Originally Posted by
Caddyshack
Good explanation, like that.
Why don't 2wd break boxes like 4wd though? Is the 2wd box stronger as same sized housing contains more strength as doesn't need room for transfer?
Tyres will spin at same torque, yes but if a 400 bhp / 400 ftlb car was 4wd the rear diff only sees 66% of the gross power / torque. 2 wd gets all the torque so the gross engine output on a 4wd can be higher before rears spin. Plus a 2wd has 2 tyres contact patch, a good 4wd with lsd (front and rear) has 4 contact patches to distribute the power.
Putting it another way the 2wd has more torque to deal with on 2 tyres, the 4wd rear only has 66% of the same torque therefore a 2wd will be more willing to spin the rear wheels than an exactly the same powered 4wd twin lsd car will spin the rears. So a 400 bhp car will spinnthe power away where a 4wd is more likely to have 100% traction; albeit spread across 4 tyres.
I am not arguing with you by the way, I am here to learn.
With the tyres spinning at the same torque I meant at the same wheel torque. Obviously a 4x4 can have more engine torque before the wheels starts spinning. Due to that the gearbox on a 4x4 has a harder life than a 2wd gearbox, at least in the lower gears.
Diffs will break quickest in lowest gears (first, second) as the torque to the diff is than the highest (engine torque amplified by the gearbox). The torque to the gearbox and clutch is always the same (just engine torque), so for the gearbox it matters less in which gear it is. Clutches will start to slip mostly in high gears as the 'resistance' from the car accelerating is then highest. For a 2wd this 'resistance' in low gears is quite low as it will simply spin the wheels, for a 4x4 it is much higher as will will not so easily spin the wheels. I guess it can be quite easy to kill a clutch when launching a 4x4, as you will need to let it slip to prevent the engine bogging down. For that reason I don't launch my 4x4, but I have no problems doing that on my 2wd V6.
I believe the T5 is breaking above 400 bhp, while the MT75 is breaking above 300-350 bhp. Officially the MT75 is rated at 300 Nm. But the 4x4 Sapph already has 290 Nm as standard and the Escort 305 Nm. In practice I believe the 4x4 box can take about 350 lb.ft / 475 Nm.