Well what breaks is not the crownwheel or pinion but the small "diff" gears
that run in the middle.
What causes it is crap suspension allowing the wheel to bounce about like a
ping pong ball as it spins when you change from 2nd to third or fourth...
Cosworth "quafe" steel gears help a bit but the break too...
But in a lighter car it should be OK unless you are an animal with too much
nitrous, and bag of cement in the boot...
See - I was talking about using the diff in a proper twin wishbone setup ;-)
As a matter of interest, what will the durability of the ex Crapi axle be
like in my locost - 500kg -ish and 110 bhp at the back wheels - centuries I
Yes - certainly heavy, especially considering the overall weight of the car.
I would have probably opted for an IRS or de dion setup, but it works well
enough for clumsy old ham fisted me.
Apparently on date Wed, 14 Apr 2004 10:54:35 +0100, "Rob"
The TR7 back axle is narrower than the SD1 it is designed like, and can take
300 horses fairly well. Oddly enough, the four speed car is the one to take it
off, because that has a more useful 3.45 ratio compared to the 3.90 you'll find
on the five speed cars.
Dunno how you would fit a V8, gearbox and diff in front of the axle, and still
leave space for a driver, in a Punto.
Apparently on date Thu, 15 Apr 2004 18:15:50 +0100, "Stuffed"
You may well be right there, now I come to look it up. Ok, forget the 4 speed,
the 5 speed is the one to have, and change the internals if you don't intend to
hillclimb with it. :)
The 5 speed is up to the job, it's taking 220 here and I know of a 5.0 engine
with reportedly 400 bhp that is driving through the regular axle.
By contrast, the sprint engine is well under that sort of power and I believe
came with a regular diff and gearbox, given that both are basically the SD1 box
and diff with a different final drive to aid acceleration of the smaller
engine. As compared to the rover v8 engine they were usually driven by, of
Time for some physics!!! at least rear drive physics...
Axles don't break because of horsepower. They break because of torque.
The maximum torque you can put through one depends on the vehicles grip and
the weight or weight transfer on to it. (This weight transfer CAN be greater
with more power though...)
So provided you can dump the clutch in your 1.3 and get a squeal from the
tyres it really shouldn't matter how much power your 5 litre motor has
beyond this because the wheelspin is a torque limiting safety valve.
Thats why a 1.6 / 2.0 litre transit has a HUGE diff, and the same engine in
an old cortina has a crappy small 6 inch one!
The max torque the axle sees depends completely on to gear ratios too. A
gearbox is simply a torque multiplier. And bigger motors generally have
taller first and second gears as they already have the motor torque to pull
it... So there is not so much of a torque increase at the axle as would be
expected. The greater aceleration comes from each gear accelerating you to
a higher final speed.
So that helps even things out too.
So if the cars light, and a 1.6 can spin the wheels in first gear then a big
nitrous V8 can "only" do the same. Once wheels spin, weight transfer and
torque actually decrease. But it goes on for 3 miles and 4 gears further!
So I wouldn't worry too much...
What really bust all my diffs was
a) more grip because of wider tyres and lower pressures than stock.
b) more rearward weight transfer due to crap soggy suspension! (on purpose
c) Heavy and hard gear changes from 2nd to 3rd because I like to feel it
squirm and see the smoke at the lights...
The suspension/weight transfer/grip thing is closed loop and really helps it
"hook up" which is the reason for breakages...
Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here.
All logos and trade names are the property of their respective owners.