Chassis Torsional Stiffness

[Peter Delaney]

Hi Julian,

I think that your description & setup is spot-on - from memory, it is the same as Trevor Booth described quite some time ago in another thread.

One that is standardised, can be done on a completed car (ie, meaningful in the real world), and is recognised by the various authorities administering the ADR's (Australian Design Rules).

The ADR's are based on internationally recognised standards, so you can bet that somewhere in the fine print, there will be a spec on how to measure torsional rigidity, & that this will be it !

BTW, how did the "monocoque lawnmower" in the back-ground handle the test ? !!

Kind Regards,

Peter D.

 

[DavidTC]

Hi guys
Just some useful background info to go with the generally quite well informed debate.I raised the issue of torsional stiffness some time ago and I have been interested in the latest thread. There is lots of data out there; Ferrari for example always produce figures for their cars (Ferrarisima is a good source of data). The stiffest chassis they built was probably for the F1 design of the F 50, which I think they quoted at F1 levels of stiffness ~ 35,000Nm per deg. I am more familiar with the Koenigsegg cars and have recently compared their design with the CAV GT40 S/S monocoque. There is no reason that they should not have similar levels of stiffness it just depends on the weight penalty you are able to tolerate. The Koenigsegg C Fibre chassis has a quoted stiffness of 28,100 Nm/deg but only weighs 62Kg, some 150Kg less than the CAV of similar stiffness. Message is a well designed steel monocoque is pretty good, it is just that it is too heavy for a car like the Koenigsegg capable of over 250 mph. Remember also the aerodynamic drag and RR drag consume something like 650bhp at these speeds hence the interest in weight reduction and increased power/torque cf Koenigsegg's 1018 bhp bioethanol road car.
If people are interested there are upcoming articles I have authored on this appearing shortly in Materials World and in Race Tech.
Regards
DTC

 

[Sandy]

David,

Just out of curiosity what is the break out of the rolling resistance vs aero drag in percentage or hp?

Sandy

 

[DavidTC]

Sandy
At 250mph the Aerodynamic drag is 87 % of the total. Just for interest the CdA for an F1 car is double that for a slippery shape like the Koenigsegg.
David

 

[Chris P.]

David -

Do you have graphs for RR for typical sports car tire sizes? I would be interested in getting numbers for those. I did a senior paper for my aerospace degree on measuring aerodynamic drag of our school's solar car by measuring battery drain and subtracting out RR and other losses. I measured the RR using a coast down method and found that it was just slightly off linear (increasing with speed). These were bicycle tires, however.

- Chris P.

 

[Sandy]

Thanks David, interesting about 85 hp, what does it look like at a meager 125mph, is the relationship linear?

Sandy

 

[DavidTC]

Hi
From the figures I have the RR is 27 % at 100 mph and 19 % at 150 mph. You have to have specially rated tyres to run at ultra high speed in the case of the Koenigsegg on 19in rims front 20 in rear (Mg/C/F composite construction), so the tyre contribution to RR is probably different than for the tyres most people are used to.
Diferent authorities require different coast down regimes (eg US vs Europe) and these are fitted to different equations for the RR part.For the US this might look like k x v*2 + K for Europe it might look like k x v*2 + l x v + K. The exact equations unfortunately are proprietary.
David