DUMB QUESTION no 101

Your certainly on the right track Voxson!!! The front wheel that hasn't lost traction would be going at vehicle speed, so the front wheel that has lost traction would be going double speed.

However,that would also mean that the rear wheels would have to turn faster, but they cannot (assuming both rear wheels have traction). So, one of the rear wheels must also lose traction. Then one front and one rear wheel would be spinning at twice the opposite wheels speed! Basically identical, but in reverse, of what happens when you spin your wheels going uphill.

How do I know this? Easy, I cheated!!! I have a remote control 4WD and I simply tried all the combinations of simulating different things to work it out. I tired to figure it out in my mind but my head then started to spin!

Cheers

Captain

Hang on, I think I'm missing something here. If a wheel is skidding downhill, it surely is NOT turning at all, yes? Then, how can the opposite wheel spin at twice the rate of the stationary wheel? What's twice zero??

Please explain before I too have to go out and buy a remote control toy 4WD and get my head spinning :-)
Klaus

Hi V8troopie,

The wheel thats "stationary" is not actually stationary! Its still being driven at engine idle speed, but its turning slower than the speed gravity is dragging the vehicle downhill, so to an observer it looks like its stationary because its turning slower than rolling speed. This one was easy, did not have to grab the r/c 4WD to figure it out ;-)

If the engine was to stall and one front wheel (and by default, also one rear wheel) was stationary, the other front (and rear) wheel would turn at vehicle sliding speed.

Cheers

Captain

If you look at a vehicle which loses traction in these circumstances you'll see what happens is the wheel that loses traction is actually driven in reverse by the action of the diff as the vehicle speed increases. First time I saw it (back in my Army days) thought I was seeing things 8--; but work it out , if the vehicle speed increase and one wheel has full traction and engine revs don't rise then the extra speed is fed to the wheel with no traction. The diff gears dictate it must spin backwards.

To quote my 10 year old, "My brain aches!!"
Cheers

MrBitchi is correct in theory the wheel in the air going down hill will spin in reverse.

Gaz

Hmmm... not sure about that Gary.

If one wheel is in the air, drive can go to that wheel so it will spin forward at engine speed (providing one back wheel also loses traction). The front wheel on the ground will simply travel at vehicle speed (ie it will overspeed compared to driven speed, but still be going forward).

However, if both front wheels are on the deck, but one front loses traction, then the one with lost traction will turn "backwards" IF a) one rear wheel also loses traction AND b) how fast the vehicle now "slides" downhill compared to engine idle rpm. Note that backwards is a relative term here. The wheel is turning slower than ground speed (actually at engine idle rpm/gear speed), but is still travelling in the forward direction. This gives the illusion of turning backwards.

The whole question is actually very interesting. By playing around with my model 4WD, I have found that many of the above scenarios are true, it all depends on the relative overspeed of the vehicle sliding, whether a wheel is in the air or simply loses traction while on the deck and also what the rear wheels are doing (if both rear wheels have good traction, they dictate the speed of the drivetrain, then it further depends if a front wheel is in the air or both on the deck)

I have spent too much time on this already, but it really intrigued me (like when you get a song in your head and cannot stop it !!!). Ultimately there are so many different scenarios depending on what wheel has traction etc... that there is no right or wrong answer without imposing more conditions on what other wheels are doing.

Just hope your heads not spinning like mine over this ;-)

Cheers

Captain

Yep you're right. The free wheel only goes backwards while the wheel with traction is going more that twice the speed than if all wheels had traction - this so long as both rear wheels have traction too.

The point is though - without braking (which changes everthing) and in a downhill situation the vehicle will let go and like be going faster than twice the fully tractioned speed and therefore the free wheel will be in reverse. That why I said "in theory" in practice the driver will likely brake and/or the the free wheel is unlikely to remain free for long. So I guess it would be relatively rare for the free wheel to be going reverse for long if at all. But we are theorising - or at least I was.

But I'm not sure I agree that the there needs to be one rear wheel in the air too. But I don't have a remote control car with me *grin*

Gaz

I can vouch for the 'accelerate to regain traction' trick. I was on a very muddy slide in my previous 4b (Jeep) with tyres that shouldn't have been anywhere near the place. Got into a slide with no control. Hit the pedal and regained traction. It saved my bacon. But wouldn't go looking for opportunities to try it unless you found a safe place.

Gaz