Chassis Rigidity - Replicas v. Origianls
- Thread starter Hoota Caroota
- Start date
Try looking here, plus I think there have been a number of other discussions on this. Have you tried the "Search" function John?
http://www.gt40s.com/forum/gt40-tec...wheels/21354-chassis-torsional-stiffness.html
http://www.gt40s.com/forum/gt40-tec...wheels/21354-chassis-torsional-stiffness.html
DRB have figures, as chassis have to be tested that they achieve the torsional rigidity required by our Australian Design Rules ( ADR's), for road registration.
Thanks for the link, Brian. It was an interesting discussion.
What brought my question to mind was the claim by SPF that so many of the parts on their cars are interchangeable with the originals. I understand their chassis aren't as complex as originals and it got me to wondering about rigidity.
In the discussion at that link, someone said the Mk IV GT40 had a torsional rigidity of just a bit over 10,000 ft-lb/degree of twist. I wonder if MK1s and MK11s were ever tested. I wonder if an SPF was ever tested.
What brought my question to mind was the claim by SPF that so many of the parts on their cars are interchangeable with the originals. I understand their chassis aren't as complex as originals and it got me to wondering about rigidity.
In the discussion at that link, someone said the Mk IV GT40 had a torsional rigidity of just a bit over 10,000 ft-lb/degree of twist. I wonder if MK1s and MK11s were ever tested. I wonder if an SPF was ever tested.
MK1 & MKII Monocoque was also in the 10000 ft lb per degree region, so if you can find out exactly what sort of test rig apparatus was used by Ford & replicate it, you should then be able to hook it up to your ??% accurate SPF replica & see if the chassis is similar. This info gleaned from book..Lola T70 coupes by Ian Bamsey.... interestingly the T70 coupe only had 3200ft lb per degree & Broadley felt at the time that the extra 6800ft lb per degree was no real advantage.
Really, I think there are things that you could be concerned about other than chassis stiffness!
Really, I think there are things that you could be concerned about other than chassis stiffness!
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I do remember that the MK I/II tub was reckoned to be almost twice as stiff as the F1 monos of the same era.
Who said that the SPF tub is "simplified" as compared to the FAV/Abbey cars? The Safir MK V WAS changed to allow more "flat sheet" construction and reduce the need for stamping dies, but the SPF tub uses a buttload of dies. The Safir cars used a fabricated roof panel for example (Autokraft hand formed the early cars roofs for Peter Thorpe until he and Brian Angliss had a falling out) but Hi Tech uses their ex-GM 100+ ton press to stamp the roof panels as well as many other non flat fabricated pieces.
The GT tub is very stiff, but do a little research and you can measure any deflection with a jack and some simple items if that is what you want to know.
Who said that the SPF tub is "simplified" as compared to the FAV/Abbey cars? The Safir MK V WAS changed to allow more "flat sheet" construction and reduce the need for stamping dies, but the SPF tub uses a buttload of dies. The Safir cars used a fabricated roof panel for example (Autokraft hand formed the early cars roofs for Peter Thorpe until he and Brian Angliss had a falling out) but Hi Tech uses their ex-GM 100+ ton press to stamp the roof panels as well as many other non flat fabricated pieces.
The GT tub is very stiff, but do a little research and you can measure any deflection with a jack and some simple items if that is what you want to know.
Seymour Snerd
Lifetime Supporter
Being intimately familiar with the SPF tub right now, since I spend my days looking for places to mount/route/grommet bits and pieces, I must say I'd be surprised it there is much simplification. There are a lot little pieces in the SPF tub that are clearly there for stiffening reasons and which SPF could easily have left off if they wanted to, saving themselves both time and materials.
So if anyone has any specifics to such claims of simplification I'd would love to know them.
I've had my SPF MK11 for about a month now and after driving it fairly hard every day for the first three weeks, snow came to the neighborhood last week and shut me down. So I've been having to just remember driving the car and think about the damn thing.
The closest I've come in the past to the GT40 is driving hard in a Pantera. But that car is nothing like the SPF. Mike Drew, who has a Pantera and a lot of seat time in SPFs, described the difference between the two cars by saying the GT40 handles like a go cart. When he told me that I thought, "Hmmmm. I think the Pantera handles like a go cart."
But now I know what he means. One reason I bought an SPF instead of a CAV was because I wanted to get as close the real experience as I could. So when I was sweeping the snow the other day but thinking about the incredible grip the GT40 has compared with the Pantera, I began wondering about chassis stiffness. I can definitely feel the difference between the two cars and I'm happy to hear that my experience with the GT40 is the same (relatively) as, say, Dan Gurney's.
The closest I've come in the past to the GT40 is driving hard in a Pantera. But that car is nothing like the SPF. Mike Drew, who has a Pantera and a lot of seat time in SPFs, described the difference between the two cars by saying the GT40 handles like a go cart. When he told me that I thought, "Hmmmm. I think the Pantera handles like a go cart."
But now I know what he means. One reason I bought an SPF instead of a CAV was because I wanted to get as close the real experience as I could. So when I was sweeping the snow the other day but thinking about the incredible grip the GT40 has compared with the Pantera, I began wondering about chassis stiffness. I can definitely feel the difference between the two cars and I'm happy to hear that my experience with the GT40 is the same (relatively) as, say, Dan Gurney's.
Years ago I had a circle track driver who bought a Pantera and brought it to my shop for evaluation. He told me it "didn't handle quite right", so I drove it. I told him it was one of the better driving Panteras I had driven. He asked "You mean it's SUPPOSED to be like that?!?!?"
I was in love with Panteras in my younger days until I had a drive in a GT40 and found out what mid-engined was meant to be like!
The SPF chassis is complicated and stiff!
I was in love with Panteras in my younger days until I had a drive in a GT40 and found out what mid-engined was meant to be like!
The SPF chassis is complicated and stiff!
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Jim Rosenthal
Supporter
Those photos look a lot like the assembly photos of 1149s tub. The blue paint is interesting; original GT40 tubs were painted that color-Ford blue, I suppose. However, the rear metalwork and sway bar do look different.
Jim,
The rear framework IS different as that is a MK II tub. And yes, all of the early cars were blue painted. Superformance offers painted chassis as an option. I really like the blue myself.
The rear framework IS different as that is a MK II tub. And yes, all of the early cars were blue painted. Superformance offers painted chassis as an option. I really like the blue myself.
Great pictures, Rick. Thanks for posting them.
For comparison purposes, here are some chassis pics of 1033 (on a trailer before it was restored), 1036 (on a stand), and an ERA chassis (speaks for itself).
For comparison purposes, here are some chassis pics of 1033 (on a trailer before it was restored), 1036 (on a stand), and an ERA chassis (speaks for itself).
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Getting back to rigidity, it's interesting to note the stiffening ribs along the sills in both the original and SPF chassis. They have to add a lot to the torsional rigidity of the structure. With the SPF, including them is a nice detail that stays true to the original and surely must add to the expense of construction.
The ERA chassis doesn't have the stiffening ribs nor do CAV and RCR.
(The pictures below are taken from the CAV and RCR websites. The first two pics are CAV's chassis, the third RCR's.)
The ERA chassis doesn't have the stiffening ribs nor do CAV and RCR.
(The pictures below are taken from the CAV and RCR websites. The first two pics are CAV's chassis, the third RCR's.)
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The RCR car is done out of heavy gauge aluminum and the ribs wouldn't really add much. I believe the CAV car is also heavier stainless and again would not get much added stiffness.
The sheet steel used on the SPF and FAV cars needs the ribs for stiffness and to prevent "oil canning" on larger flat surfaces. Note the SPF and FAV tub also have ribs on the top of the sponson sills.
Of course the welding systems used differ between the RCR alloy, the CAV stainless and the mostly resistance welded SPF GT40 tub. There are some MIG/TIG type welds on the SPF/FAV tubs, but there are a buttload of spot welds.
The sheet steel used on the SPF and FAV cars needs the ribs for stiffness and to prevent "oil canning" on larger flat surfaces. Note the SPF and FAV tub also have ribs on the top of the sponson sills.
Of course the welding systems used differ between the RCR alloy, the CAV stainless and the mostly resistance welded SPF GT40 tub. There are some MIG/TIG type welds on the SPF/FAV tubs, but there are a buttload of spot welds.
Jim Craik
Lifetime Supporter
This is a very interesting discussion.
I would love to know just how different the Abbey and the SPF tubs are.
During the months I was waiting for the (very slow boat) to deliver P-2264, I spent a lot of time going over books and photos of both tubs. They certainly look the same to me.
I would love to know just how different the Abbey and the SPF tubs are.
During the months I was waiting for the (very slow boat) to deliver P-2264, I spent a lot of time going over books and photos of both tubs. They certainly look the same to me.
Seymour Snerd
Lifetime Supporter
I was just doing some work on my rear-hoop cross member and in peering into the hoop ears noticed a bunch more spot welds I hadn't seen. So I think it's at least 1.1 buttloads.
Chassis ridigity is always a good discussion, but, it always raises a few questions.
1. What figure are you looking for.
2. How do you measure it.
For me the first is that you are looking for a stiffness a magnitude above the stiffness of the suspension. Effectively what you are looking for is the suspension to do the movement where you can control is with the shocks, and tweek it with the tyres, arb's etc.
F1 tubs aren't as stiff as touring car shell, primarily because a toruing car will have a massive advantage in terms of the chassis width so its sectional properties are higher. Stiffness to weight ratio obviously being in the F1 cars favour. You might have 15,000 Nm/degree in a carbon single seater tun and well over 20 in a toruing car. If you are running massively stiff suspension like these cars do then the very stiff shell is a big advantage and you can argue the extra weight to achieve it is worth it.
The second point about how to measure it is very interesting, as it makes a huge difference to the figures you get. In all my time in racing i've constantly debated with many other engineers the best way to do it. The answer seems to be that there is no ideal way to do it, but, you must do it the same to be able to compare. So any figures that you see written may vary greatly from what you would get testing it with your preferred method. You get this with FEA of parts too. The restraining and loading method alters the result so much that you can condemn a perfectly good item if you don't get the FEA right.
Ross Brawn is quite famous for not being overly worried about torsional stiffness, he prefers to analyse the weakest link in the chassis' stiffness and concentrate on that. He believes the chassis is only as stiff as this weakest point. I can kinda see what he is getting at.
Added this bit after reading some of the other thread.
The reason that the method of restraint matters is that the stiffness of an item is largely dependent on its sectional properties. The axis it twists about referencing the distance the extremities of the chassis are from this axis is what largely determines its stiffness. Imagine a very simple system like a driveshaft. You load one end about the centre of the shaft and restrain the other end about this same axis, so the cross section of the shaft is also located about the axis, all points being as far away from the axis (known as neutral axis ie point at which no twist occurs).
If you now think about how to measure a chassis, how you restrain it and load it determines where this neutral axis will be, and therefore how far away the structure of the chassis is from the neutral axis, thus the result you get will dramatically alter.
D.
1. What figure are you looking for.
2. How do you measure it.
For me the first is that you are looking for a stiffness a magnitude above the stiffness of the suspension. Effectively what you are looking for is the suspension to do the movement where you can control is with the shocks, and tweek it with the tyres, arb's etc.
F1 tubs aren't as stiff as touring car shell, primarily because a toruing car will have a massive advantage in terms of the chassis width so its sectional properties are higher. Stiffness to weight ratio obviously being in the F1 cars favour. You might have 15,000 Nm/degree in a carbon single seater tun and well over 20 in a toruing car. If you are running massively stiff suspension like these cars do then the very stiff shell is a big advantage and you can argue the extra weight to achieve it is worth it.
The second point about how to measure it is very interesting, as it makes a huge difference to the figures you get. In all my time in racing i've constantly debated with many other engineers the best way to do it. The answer seems to be that there is no ideal way to do it, but, you must do it the same to be able to compare. So any figures that you see written may vary greatly from what you would get testing it with your preferred method. You get this with FEA of parts too. The restraining and loading method alters the result so much that you can condemn a perfectly good item if you don't get the FEA right.
Ross Brawn is quite famous for not being overly worried about torsional stiffness, he prefers to analyse the weakest link in the chassis' stiffness and concentrate on that. He believes the chassis is only as stiff as this weakest point. I can kinda see what he is getting at.
Added this bit after reading some of the other thread.
The reason that the method of restraint matters is that the stiffness of an item is largely dependent on its sectional properties. The axis it twists about referencing the distance the extremities of the chassis are from this axis is what largely determines its stiffness. Imagine a very simple system like a driveshaft. You load one end about the centre of the shaft and restrain the other end about this same axis, so the cross section of the shaft is also located about the axis, all points being as far away from the axis (known as neutral axis ie point at which no twist occurs).
If you now think about how to measure a chassis, how you restrain it and load it determines where this neutral axis will be, and therefore how far away the structure of the chassis is from the neutral axis, thus the result you get will dramatically alter.
D.
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