

Best to all, you guy's are great,
Scott

 

[Julian]

Interesting discussion. A few points to consider before I'll show you all one way (the 'correct' way) to test for torsion. No cheating and scrolling down to the photo now!
What is the point of chassis stiffness? Well, for a car with functioning suspension (not talking about weird stuff like dragsters, F1 or similar), it allows the suspension to do the job it was tuned to do, and not the chassis flexing excessively so as to dilute each minor suspension tweak. Given the nature of cars we are talking about (road/circuit race), it is important to maximise chassis stiffness, within reason. Meaning stiffness vs weight is a compromise, like all things. However, some intelligent design will allow maximum stiffness per given weight.
The other point of chassis stiffness is that the registration authorities here in Australia tend to require documented proof! So it takes on a whole new importance down here!
So what are some realistic numbers for actual kit cars? Well, it just so happens I have a few results to hand. These are real results, not internet hearsay. Having said that, the results list posted earlier in this thread seem realistic to me.
OK - extensively modified (extra bracing) Locost +100 chassis 5400 Nm/deg
Locost type chassis with live rear axle (32 mm sq tube used, not 25 mm sq tube as usual) 4300 + Nm/deg (test was performed without dial gauges at rear (fixed) axle line) so this value is a bit conservative. Could be 5000 - 6000 Nm/deg in reality
Similar chassis (same 32 mm tubing) but IRS design 5950. Same conservative testing so probably 7000 - 8000 Nm /deg in reality.
Cobra replica chassis (kit car manufacturer supplied) 8000 Nm/deg
Cobra replica chassis (scratch built) 6500 Nm/deg

These are actual tests. Much more testing has been done using FEA computer analysis. A std book Locost chassis is around 2300 Nm/deg when computer analysed for instance. A Westfield would be similar given that a Locost is essentially the same thing. Computer analysis is great when designing chassis for 'what if' scenarios as it is much quicker and easier to virtually add a bit of cross bracing or chaning the chassis memeber sizes than doing it for real! Plus you also see the weight effect of making changes, though that's pretty easy to do on paper anyway.

The value for a DRB GT40 is 7250 Nm/deg, which again is realistic.

Manufacturers, as previously stated, don't tend to give out actual torsion results, but love the % increase reporting with each new model. I do remember reading that the new (at the time) BMW Z4 was three times torsionally stiffer than the old Z3 with a figure of 14500 Nm/deg. So the Z3 would be a little under 5000 Nm/deg one could infer. I also remember reading very recently a newish car was claimed to be around 30000 Nm/deg, which was apparently some sort of record for a production car. Wish I could remember what car it was though!

So, how to test torsional stiffness? Well, unlike the photo shown earlier, and also unlike the CAV GT40 test method, the load should be transferred to the chassis using the wheel hubs and suspension, the same as any real life load would be. I had trouble swallowing the amazing results for the CAV, but when you see how they tested it, well it's not comparing apples with apples now is it? Nice test setup though.

Have a look at the attached photo, which is actually of the +100 Locost mentioned above being tested. Note the chassis is fixed to ground via rear hubs. The torsional load is applied through the front hubs. If you're very observant you may note the front pivot is located on rollers also to take out any chance of loads building up due to restrained lateral movement under load. The other trick is to replace the spring/damper units with solid bars to prevent suspension movement. Dial gauges are located down both sides of the chassis and simple calculations allow conversion of vertical movement to angles. In this way the chassis siffness can be checked for abrupt discontinuities along it's length. Also a pair of dial gauges is located at the rear (fixed) axle line to subtract out the minor twist that will occur there even though it is 'locked' to the ground.

Think about what happens to a chassis due to suspension load inputs. In the case of double wishbone suspension (typical on front of most kitcars) a load travels up the spring/damper unit, but also loads are applied into the chassis through the wishbones. So, you could make a chassis really good at resisting forces through the damper mount, but forget about the wishbone mounts if you tested just applying loads to the damper mounts!

Anyways, there's a lot more that can be said, but it gets a bit 'engineering nerdy' and I can't be bothered! Lazy me...

 

[Fred W Ballinger]

Wow, that ran a bit.......

Thanks for your well made comments Julian. I will agree that my rig was rough and ready, but I like to think my results at least give me a indication of how stiff my chassis is.

Did you do any tests with a drivetrain in? As stated by various people above, a real world test would be ideal, but it would be very inconvienent to build up (or model) a whole car when you are trying to do comparative or iterative tests on a chassis design.

WRT to the CAV test, I though that the fact that the load they applied was such that they only recorded just over a tenth of a degree of actual twist will make them very suseptable to any errorr. They would have only recorded about half a mm on their dial gauges (up one side, down the other, at the 300mm from centre span distance they used.

Cheers

Fred W B

 

[Keith]

:lurker:

 

[speed220mph]

To give you all some reference, the 75 pound tub of the Mk IV GT40 had a torsional rigidity of just a bit over 10,000 ft-lb/degree of twist.

Having done a fair amount of chassis bending and twist tests, there comes a point that more doesn't give any benefit. As for the "Flexy Flier" argument goes, if you're doing a Baja racer, a flimsy structure can be OK. As for hard surfaces, this is definately not the case. Regardless, it is best--as said--to design the complete package to let the suspension do the work both for both on and off-road applications. I've seen cars that wouldn't respond to changes in sway-bar stiffness simply because the chassis was so flimsy it would twist and not transfer weight in roll.

A final point: When doing a bending or twist test, it's critical that there is a continuous deflection over the entire length of the structure. This is determined by using many dial indicators place every six or 12 inches between the front and rear wheel centerlines, performing the twist or bend, reading the deflections, then graphing the results. The curve should be smooth. If it isn't, the weak points are strengthened and the structure is restested.

 

[Scott Calabro]

Julian,

That rig is COOL !
When I get a kit I'm going to build a rig like that it just for the hell of it !

EXCELLENT post BTW

Regards,
Scott

 

[Randy V]

....and when you bend it during your testing and it doesn't come back?


BTW - I never made mention to the chassis being as a noodle or Flexi-Flyer (etc).. Calculated compliance is exactly that.. It's compliance that is accounted for. It's acceptable.

Some potential points to ponder...

How many drivers get killed in;
1) Road racing
2) Off Road racing
3) Drag Racing
4) Stock Car racing

????

Make that chassis as stiff as you will - just remember that you are making a human being PART of that chassis...

Exclusive of the springs - It would appear that some may think of the suspension as that which attaches to the chassis on one end and the wheel on the other. Have you given thought to the tires and wheels actually playing a role as springs / compliant members?

Most people think of suspension in terms of the spring's ability to suspend the vehicle while carrying the additional loading of body roll and weight transfer. Little creedance is paid to lateral forces that are momentary.
Let me put this into a slightly less than "perfect world road race" scenario- You set the car up perfectly for the corner and flawlessly hit your apex. The car is drifting / tracking out perfectly when you encounter just a little bit of sand from another car's "off".. Traction is compromised momentarily and the track out angle increases 10% - there's the FIA curbing to catch you (you hope), gators on the other side of it.. Your car hits the curbing and rides up to the gators.
((Stop))
What's your suspension doing for these lateral impacts? - what's absorbing them? If the tire takes 20% of the impact, there's 80% left for the wheels, control arms, balljoints and (whups) the chassis..

It was only a slight misjudgement - what will it cost you?

What happens if you get a tap in the quarter panel from a fellow competitor and you find yourself in worse shape and take a full OFF at a high rate of speed through a gopher hole strewn pasture that is slamming the weight of the car laterally into the suspension - literally hammering it... Do you just write off the race, the car or would you rather be able to recover and continue on.. Not to mention being able to race that car again..

If life for competition cars precluded ruts, curbs, contact with other cars and driving only took place on pristine racetracks - you'd be perfectly justified in requiring a chassis that had absolutely no compliance in the mounting points for the suspension (remember the soul that is harnessed to your work though)..

If you wake up and find yourself in less than a perfect world and driving in less than perfect conditions - you may want to consider having just a little bit of compliance in your chassis.

Meanwhile - I'm going to join some of the others and munch on some :lurker: ...

 

[Jac Mac]

Bring plenty of popcorn Randy, we could be here on the fence for a while yet!

Jac Mac

 

[Mark Worthington]

Excellent post, Julian.

Randy, that was an interesting description of an off-road excursion, but what does it have to do with chassis torsional rigidity? If I read your post correctly, you think some chassis flex is a good thing as it might save your ass if you've used up 100% of the compliance in your tires and suspension but you need 100.05% to keep from going off the road? I submit that, in a situation such as you describe, a stiffer chassis would have let the suspension and tires do a better job maximizing grip such that you wouldn't have gotten into that situation in the first place. I agree with you that chassis stiffness is not as important on dirt surfaces and on karts (where the tires are the suspension). But I would want to make my road car chassis as stiff as possible for a given weight.

Oh, yeah, I had a couple other questions/comments about a post you made earlier:

The wing tips on a 747 flex up/down up to ~4 feet.
Buildings are engineered and built to sway a certain amount in the wind.

True, but an airplane wingtip or a building is not at all like a GT40 chassis in contact with a paved road surface through a tire/suspension system working in series as defined by Hooke's Law. The comparison is moot.

These chassis, when new, were rockets and handled quite well. As the chassis' aged, they would work-harden and get stiffer overall. They handling of these chassis were exceptionally difficult to get hold of. Part and parcel to the hardening of various parts of the chassis at different rates. We finally got a handle on how long it would take Mild Steel to stiffen and how long it would take CM to stiffen up. We could get 20 races from the mild steel and half again more from the Moly.

We would sell these used chassis to "Budget" racers that would run them until they broke

Ummm, work hardening, by definition, occurs as a result of plastic deformation. Therefore, I don't think your chassis are work hardening (unless they're so flimsy that they actually do deform beyond the elastic limit!!), rather, it sounds like plain-ole metal fatigue to me.

And are you guys really getting NASCAR horsepower levels (800+ hp) out of LMSCs? That must be a helluva ride.