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kit car torsional stiffness question (with a twist!)

kit car torsional stiffness question (with a twist!)

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!)

However the drive is split, the front transaxle is likely bolted to the motor, so about 1/2 of that torque will be taken up between the two, and the assembly will want to rock around the transaxle. The other 1/2 will be multiplied by the drive ratio and transmitted to the rear differential and resisted by the frame.

RE: kit car torsional stiffness question (with a twist!)

the front diff is not mounted to the engine, its bolted to the chassis. the rear diff is likewise bolted to the chassis at the rear. the car has wishbones on all four corners - no transaxles.

RE: kit car torsional stiffness question (with a twist!)

A car can have wishbones and a transaxle.

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!)

all corners have double wishbone suspension. there is a drive shaft from the transmission to the back diff and another drive shaft to the front diff from the transmission. there is a centre differential which splits the torque 60/40.

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!)

That sounds likely. You could do a free body diagram and see what that tells you. The arrangement sounds easier to draw and FBD than to sort out by discussion. Observe the rotation of the different drive and output shafts to determine the correct sense of torque.

RE: kit car torsional stiffness question (with a twist!)

well both drive shafts turn in the same direction if that's what you mean?

obviously, so do both pairs of half shafts!

RE: kit car torsional stiffness question (with a twist!)

So you've got an engine and transmission unit bolted such that the engine's torque reaction comes out somewhere along the chassis' X-axis and two differentials bolted at two other locations. Sounds like a parallel combination of springs in series, especially given the anticipated [low] magnitude of chassis torsional stiffness.


RE: kit car torsional stiffness question (with a twist!)

the diffs are solid mounted and the engine/transmission unit is mounted on rubber mounts.

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!)

No matter what the directions of the various moments are, the total imparted to the hard points in the chassis will not be zero, if that's what you are asking.

You'll also have couples between them.

RE: kit car torsional stiffness question (with a twist!)

Let's simplify the problem. We are concerned about the body twisting between the front and rear halves. The engine and front differential are in the front half. Any torque supplied to the front wheels will reduce the torque to the rear of the car, so twist of the body will be reduced.

RE: kit car torsional stiffness question (with a twist!)

well that's the same as reducing engine torque but doesn't address the question!

RE: kit car torsional stiffness question (with a twist!)

The answer to your question is no.

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!)

The car has no roof. The OP's concern is with twist of the chassis due to this fact. The engine and transmission mounts are not a factor in this. They are only relevent to twist in the front half of the car, not twist between the two halves. A front wheel drive car can drive without a back half.

RE: kit car torsional stiffness question (with a twist!)

The actual configuration of the alleged car is so far not in evidence.

Mike Halloran
Pembroke Pines, FL, USA

RE: kit car torsional stiffness question (with a twist!)

The powertrain exerts forces at the points where rotation is resisted, i.e. at the tyres and at the engine mounts. For a longitudinal (north-south) engine,the mount forces will give a twisting torque on the chassis, reacted by vertical forces through all the tyres. Nothing else matters. A simple thought experiment involving a transverse engine might help.


RE: kit car torsional stiffness question (with a twist!)

Are there, on TEH INTERNET! any pictures, complaints of said kit car, built per mfg instructions shown twisted up, with rippled body panels, cracking at stress risers, shedding paint, or with doors that closed properly when made, but not any more, or other evidence of damage related to winding the motor up all the way in 1st gear (max driveshaft torque)?

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!)

this is the car but i'm not sure how it helps you.

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!)

You could talk to the people who know it best- the manufacturer and other owners, and see what the record is regarding installed power, usage and structural performance. Ideally, you'd want to find someone who put a bigger motor in, and races it every sunday for the past 5 years, and has had no frame problems.

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!)

The center diff will have to be fixed to resist the same torque as the motor / trans mounts. This might be a place to inspect for signs of deformation or cracking. In 4wd configureation, the front and rear diff's will each have to resist 1/2 the torque as with the same engine in rwd.

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!)

450 ft. # of torque into a Lotus 7 clone! Holy Cow Batman. I don't think that anything about the original design was intended to withstand that sort of loading. My crystal ball says tube bending all over the frame after loading it up a couple times.

RE: kit car torsional stiffness question (with a twist!)

it hasn't bent yet that i am aware of

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!)

A couple of points.

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!)

The two engine mounts will be where the engine side torque gets applied to the chassis. If that came out at the center diff you'd be risking breakage of the transmission/center diff casting, especially on hard launches from a complete stop.

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.


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