kit car torsional stiffness question (with a twist!) 1

[dexion7]

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?

 

[moon161]

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.

 

[Screwman1]

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.

 

[dexion7]

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....

 

[gruntguru]

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

 

[dexion7]

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

 

[NormPeterson]

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.


Norm