Rear Drive Rant #2

     OK, I said I wasnít going to do this because itís fraught with danger for the uninitiated but Iíve been goaded into it by events in the last few days. I wonít go into that but I will say Iím, if not pissed off, at least a bit disgruntled. Bevelboxes arenít terrifically hard. The thing is, if you get them wrong they are very expensive. This is the main reason I havenít up until now tried to give any explanation of how to set up the mesh on the gears or rebuild the box.

    A set of spiral bevels, as used in a car differential or the final drive of a Guzzi, (or BMW, or Lilac or the cam drive on Ducatiís, Nortonís, Velocettes, etc.) require that the gears, which are manufactured in pairs, be set up so that when they are working against each other they donít move in relation to one another in any way apart from in direct, if you like, ĎPressingí contact. The way they are manufactured involves in itís final stages a lapping process where the gears are set up in a dolly that places them in their ideal running format and they are then lapped together to make a mating pair.

    Once this has been done whenever the gears are used they must be set up in precisely the same configuration, or very, very, close to it. This is the reason that the gears have to be shimmed up. What you are doing is getting as close as possible to the ideal. Within the manufacturers parameters there is always going to have to be a tolerance. This is an acceptable limit away from the ideal where the components are going to give an acceptable service life. In any mass-produced vehicle there are dozens of these tolerances on moving parts. In essence your whole vehicle is a series of compromises. Probably there is no place where this can cause more angst than in your bevel box.

    Firstly lets look at your pinion. This is the gear that the drive shaft connects to, the bit with the splines that sticks out of the front of the box when you disconnect it to the swingarm. The first thing you will see is that it has a FB nut on it! The FB nut is actually a device that clamps together the bearings that the pinion rotates in. If you can imagine it as being a bit like your steering head bearings with two tapered roller bearings at opposite ends of the shaft but because of the loadings on that shaft the bearings canít just be tightened down until they feel right they have to be separated by a spacer so that when the nut is done up Bloody Tight the bearings will not either bind or flop about like the proverbial cock in a sock. Most people will of experienced loose steering head bearings that chatter when you brake and upset the handling of your bike? Imagine the result of loose bearings on the teeth of a gear that has to match perfectly with another one in two different planes. Nah! Doesnít bear thinking about does it?

    So the first thing you have to do is set the pre-load on the pinion bearings. Ideally, when the pre-load is correct the pinion will spin completely freely but with no movement in any plane. You shouldnít be able to move the pinion fore and aft or rock it in the bearings. Here though, we encounter the problem of tolerances. Guzzi pinions have a tube spacer that fits between the bearings. This is a bit too short to space them correctly so after this has been installed you add a series of shims which are supplied in 0.05mm and 0.1mm thicknesses. Essentially you add shim until you have way too much end float between the bearings and then subtract it in 0.05mm amounts until the bearings are AS CLOSE AS POSSIBLE to the ideal. Obviously you canít remove too much because if you do this the bearings will bind, there isnít enough clearance betwixt the rollers and their races, so they will remain in a semi-crushed situation which not only will impose enormous stresses on them but will also mean that oil canít get to the working surfaces. Remember, in any rolling element bearing the primary function of the oil is to remove heat, NOT, to lubricate! This is VERY important. Because of this it is perfectly permissible to have a tiny amount of end float in the bearings. If they are set up right it will never be more than a maximum of 0.1mm and should be limitable to <0.05. While this means that the pinion head will move in relation to the crownwheel teeth it will be Within Tolerance and unless you are willing to use a surface grinder to grind up a shim of precisely the right thickness itís as good as youíll get and most importantly will be QUITE SATISFACTORY in service!!!!!! It is vital thou to err on the side of slightly loose rather than too tight or you end up with a disaster.

    Once you have the pinion bearings pre-loaded correctly you can move on to shimming up the mesh on the gears and here is where a lot of people begin to get confused because you now need to get both gears, the crownwheel and pinion, set up so that they work together correctly and to do this you have to adjust the mesh in two different planes. Firstly you can move the head of the pinion closer or further away from the axis of the crownwheel by placing shims between the head of the pinion and the rearmost bearing. Secondly you can move the crownwheel towards or away from the axis of the pinion by using shims between the crownwheel bearing carrier plate and the main box itself.

    The Guzzi system is really very simple as it relies on the forces acting on the gears to keep them as far apart as possible. This is important to remember as it requires that when the mesh is being checked that everything be bolted up tight and the crownwheel carrier itself be pulled away from the pinion as far as possible.
For the purposes of this diatribe Iíll assume that people are only going to be replacing standard gears in whichever box they are using. If you differ from this norm you open up a can of worms as there are a variety of different sizes and varieties of shims, gears and bearings that make setting up, say, an 8/35 gearset in a bike with a disc brake swingarm considerably more problematic. Itís not insurmountable, but it requires more work, different thicknesses of shims, (Which arenít available ĎOff the Shelfí!) and a clearer understanding of WTF you are doing.

    My first recommendation would be, before you attempt this, get a good bevelbox on the bench. Drain it of oil, bolt the pinion carrier into it by using some spacers and a couple of nuts and then with the side that would face the wheel downwards and holding it by the edges of the outer flange give it a sharp bash down onto the workbench. This will ensure that the crownwheel carrier is settled in itís bearing. Now lift it gingerly and rock the pinion back and forth. You will feel a tiny bit of play between the teeth of the pinion and the crownwheel. This is the BACKLASH and this, above all else, is the most critical element of the setup. Take your time, repeat the biffing of the box on the bench and try again. Get the feel of how much backlash there is on a good box because this will give you a decent guide as to what to look for as you set up your new gears.

     With a 7/33 gearset as used on most Tonti framed bikes you will have a choice of 3 pinion shims from memory. Check those you have ordered, (Yes you will need them all, sodís law dictates that it will be so!) with a micrometer. Now, stick one of Ďem between the pinion head and the back bearing, tighten down the FB nut on your pre-loaded bearings and then BOLT it into the bevelbox.

    Assuming you have already removed the old crownwheel on itís carrier by removing the plate on the wheel side of the bevelbox you can now undo the eight bolts that hold the crownwheel onto the carrier and discard them. I strongly recommend replacing these as a failure of such a highly stressed item will be very expensive. Then clean the carrier carefully and inspect the bearings. If in doubt, replace. Now the new crownwheel can be installed and bolted home. I never use the poxy locking plates Guzzi periodically use. They stink! I use Red Loctite and Schnoore washers. I havenít had a bolt come loose yet!!!! Make sure that the surface between the carrier and the crownwheel is perfectly clean, this is important.

    Next you can invert the crownwheel and itís carrier and drop it into the bearing in the bevelbox. Look at where the crownwheel teeth and those of the pinion meet. While it isnít always the case that mesh will be right if you align the edges of the teeth itís a pretty damn good starting point. I will swap pinion shims until it looks pretty close before Iíll move on to the next stage.

    This is the bit that scares people!

    Put some colour on the pinion teeth. Despite what people say you can use just about anything. Kiddy poster paint is good, or engineers blue, or even a thin coat of nickel type anti seize. Make sure the teeth of both crown wheel and pinion are dry, select a big shim and two gaskets, one of the ones that goes between the crownwheel carrier plate and the box itself, (use a thick one first.) and drop the plate over the crownwheel carrier. Loosely do up four of the bolts in a crosshatch pattern and then do the biff-on-the-bench thing to settle the carrier into its bearing. Test to see the pinion isnít binding by giving it a rock and if ití OK tighten down the rest of the way. Repeat the ĎBiff-on-the-benchí trick then clasp the box to your (in my case) capacious gut and while pulling the crownwheel carrier OUT, in other words as far away as possible from the pinion, rotate the pinion with your fingers of the other hand. (At the same time you are doing this feel for the backlash by rocking the pinion. If it feels rough itís too tight, if it lashes bout like a shark in a feeding frenzy itís too loose.) Do this for a full 4 revolutions of the pinion in the direction of drive and then go backwards about a turn. Now undo the bolts, take the plate off and withdraw the crownwheel and its carrier.

    By looking at the teeth of the crownwheel you should be able to discern where the colour has been transferred to the crownwheel teeth. This is the point of contact, (POC.) and it should be near as dammit in the centre of the CW tooth. If it isnít then you have to adjust the shimming.

    The thing is it all works exactly the opposite way to how you think it should.
Someone recently gave me a nice little maxim.
ďFace and Flank, move the crankĒ (Pinion.)
ďToe and Heel, Move the wheelĒ

    If the mark is too high on the tooth or too low, you donít move the crownwheel by using the big shims. You move the pinion in or out. Conversely if the POC is too far towards the ĎBackí or ĎFrontí of the tooth you donít move the pinion, you move the crownwheel.

    One of the reasons that rebuilding bevelboxes is expensive is not that all the bits are pricey, (So bloody look after them OK!) but also because you almost always have to take them apart and put them back together half a dozen times to get it right. Even then, now and again, youíll get a set of gears that is a right frontbottom and it will take seemingly forever to get it right. Why? Buggered if I know? Itís just the way!!!!
Before you finally button it up you should of course strip it again, clean it thoroughly and replace both the oil seals- that goes without saying

    As Iíve often said before, Iím not keen on sticking these sorts of articles up on the Ďnetí because sure as shit ainít sugar, one day some knobhead is going to get in over his head and itís going to be all my fault and heís gunna soo!!!!! SO. I take NO responsibility for anyone else who tries this. You need the full gamut of shims, new gears and a fair bit of nouse to build a bevelbox. Most mechanically minded people can do it. If you canít work out where TDC is and have trouble remembering your second name? Forget it.
Any questions? Iíll try to answer them.

Regards,
Pete Roper
motomoda@austarmetro.com.au
Moto Moda
23,King St.
Bungendore NSW 2621
Australia