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D2 - Transmission Gearboxes, diffs, drive shafts.

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Old 1st December 2019, 05:25 PM
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moltuae moltuae is online now
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Originally Posted by MikkiJayne View Post
Because turning the wheels in opposite directions will result in zero rotation of the output shaft of the diff. Turning the wheels in the same direction results in the output of the diff turning. With Torsen it makes no difference as the geared centre diff will just be driven by one axle instead of the engine. No big deal for a short time. Something with a viscous centre diff (Subaru?) would probably also be fine.

Think of a TT though, where the rear wheels are directly coupled to the front wheels by way of a clutch pack, or indeed a Land Rover where they are directly coupled by the transfer box. If you turn one axle forwards the output from that diff must turn the other axle forwards. If that other axle is on the ground something has to give. In a TT the clutch pack breaks - what happened to my MoT guy. I expect a Landy would either drive off the rollers, stall them, or just lose traction.
Thanks for the explanation

I get how having the wheels of one axle on the ground (while the other axle is turning) might be bad for some AWD systems. But I thought we were saying that the axles should (ideally) be turning in opposite directions? That I don't get. I mean, if all 4 wheels are turning in the same direction, is that not the same as the vehicle coasting along the road? Or are we talking about protecting the gearbox here? I can see how opposing rotations might cancel out at the diff, preventing rotational forces from transferring to the gearbox.
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Old 1st December 2019, 06:19 PM
MikkiJayne MikkiJayne is offline
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Wheels on the same axle should rotate in opposite directions

So lets say we're testing the front axle - spin one wheel forwards and one wheel backwards which will result in net zero rotation of the front diff. In that case its perfectly fine that the rear axle isn't turning.
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Old 1st December 2019, 08:55 PM
paulrstaylor paulrstaylor is offline
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Bear in mind, 10 mins before the brake test they lift each axle and turn each of the wheels by hand, if the car has a "proper" diff you see one wheel turning backward as the other turns forward. Nice reminder to the tester......
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Old 2nd December 2019, 01:07 PM
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Originally Posted by paulrstaylor View Post
Bear in mind, 10 mins before the brake test they lift each axle and turn each of the wheels by hand, if the car has a "proper" diff you see one wheel turning backward as the other turns forward. Nice reminder to the tester......
Here in NI, the tests are conducted slightly differently. The first stop in the building is where the emissions, lights, wipers, seat belts, etc. are all checked by the tester. The next stop is for the brakes and suspension, which is all automated. All the tester does is follow the instructions on screen and press a remote to go to the next step in the checks. The third and final stop is where the car goes up on the lift, with even that mostly automated, as the pads in the lift do the twisting and turning of the wheels whilst the tester is underneath checking the various components.
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