Jim Dunleavy

As I understand the scooby four wheel drive system there will be a lot of transmission resistance to one set of wheels rotating while the others are stationary. If this is the case what happens when just one pair of wheels are on the brake tester?

[This message has been edited by Jim Dunleavy (edited 09 September 2001).]

mutant_matt

The transmission breaks

Seriously, road test only!!!

Matt

scoobyboy

it can be done i have seen mot testers do it . providing it is done quick i see no problems .

although we did have a turbo delivered to us once which had been spec lifted from a garage we had brought it from so it was rear wheels only on the deck and it did burn the centre diff out!

slimjim16v

do NOT allow them to put a permanent 4wd car on one set of rollers, it will cause damage. they have an in car brake tester they can use on a road test.

scoobyboy

if it is done quick it will be ok the trans mission can handle it coz it has a viscous centre diff if it had a mechanical diff thn no.

slimjim16v

the diff is designed for a certain range of slip, can't remember, but say 25 - 75%.
if you want to let someone put 100% slip through your diff, be my guest.

Jim Dunleavy

Thanks for all the comments, it confirms what I suspected.

AndyMc

I thought the brake test machines turn the wheels in opposite directions to stop the car rolling away or needing to be tied down.Eg if the front is being tested one front is rotated forward and the other is rotated backwards.

If so then the center diff will stay still as will the diff at the end of the car not being tested.The diff at the tested end will turn fairly slowly which is no problem for open and viscous type diffs but could damage mech diff if they are capable of locking fully.Most don't lock 100% however because although it would provide better traction it make the car a pig to drive.Does anybody know if the mech diffs fitted to some of the imports can lock 100%?

So like Scoobyboy I can't see a problem,also wouldn't there be a warning about this in the owners handbook if it was going to damage the diffs? (yes I know it does warn about towing with only two wheels on the ground but this is different to whats being discussed here)

Andy

scoobyboy

thank you andy

Jim Dunleavy

I used to be a tester and all the rollers I saw then turned in the same direction.

scoobyboy

the rollers both go in the same direction when the front wheels are in them you apply the hand brake i think scoobys pull about 200 kilos on both front wheels then the wheel will lock and skid on the rollers but it won't jump out . but when you check the rears if they lock on the hand brake the rollers do push the car out coz theres no other secondary braking to hold them in unlike the fronts

AndyMc

Hi Jim

If the rollers turned in the same direction wouldn't you be limited in the torque you could apply during the test simply because the car would roll out of the rollers.I would estimate that it would take only 100-200 lb/ft of torque to make the car lift itself off the rollers and this is nowhere near the torque that the brakes on a modern car can produce which is well over 1000 lb/ft. Did you have to tie the car to the rollers to stop it moving?.

Another way of looking at it is that its not that hard to drive a car off the rollers,I have done this many times,you have to rev the engine a little but you do not need anything like full power.If you were to try to drive off in first gear with your foot hard on the brakes(pretending for a minute that you have three feet) then no matter how hard you tried you would only succeed in burning the clutch out,the brakes are a lot more powerful than the engine even in first gear where most torque is produced.What I'm saying is that if both rollers do go in the same direction you would only be able to check that the brakes can generate say 25% of their potential stopping power.This seems unlikely to me.

If I was going to design a brake test machine I would have the rollers going in opposite directions.This would try to make the car twist sideways but the tyres at the other end of the car would stop this from happening.You could apply much more force only limited by the tyres grip on the rollers so the full power of the brakes could be tested.This also has the advantage of not requiring the car to be tied down saving the garage precious time.

Of the two possibilities above I think mine is more likely,these are however just my thoughts and I could well be wrong .

If this post doesn't kill the thread stone dead then nothing will

Andy

AndyMc

Oh well,I must be wrong about the rollers going in opposite directions,sorry for confusing things!.I still think it is a strange way of testing though.My car is due its first MOT in a few weeks so I'll take it to the garage where I thought I saw the rollers going in opposite directions to see if do.

Back to the other point though, even with the rollers going in the same direction Imprezas with viscous center diffs would be fine as would those with the adjustable diff in the full open position.

Andy