Spaceframe Comparison

1074R/001 · May 17, 2009

Paul,

You're not changing the general focus of the thread. It's about good spaceframe design concepts for a GT40, and your input is good. It'd be nice if the other current producers had as much info to hand or were as open with it.

Stew

JohnMcL · May 18, 2009

Hi Stew,
Check the Bailey-Edwards. Their website mentions 18,500Nm/deg for their frame. Doesn't say anything about weight of the frame and if this is with /without integral roll cage.
Regards,
John.

jimbo · May 18, 2009

Just for comparison, what is the torsional stiffness of a standard mild steel GT40 monocoque?

ross nicol · May 27, 2009

Thought I would come into this discussion to back up how Paul (The New Roaring Forties) has described my RF race car. The chassis is unmodified apart from minor alterations to allow fitment of the following
1/ Diagonal removed (RHS of engine/bellhousing) and rear rail sculpted for ZF transaxle clearance,
2/ Steering rack mounting altered for Tilton pedal box
3/ Coil over top mount at rear beefed up
4/ Top link (at rear) double shear mount removed in favour of single shear rose jointed link. Bolted through original hole in chassis frame.

An 8 point roll cage is fitted.

As Paul pointed out the chassis in my car is basically as it came from the factory. The only further mod I have considered is to alter the inboard mounting for the front upper wishbone, to orient the Rose Joints in line to feed loads to the chassis better.
I believe by keeping my chassis standard I am able to give valuable feedback to Roaring Forties from the race track.
I find the car responds well to minor suspension changes and I believe the chassis stiffness (being high), is responsible for this.

Ross

JIMMYMAC · May 31, 2009

jimbo said:
Just for comparison, what is the torsional stiffness of a standard mild steel GT40 monocoque?

12500 lbf.ft per degree of torsion.

Ron Earp · May 31, 2009

JIMMYMAC said:
12500 lbf.ft per degree of torsion.

1074R/001 said:
Winston Cup car in the 12000 ft.lbs per deg area is considered pretty good. Not that they're F1 or anything, but pretty stiff non-the-less.

The problem with the torsion measurements, and this has been discussed at length on this board, is the many different ways of measuring the value. This makes them essentially non-comparable.

I think anyone who has seen a modern Cup car and a GT40 mono is going to have a hard time believing the latter is the same as the former. IIRC the Cup cars are 1.75" tube at 0.090" with all side bars at 1.75" at 0.120". Quite stout. I doubt there is any torsional comparisons on the same test rig with a Cup car and a GT40 chassis.

Ron

jac mac · May 31, 2009

Hmm, in the book 'Lola T70 Coupes' by Ian Bamsey, figures of 10000 ft/lb per° (GT40) and 3200ft/lb per° (T70) were given by Broadley. I would venture to suggest that 'he' would have checked them on the same rig. Like one notable forum member mentioned though, you cannot always believe everything you read or are told!

Tell you what, send one tub + 4 corners of everything for each one to me & I will test them all in a back to back manner.

Ron Earp · May 31, 2009

jac mac said:
Tell you what, send one tub + 4 corners of everything for each one to me & I will test them all in a back to back manner.

I think that is the only way we'd get something repeatable and compatible. Get GT40 P1075 at the same time so you've got a good original reference point. I imagine pretty much all the chassis out there are more than sufficient for purposes - either imagined or actual.

I'd hate to know what my wet noodle 260Z chassis comes in at but it doesn't matter because it isn't bench raced. Instead, like Ross's car, it is raced and at that point there are 100s of other aspects that will influence how good of a race car it is, including the nut behind the wheel.

Russ Noble · May 31, 2009

It's a bit like quoting dyno numbers, unless you're using all of it all of the time it's a bit irrelevant! Bragging rights maybe?

Who has a problem with chassis flex in any GT40, replica or otherwise, at any point in any situation? I would venture to say most of the replicas out there are more than adequate for normal use.

Sure it's nice to know you've got a stiff chassis in the same way that it's nice to know you've got 500hp, but how often do you push it to the limit to fully exploit either or both? Maybe someone has found that 500hp, or whatever they've got, is not enough, but has anyone found the torsional stiffness of their chassis is inadequate?

Doug68 · Jun 1, 2009

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urple; mso-themecolor:followedhyperlink; text-decoration:underline; text-underline:single;} .MsoChpDefault {mso-style-type:export-only; mso-default-props:yes; mso-ascii-font-family:Calibri; mso-ascii-theme-font:minor-latin; mso-fareast-font-family:Calibri; mso-fareast-theme-font:minor-latin; mso-hansi-font-family:Calibri; mso-hansi-theme-font:minor-latin; mso-bidi-font-family:"Times New Roman"; mso-bidi-theme-font:minor-bidi; mso-fareast-language:EN-US;} .MsoPapDefault {mso-style-type:export-only; margin-bottom:10.0pt; line-height:115%;} @page Section1 {size:595.3pt 841.9pt; margin:72.0pt 72.0pt 72.0pt 72.0pt; mso-header-margin:35.4pt; mso-footer-margin:35.4pt; mso-paper-source:0;} div.Section1 {page:Section1;} --> </style> Quoting from this article on the design of the Bentley LMP:

"As regards the overall level of stiffness, it is an order of magnitude higher than the roll stiffness of the suspension, which is usually a good rule of thumb to work to…"

This has been about the only sensible statement I've ever found on "How stiff is stiff enough?"

I'm not sure it’s at all easy to spot anything other than a grossly flexible frame in a car, but I bet lots of people throw plenty of money at "fixing" suspension issues that aren't the suspensions fault, if you know what I mean.

Doug68 · Jun 1, 2009

oops.

Russ Noble · Jun 1, 2009

As an aside, does any one know how stiff a top gokart chassis is?

I haven't looked at one lately but it seems to me that the ones I saw years ago would be as floppy as you could ever get?

And does anyone have any lap time comparisons with 'normal' single seaters or sports racers? On 'road racing' circuits that demand 'proper' handling, that is, not left-turn-only specialist power circuits.

They seem to be pretty fast..... Imagine how much better they would be if you could stiffen the chassis up a bit.....:lipsrsealed:

JIMMYMAC · Jun 1, 2009

jac mac said:
Hmm, in the book 'Lola T70 Coupes' by Ian Bamsey, figures of 10000 ft/lb per° (GT40) and 3200ft/lb per° (T70) were given by Broadley. I would venture to suggest that 'he' would have checked them on the same rig. Like one notable forum member mentioned though, you cannot always believe everything you read or are told!

Tell you what, send one tub + 4 corners of everything for each one to me & I will test them all in a back to back manner.

Jac,
TRUE ! 'don't believe all you read'
Several people quote 12500 lbf including Ludvigsen.

jac mac · Jun 1, 2009

Russ Noble said:
As an aside, does any one know how stiff a top gokart chassis is?

I haven't looked at one lately but it seems to me that the ones I saw years ago would be as floppy as you could ever get?

And does anyone have any lap time comparisons with 'normal' single seaters or sports racers? On 'road racing' circuits that demand 'proper' handling, that is, not left-turn-only specialist power circuits.

They seem to be pretty fast..... Imagine how much better they would be if you could stiffen the chassis up a bit.....:lipsrsealed:

I trust you also noticed the lack of springs/dampers on the go carts Russ.

A.J. · Jun 1, 2009

Jac Mac is quite correct,the flex in the kart frame amounted to what passes as 'suspension', as with rigid spindle/axle mounts,some flex provides the roadholding( I don't think you'll find a 'sloppy' amount in most race karts).Also,the kart rear axle is a solid bar,wheel to wheel, so most turns are accomplished by power sliding to a greater/lesser degree. Oval track sprint cars bend a bit too,having the same handling characteristics.

albanygt40 · Jun 7, 2009

It achieves a little over 12kN / deg in torsion at the wheelbase through the suspension pick up points which is the correct way to measure it.

Are you sure this is correct 12 000nm/degree. That's nearly double a DRB.
I have seen and tested a lot of chassis's the way you're talking and it takes a lot of cross bracing to achieve the figures you're talking from a spaceframe with no trans tunnel . The car in pic achieved about 5400nm/degree. I know it's the placement of the bracing but this chassis was FEA enhanced in the design stage so steel was put in sweet spots.

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Russ Noble · Jun 7, 2009

jac mac said:
I trust you also noticed the lack of springs/dampers on the go carts Russ.

That's right.

The point I was making is that a rigid chassis is not the be all and end all!

Chassis flex as in a gokart just contributes to/is the overall suspension. In a GT40, if you can make your chassis rigid it is obviously one less variable to deal with when setting up at the track. We're not talking about F1 here where everything including the drivers is honed to the nth degree, we're talking GT40's with, at the very most, weekend racers just out for a play! All be it pretty serious at times....But in all honesty, who amongst us could tell the difference between a chassis with 'x' torsional stiffness and one with '2x'? And even more so on the road! At the track you just alter springs, shocks, anti roll bars, brake balance and roll centres etc to change the handling characteristics to your liking! Who cares if you are also compensating for chassis flex? It's just part of the total equation.

And for the road it's almost irrelevant so long as the chassis is sufficiently stiff to eliminate axle tramp, scuttle shake and doors flying open!
:flameon:

Bobski · Jun 7, 2009

Wow, that a nice rig Dave S, do you test many chassis with it?
You are correct in saying that the RF has almost twice the stiffness of the DRB. Back when I was looking, the figures that were quoted to me by BRD was just under 7kNm/deg where as per my engineers report, the RF achieved just under 12kNm/deg naket. With all makes, one can add roll cages and thicker material to skin the chassis to add stiffness. It would be interesting what the RCR achieved as it was recently tested to pass ADR specs.

albanygt40 · Jun 7, 2009

It works very well and tells no lies. Many different cars have been on it.
Most memorable was a Morgan copy we nicknamed 'RamRaid'. It came in at about 12-13knm/degree.
I remember the chassis in the pic use to make a 'BONG' noise when it was loaded to 4000nm/degree then it was quiet all the way to 5400 which was one degree then as you unloaded it would 'BONG' again on the way down.

We also had/have access to FEA testing the chassis to compare results and sadly what worked on the computer didn't allways transition to
mechanical testing. I don't have much to do with testing anymore as kit manufactures usually provide paper work for us.

The rig was built using Gov regs for testing Limo's and convertables. It
is a PITA to set up.

RamboLambo · Jun 8, 2009

I would have thought the chassis only needed to be stiff enough to resist input force from the greatest load introduced to the spring mount? Anything more would be a waste of engineering.

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