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Transmission knock & backlash
marv - 15/12/14 at 06:35 PM

Hello everyone.

This is one of my few posts to the forum, mainly because I have probably spent far too many hours/days/weeks/months & now years! reading other people's posts to solve my problems, but I'm now stuck! I'm currently building an MK Indy with a ZX10R powerplant. I had the car up on axle stands to check the electric reverser operation and a few other bits and pieces. I've discovered an intermittent, loud knocking/banging noise with coinciding vibrations through the transmission and chassis. After looking through a lot of the posts here, I have come to the conclusion it may be to do with the inherent backlash of a BEC.

My questions is this. Is it the same noise/vibration and play that everyone else gets, and if so is it excessive or normal?

I appreciate the nature of the BEC transmission, it really does bang! into first gear when selected, which I understand is normal. So I've ensured the prop is not fouling, the bolts are tight, u/j's are in good condition, the spline connection from MK (which had some play in it) has been checked by Baily Morris and although its the 'most they have seen' with their direction, a liberal application of grease has cured the play. The strange thing is, when using the electric reverse there is no knocking.

Follow the links for some Video of the noise and the play in the rear diff.

The car was in 1st gear, at idle with the rear wheels off the ground, and I also dipped the clutch in and out a few times.

https://www.youtube.com/watch?v=qtaPAtbD0rg

This time the car is in N and I'm simply moving the prop by hand, back and forth within the play of the diff.

https://www.youtube.com/watch?v=BBtKCTEEbE8

https://www.youtube.com/watch?v=IbpPfyrPxoI

Let me know your thoughts!

Thanks

[Edited on 15/12/14 by marv]

dave_424 - 15/12/14 at 07:19 PM

I don't have a BEC myself, but have a chain driven motorcycle engined vehicle. I would not be worried about the backlash in the diff, remember that there is more than just the lash between the pinion and the crown, there is also the other gears in the diff, the CV's at each end and to a small degree the splines on the driveshafts to the CV joints.

Also with the knocking noise when running on axle stands, I would say this is normal since there is very little resistance on the running gear (car not on the floor) it allows all the driveline lash to show itself, with the BEC gearbox it's very large, with the engine off and in gear, you can rotate the prop and see the large amount of lash in the BEC gearbox due to the engagement dogs.

Possibly a bit dangerous but I would try and do the same running on stands but either lightly apply the rear brakes or something else to put drag on the rear wheels, you should see that this gives smooth running

MK9R - 15/12/14 at 07:28 PM

All totally normal

mark chandler - 15/12/14 at 07:28 PM

Never run it with the wheels in the air like that, the horrible clonking noises will drive you to distraction as the cogs crunch away, mine sounds just the same

If you load the brakes up when it's running the noise will all but disappear, when I stuck on my turbo I had it up to maybe 100 mph with the chassis lifted on big blocks off wood and stamped on the brakes to check the boost, 5 seconds of that convinced me that rear brakes will not hold back the engine and the smell was horrific as the pads overheated, I did see a couple of PSI

Nothing to worry about, it's fine.

aka Keith - 15/12/14 at 07:32 PM

as Dave says, that looks normal.

When I took my BEC onto stands and run the engine in gear I get exactly the same. I believe what happens is that the drive spins the wheels and as there is no resistance they actually accelerate and end up spinning faster than the drive shaft (clunk), then slow down (clunk) and the speed up etc etc.(clunk clunk).

Cheers
Craig

adithorp - 15/12/14 at 07:55 PM

Normal if you've got it jacked up. Combination of backlash in the drive train and no flywheel to smooth out the firing pulses from the engine. Lift the revs and the rattle will reduce.

Scared the crap out of me first time I ran it in gear on stands. I just avoid doing it now.

marv - 15/12/14 at 08:55 PM

quote:
Originally posted by mark chandler
If you load the brakes up when it's running the noise will all but disappear


I tried that, but running at idle, using the handbrake and got a fair bit of vibration and wobble, and some slight knocking, but that might be down to putting too much load on the engine, I kept going until the engine stalled Maybe under normal acceleration and power/load it will be ok.

Thanks for all the replies, I think a test run ASAP is required... better find a quiet lane..

Cheers.

40inches - 16/12/14 at 09:57 AM

I asked the very same question 2 years ago

HappyFather - 16/12/14 at 07:42 PM

I never run mine on stands. But if I drive it in 1st and going so slow I'm almost stalling, it goes like that. I press the accelerator peddle and the problem is solved.
Bottom line... I'll have to drive it fast!