Lucy W wrote:
You may like to purchase the Bosch Automotive Handbook published by the Society of Automotive Engineers, the 6th Edition (I believe is the latest ISBN 0-7680-1513-8) explains this issue on page 862. But don’t take my word for it have a go yourself..
I think you'll find it's on the 7th edition now - it's been very substantially revised in the 7th edition - that's why there are quite a lot of 6th editions floating round Amazon and such places! (and, as you'll hopefully gather, YES, I'm quite familiar with it)!
Lucy W wrote:
The key is that the foot brake can never give the spinning wheel MORE resistance. If that were the case, differential lock would never have been invented, it would have been left to brakes and gearing.
The resistive force you need to apply at the spinning brake is as high as the resistance of the car, and it keeps increasing the more you brake the tractive wheel, so the spinning wheel can never catch up.
As I have said in my previous post, we can both agree on this in theory. In practice (for reasons that I've tried to outline above), it seems to make a bit of difference!
Lucy W wrote:
(1500kg at 10mph on a soft field going up an incline of 5 degrees will have an overall resistance of approx 6400N and a with max engine torque of 160Nm, a typical car will be able to create a maximum of 8,800N of forward motion) rear brakes are going to be a problem, in fact 2x6400N i.e. 12800N is required and hence the car can not transfer torque via the diff as it is limited by the combination of its torque and gearing. Whereas on level tarmac, spinning wheel on ice, then the overall resistance is 209N so I guess that the rear wheels could be “dragged along” if the traction of the good wheel is sufficient.
I'd love to see those sums! I'm intrigued by the great conviction with which you pronounce the resistive force on ice to be 209N, I must say! Remember, however, that in the instance I'm refering to, I was trying to back a 2 tonne car up a 1-in-5 slope!
Lucy W wrote:
Of course your right about the practicalities of hose clamping, I never really though of that. Yes you would be in and out of the car all the time on a severe road as the ice patched swapped from left to right. I guess it limited in practice.
And up to a couple of times per second too! Yes, it's completely impractical!
Lucy W wrote:
But have a go yourself. I hope you will find this sort of experimenting interesting. It may seem basic, but in fact it is exactly what an expert would do albeit they may have access to a rolling road.
I did. Read the post. If I hadn't, I'd still have been there spinning one wheel until Spring came! Quite odd really, I wouldn't have though it would make as much difference as it did but I was desperate and wasn't exactly spoilt for options, so I gave it a go!
Lucy W wrote:
Good luck, it might all sound silly, but the best way is to experiment and understand how your car functions. Seeing is believing.
Indeed it was!
Lucy W wrote:
P.S. A danger of clamping brakes, certainly if you do three to cut out the rear, is if on an iced road, the one wheel with any braking may stay up on ice as you move forward and you will be somewhat helpless!
I have also pondered if a flexi-pipe or at some other point a manual valve could be fitted to make life easier than clamping although I expect this may fall foul of the MOT test.
It would almost certainly contravene the C&U Regs. The drag racing boys like to fit "line locks" to their rear brakes so they can do good burnouts. They work the same way. I have to put "fail valves" in the braking system when I'm doing the type approval braking tests so that I can simulate failure in one or both braking circuits. I guess I could have four - one for each wheel, but I'd never be able to operate them fast enough in the situation I'm talking about!