Gearbox crunching

[kipsy]

Hola, I've got a 325 rally car that's running a 320 box (better ratios) and when going down the box it's crunching into 2nd and 3rd. I do heel and toe and the car is not fitted with a quickshift. Is the box knackered or any ideas?

[GeoffBob]

Assuming your clutch is disengaging as it should, then it sounds to me like the synchroniser rings have stretched.

Inner face of each ring is supposed to bind against the cone shaped face of the adjacent gear in order to slow the two to the same speed prior to the teeth on the selector ring engaging those on the gear. After much abuse synchroniser rings stretch to the point where the inside tapered face no longer binds with the cone shaped face on the side of the adjacent gear. And so, when you slide the selector ring across (as you shift), the two are still turning at different speeds, and so they crunch.

This still happens on an up-shift but is less noticeable since its easier to synchronise the speed of the gear and selector ring to each other (as the engine naturally sheds rpm’s when you press the clutch). Get your heel-toe action spot-on and you’ll be able to avoid the crunching on a downshift, since it is you that will be providing the synchronisation, not some soggy worn out ring.

And remember, always blip the throttle with the box in neutral with your left foot off the clutch. That way you will actually be working towards bringing the speed of the selector ring to match that of the adjacent gear during a down-shift. Simply blipping the throttle while the car is still in gear with your foot on the clutch does nothing to aid synchronisation.

HTH

Edit: You did say this is a rally car didn’t you? I don’t advocate driving this way on public roads. If your car doubles as a road car then your box needs fixing/replacing

[kipsy]

Yeh for rallying. Doing welsh Tarmac championship. Will change box but too many other bits too do first. I generally heel and toe to match revs to next gear wheels speed etc as not to stress axle and running gear by trying to cause the wheels to slide (or tramp) and to make it easier to get into gear.

[GeoffBob]

.... and to make it easier to get into gear.

As above, be sure to blip the throttle with the box shifted to neutral with your foot off the clutch. That way the gearbox input shaft, and hence the lay (counter) shaft, will follow the speed of the engine, thus making it easier to shift. Blipping the throttle with your foot on the clutch (gearbox disengaged from the engine) and the box in gear will only raise the speed of the engine and does nothing for the gearbox.

Apologies if I am teaching you to suck eggs, but you would be surprised at how many don't know this. If you haven't shifted this way before, try it! You'll find it will make a world of difference and (when you get around to fitting a new gearbox) will make your synchroniser rings last longer.