Differentials, Tailshafts, and Brakes
Shortcut to -
WAY too much to talk about here, but one clever trick that helps the handling of a live axle read end car is to bend the diff housing. You have to put them into a very large hydraulic press and rig up some strong bracing to hold them, but it is possible to get a half degree or so negative camber into the housing an also a millimetre of toe-in. It may not sound like much, but those small amounts will helps immensely to make the car more stable at all times. It's also rather surprise just how far you have to bend the diff housing to make it stay bent!!
There's nothing terribly tricky about the internals of a differential, so all you have to do is to set up the clearances as per the manual.
One thing that's always annoying is
getting the darn filler and drain plugs undone! They always seem to
strip the head ... The way out of that is to weld on a large nut
to the plugs so you can get a big spanner to undo the little darlings
BTW, when you weld the nut on, make sure that you remove the EFI computer from the wiring loom and also disconnect the battery. Don't say I didn't warn you.
One good gadget to have in the rear end is an LSD, or Limited Slip Diff. What this thing does is create a link between the rear wheels (or front wheels, in a FWD car, and often between the front and rear wheels as well in a 4WD) so that instead of letting them spin freely with respect to each other, they have a certain amount of friction. I'll be talking about three types, but only two in any detail. The cheapest and certainly the nastiest to drive is what we call in Australia as the 'CIG Locker'. What this means is that the centre of the diff has been welded up (By using an oxy welder, almost invariably powered by CIG gasses, gettit??) so that the rear wheels are solidly locked together. This makes the car very hard to turn around slow speed corners, as the inside tyre wants to rotate at the same speed as the more heavily loaded outside, and so tends to push the car in a straight line - big time understeer! If you have enough power, then you can get both rears spinning nicely, and so create and control the oversteer that will then develop. You really have to drive the car on the throttle a lot more than a 'normal' car, and so they can be a quite tricky unless you get used to it quickly.
|What's far more common - and pleasant to drive - is the Salisbury (pro: Sawls-bree) LSD. All they are is a conventional open diff, but with a clutch pack on the axle diff gears so that there is friction created between the rear wheels when they rotate at different speeds. In an exploded view, they look a bit like this diagram shows.|
|After a while - depending on how you drive the car, etc - the clutches will wear out and the 'slip factor' decreases to the point where the LSD isn't a lot better than an open diff. This is a cutaway view of one|
|If you have a look at the above picture, you'll see a small shaft sticking out the side of one the diff gears - This is the shaft that controls the 1, 1.5, and 2 way action. See this pic to get an idea as to how it works.|
Possibly the best LSD of all is the Torsen 'torque sensing' type. They are a very good thing, as they always spread the torque evenly between both wheels at all times, so even if you have one tyre on ice and the other on sandpaper, both wheels will spin evenly. There are two types of Torsen, and this is a Type One.
The Type One Torsen LSD that does a pretty good job and is available on a lot of Toyota cars from about the mid 90's onwards. This is an exploded view of a complete diff -
Here's a view of the LSD gears themselves -
And finally, a view of the action that the LSD gears take to provide the LSD effect -
Fortunately, not a lot to be said about tailshafts. There's only a couple of tricks to them, and by far the most important one that people often don't know is -
- The universal joints at either end MUST be lined up exactly parallel to each other. By this I mean that the cups at the end of the tailshaft must be in line as you look down the tailshaft. If this isn't the case, then the tailshaft will destroy itself and the universal joints rather quickly! The reason for this is because as they spin around uni-joints don't follow a constant rpm. This increases with the deflection angle, so if you have them out of line on your tailshaft then one end is trying to do one rpm, and the far end is trying to do a slightly faster or slower one, every revolution! This can sometimes be felt as an incurable vibration in the car which gets worse as the speed builds up.
- Try to keep the diameter as small as possible to reduce rotational inertia. As a general rule though, the smaller the diameter then thicker the tailshaft tube metal has to be, partly negating the advantage of a smaller tube.
- The correct length is when the car is at ride height, the front yoke's back face of the tailshaft is flush with the back of the gearbox housing. Make sure that there is at least 15mm to 20mm of further travel in both directions (In & out) of the tailshaft at full bump & droop on the rear suspension.
- Most modern cars have a two piece tailshaft. This is only to reduce vibration. (Have a look at one if you get a chance - One part is always longer than the other to reduce harmonic vibration. Clever!) For a performance car, this is a waste of time, and it should be replaced with a one piece unit.
- A lot of modern tailshafts also have the uni-joints peened in so they can't be changed an so you have to buy a new tailshaft, at great expense. Not so - Just take it to a decent engineering shop and get them to re-fit the tailshaft with normal, replaceable type uni's. The cups that sit in the yokes will have to be bigger than they were, but that's no big deal compared with the alternative ...
- A universal joint is ok when it shows no signs of 'feeling tight' when you work it around with your hands. If it feels the slightest bit tight or overly 'loose' at any point, then change it.
- There are types of tailshafts that have the uni-joints held in with small U-bolt type devices. These are junk, and should be replaced with fully enclosed (The metal goes all the way around type) yokes.
A much abused device. :)
There are a few ways of increasing braking power -
- Bigger discs. A bigger disc, even with no other changes, will increase stopping power by quite a bit. They do this by increasing the leverage effect that the brakes have on the wheel. If you have a 9" disc and go up to an 11" disc, you've just increased your braking power by a good 22% (5.5" effective lever arm compared to a 4.5" effective lever arm)
- Four spot calipers. They give a much more even spread of pressure on the brake pads, and so make the pad work more efficiently. The reasons for this are that the twin pistons per pad are pushing over a wider base than a single piston ever could, and the four piston calipers are inherently more rigid than a two spot or single pot slider caliper is. The better four spot calipers have differential sized pistons, ie, the 'front' piston is bigger than the 'rear' piston. (I'll explain why shortly)
- Better pads. Modern pads are bloody fantastic. Price = Performance though, so how fast do you want to stop?
What other variations in brakes do -
- Twin calipers. Used to get more pad area on that brake, and usually has one caliper at the front and the other in the back. Not often used.
- Ventilated discs. The gap in the middle does a couple of things for the same result. The main reason is to give the disc double the surface area to help it dump its heat into the atmosphere. The other thing the gap does is to use the vanes to pump (centrifuge) air through the gap to help dump heat faster.
- Drilled discs. Exactly the same reason as a ventilated disc, but not quite as good. The size of the holes in the disc is worked out so that the surface area of the cylinder they make is larger than the hole they make in the disc, and so create a greater surface area overall to help cooling. The other advantage is that the holes help the plasma hot gasses from the pads escape somewhere.
- Slotted discs. No extra heat dissipation, but a slot that passes over the pads to help the hot gasses escape. The slots need not be put on the disc like spokes on a bike wheel, but can simply be a slot that goes across the disc from one side to the other side, just clearing the inner running surface of the disc.
As pads wear down, they will become
tapered. This is because as they are applied the disc heats up and
part of that heat is transferred to the pad itself. The problem is
that the heat is not transferred equally - The outer part of the pad
(The part closest to the outside of the disc) gets hotter because the
physical speed of the disc is higher there than it is towards the
The same goes for the part of the pad that is last to touch the disc as it goes past - The disc is somewhat warmer at that point than it is as it first touches the pad, and so the pad wears faster at that point.
Some of the taper problem can be relieved by making the discs bigger (The difference in speed between the inner and outer parts of the disc is lessened) and using a decent four spot caliper. I mentioned before that the better ones had different sized pistons - This so the front of the pad gets pushed onto the disc harder than the rear, thus reducing taper.
In any case, if you have tapered pads they must be milled flat again or thrown out. It's that simple!
So how hot can brakes get??
I guess that the above picture answers your question - It's a picture of Damon Hill's Jordan Formula One car, under heavy braking. They use carbon-carbon brakes, which means that the disc pads and disc rotors are made from a carbon fibre matrix composite. They don't work very well when cold, and so have to be used a few times to get up to a good temperature. (~800°C)
On a slightly different track, one good trick I've learned when bleeding brakes is to use a different coloured fluid so you know when the new fluid has gone through the system. Good quality fluids are usually available in either clear, red, or green colours, thus making the job easier.
Speaking of hydraulic fluid, I was asked
recently to explain how to pick the correct master cylinder and brake
When it comes time to selecting the size of the master cylinder (I'm assuming that you'll be going for the single master cylinder/balance valve option) you really only have a choice of the aftermarket sizes between about 1/2" and 3/4", and they usually go up in 1/16" increments. The most common size is usually 13/16", however. The 13/16" ones are often coupled with a vacuum booster, and that's why they're so big. (The vacuum helps you push the pedal, etc) If you go for a single cylinder, then you'll also have to, by Law and common sense I believe, have to have one that has twin (hydraulically separate) outputs. These can be commonly found on the vast majority of modern cars.
The rules for master cylinder sizes are simple - The bigger you have them, the harder you have to push the pedal, but the less it moves. (Eg, the pedal in my racing moves perhaps 1/4", then goes almost solid - I run fairly large master cylinders)
The rules for the brakes themselves are similar - If they have a large piston in them, then for the same given brake line pressure they will have more clamping pressure on the disc, thus giving better retardation. But they will also have a longer pedal travel.
Getting the balance between having the right size master cylinder and brake piston can be difficult, and it may take some fiddling. It may seem a bit 'slack', but I would suggest that the best way to get good brakes right from the start is to simply copy some-one else's that is known to work well. Don't take their word for it either - Drive the car to try it for yourself.
Another factor is the brake pedals' pedal mechanical leverage. It is typically about 4:1 to 5:1. Similar rules to the cylinder sizes, of course - The longer the pedal, the better the brakes work but the more the pedal moves. I personally much prefer a solid pedal that you have to push hard on, rather than a soft-ish one that moves a lot. I strongly believe that it gives you much better feel, and so control the brakes right 'on the limit' very accurately.
Back to the Drivetrain - Gearbox - page
For more motorsport links, try the motorsport section on my links page.
Back to the Index pagePage & contents where applicable © Bill Sherwood