kit car torsional stiffness question (with a twist!)
kit car torsional stiffness question (with a twist!)
(OP)
my car is a kit car with no roof so the torsional stiffness of the chassis if probably quite low. it has an engine making about 450 lb ft of torque which probably tests the chassis, particularity in the lower gears. however, it's 4wd so i wondered if the torsional load exerted on the chassis would be mitigated by the torsional load at the other end of the car?





RE: kit car torsional stiffness question (with a twist!)
RE: kit car torsional stiffness question (with a twist!)
RE: kit car torsional stiffness question (with a twist!)
Is there a driveshaft between the front differential and the transmission? If the answer is no, that's a transaxle.
Either way, the engine and trans will apply a moment to the chassis.
RE: kit car torsional stiffness question (with a twist!)
my question is, will the effect of having drive to both ends mitigate the chassis twisting which would likely occur in a similar 2wd car?
RE: kit car torsional stiffness question (with a twist!)
RE: kit car torsional stiffness question (with a twist!)
obviously, so do both pairs of half shafts!
RE: kit car torsional stiffness question (with a twist!)
Norm
RE: kit car torsional stiffness question (with a twist!)
is the parallel combination of springs in series suggesting that the chassis twist will be mitigated or not mitigated?
RE: kit car torsional stiffness question (with a twist!)
You'll also have couples between them.
RE: kit car torsional stiffness question (with a twist!)
RE: kit car torsional stiffness question (with a twist!)
RE: kit car torsional stiffness question (with a twist!)
The motor mounts/transmission mount still have to resist the full moment put out by the engine/transmission.
RE: kit car torsional stiffness question (with a twist!)
RE: kit car torsional stiffness question (with a twist!)
Mike Halloran
Pembroke Pines, FL, USA
RE: kit car torsional stiffness question (with a twist!)
Steve
RE: kit car torsional stiffness question (with a twist!)
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: kit car torsional stiffness question (with a twist!)
If not, and the there is a reasonable existing fleet of this kit, then I'd say a free body diagram would raise and answer similar and interesting questions to those that were addressed in the design phase of the kit.
RE: kit car torsional stiffness question (with a twist!)
its a dax rush which is a copy of the original Lotus 7 from the 60's. there are many kit car companies who made copies of the Lotus 7 varying from good to shocking. the Dax Rush was designed from the outset to support high output engines and has significantly more chassis members and gussetts than other types. i understand that around 500 2wd versions were made but only around 10 4wd like mine. i'm not aware of any of them cracking thier chassis like you mention and there is no evidnece of that on mine. i think that the torque output of the engine in mine is higher than most of them, 2wd or 4wd.
some time ago i recall someone posting a video of thier car (not a Dax Rush) and they had mounted a camera on the rear looking forwards with a ruler sat on top of the windscreen. it was possible to see chassis deflection on that basis and this is what made me consider the situation in my car. i could try the ruler thing but imagined that an expert opinion from someone on here would be easier.
i dont know what a free body diagram is but can provide any information required
RE: kit car torsional stiffness question (with a twist!)
Without knowing details and going through the design I don't think any one of us could give an analysis in a short amount of time that we're prepared to have someone else risk their safety over.
I think the history of a couple similar and heavily used cars could tell you more than all of us.
RE: kit car torsional stiffness question (with a twist!)
If the OP put a subaru engine/transaxle in, then the frame will try to twist around the driveshaft and also around the CV joints.
RE: kit car torsional stiffness question (with a twist!)
RE: kit car torsional stiffness question (with a twist!)
i wouldn't have done it in a 2wd chassis but thought it would be ok 4wd as i imagined that the chassis stress at each end would balance out.
still looking for a definitive view of the situation however....
RE: kit car torsional stiffness question (with a twist!)
1. Your car is probably traction limited at low speeds. This means that the maximum torque seen in each driveshaft will be limited to the tractive force available at the contact patches of that axle, times the rolling radius divided by the final drive ratio. (plus any inertial load due to rapidly accelerating the wheels - eg clutch dump) This means the mass of the car has more effect on the torque applied to the chassis than any engine capability.
2. The torsion applied to the chassis comprises the centre diff mounts loading the chassis in one direction vs the two final drives acting the other way. (Assumes the centre diff is part of the gearbox.)
je suis charlie
RE: kit car torsional stiffness question (with a twist!)
both the rear diff and the centre diff are viscous coupled units which have been uprated (less slip than standard) and tyres are semi slick compound. the car is not traction limited at any speed (except on damp roads - which it doesn't go out on).
the engine & gearbox are from a ford cosworth YB (with a much larger turbo) which means the centre diff is part of the transmission.
the engine / transmission unit mounts are: 1 each side of the engine and 1 at the rear of the trasmission
the wieght of the car is 800kg
RE: kit car torsional stiffness question (with a twist!)
However much torque goes forward ought to reduce torsional deflection from the windscreen back, but if the chassis is torsionally a wet noodle from the engine mounts forward you probably wouldn't get as much help as a straight proportion of torque predicts.
Norm