Don Terrill’s Speed-Talk

I had a conversation with a fellow, many years ago, that had been through some sort of driving school (non performance I believe). He asked me if I knew why the rear wanted to swap ends when rear traction was lost; either by power or locked up brakes. He said it is because "a sliding tire always wants to lead". Seems to be absolutely correct however I will leave the physics part of why up to you scientific fellas !

Simple gravity or momentum will cause the vehicle to move that way when both wheels spin together, or tire pressures different will affect it also.
Hydroplaning will also affect the sideways motion.

I actually learned this but have forgotten how it was explained to me. We had a demonstration done with a 8" long car with wheels you could lock in place so they wouldn't roll.

Roll it downhill with all 4 unlocked and it went straight downhill. As soon as you lock up the rears and try to let the car go down the incline, the rear passes the front of the car every single time. I'll tell you this. It has exactly zero to do with the crown on the road because that incline was exactly flat with no crown whatsoever. There's a physics thing behind why it happens, I just can't recall what it is.

makin chips wrote: ↑Fri Apr 26, 2019 7:24 pm I actually learned this but have forgotten how it was explained to me. We had a demonstration done with a 8" long car with wheels you could lock in place so they wouldn't roll.

Roll it downhill with all 4 unlocked and it went straight downhill. As soon as you lock up the rears and try to let the car go down the incline, the rear passes the front of the car every single time. I'll tell you this. It has exactly zero to do with the crown on the road because that incline was exactly flat with no crown whatsoever. There's a physics thing behind why it happens, I just can't recall what it is.

That's really weird, as it's exactly opposite of what forty years of front-wheel drive experience has taught me. FWD is magic in most low traction situations. The only time FWD ever bitten me was the first time I was going into a downhill curve and backed off the throttle. Compression braking on the front tires while the rears were rolling free caused the rear end to try to pass the front. Since that time, if I'm driving a FWD and there's even a chance of having to decelerate going down hill, I go into neutral so all four tires are rolling and braking the same.

It's amazing what a difference it makes when turning on a slippery surface. When it's safe to, you can start into a turn a little fast, so the car starts to plow, then pop it into neutral and it tracks like a train.
It's equally amazing that hardly anyone knows this and no one seems to teach it.

makin chips wrote: ↑Fri Apr 26, 2019 7:24 pm I actually learned this but have forgotten how it was explained to me. We had a demonstration done with a 8" long car with wheels you could lock in place so they wouldn't roll.

Roll it downhill with all 4 unlocked and it went straight downhill. As soon as you lock up the rears and try to let the car go down the incline, the rear passes the front of the car every single time. I'll tell you this. It has exactly zero to do with the crown on the road because that incline was exactly flat with no crown whatsoever. There's a physics thing behind why it happens, I just can't recall what it is.

WAG is that it is based on the coefficient of static friction being higher than that of sliding friction. There will (normally) be some, however slight, difference in the static frictional force from one locked side to the other which will cause one side to release sooner and impart a moment (rotation) to the body. The rotational movement will cause the front (assuming fixed) wheels to steer to one side further accentuating the rotation of the body.

i didn't read all the post so someone could have already covered this , i take it you are mostly referring to RWD , my quick note would be that even as the engine rev's it lifts one side of the chassis , so consider those forces transferring down to the rear and the same motion on the axle , therefore one wheel will be forced into the road getting max grip and the other one will have less . the grip pulls the rear in that direction .
Some one said the one spinning most / but both wheels are spinning at the same rate

Per a previous post, it takes so little torque to spin the tires in snow that there is no apprecaible torque reaction unloading the right rear tire.
Also, such torque reaction only occurs on solid axle cars. With independent suspension this load is borne by the chassis-mounted differential and/or transaxle.

i don't accept the solid axle thing , as if you take solid and independent , both react in the same manor id the angle of the pig's head [ to prop shaft ] is altered , be it independent or solid , the load of the chassis will differ on both sides .
by chance i was watching a bit of rallying last night , BMW car heading slightly off in the wet dirt , he tried to move forward , rear went sideways , so he decided the only way was to get the front back on the road , he slapped it in reverse and the rear went the other direction , so try this next time your in the snow , 1 then R

Imagine the ring and pinion are welded together. Let's say the engine is making 500 lb-ft. at the crank. Multiply that by the first gear ratio, say 2.5:1 so 1250 ft-lb. trying to lift the right side of the axle assembly and press down the left. The solid axle is resting on the ground and so acts like a giant Johnson bar to resist this torque, so it can't move except for the slight decompression of the tire. At the front of the driveshaft, per Newton's Third Law, this torque is balanced by an equal and opposite reaction which, since the transmission is attached to the engine and hence the chassis, causes the chassis to try to twist in the opposite direction, CCW as seen from the front. (Which is why the left front tire is more prone to catching air) The combined effect is to extend the left rear spring and compress the right one, thus reducing the load on the right side tire, leaving it more prone to breaking traction.

With independent rear suspension, the diff is bolted to the chassis and thus the CW and CCW forces merely try to twist the chassis and there is no torque-induced vertical force at the rear tires. It would be like trying to lift yourself up by your shoelaces.

PackardV8 wrote: ↑Fri Apr 26, 2019 10:50 pm

makin chips wrote: ↑Fri Apr 26, 2019 7:24 pm I actually learned this but have forgotten how it was explained to me. We had a demonstration done with a 8" long car with wheels you could lock in place so they wouldn't roll.

Roll it downhill with all 4 unlocked and it went straight downhill. As soon as you lock up the rears and try to let the car go down the incline, the rear passes the front of the car every single time. I'll tell you this. It has exactly zero to do with the crown on the road because that incline was exactly flat with no crown whatsoever. There's a physics thing behind why it happens, I just can't recall what it is.

That's really weird, as it's exactly opposite of what forty years of front-wheel drive experience has taught me. FWD is magic in most low traction situations. The only time FWD ever bitten me was the first time I was going into a downhill curve and backed off the throttle. Compression braking on the front tires while the rears were rolling free caused the rear end to try to pass the front. Since that time, if I'm driving a FWD and there's even a chance of having to decelerate going down hill, I go into neutral so all four tires are rolling and braking the same.

Its possible that I got it backwards and he had the fronts locked. It's been quite a while so I could've easily misremembered which were locked and which weren't.

In fact, I'm pretty sure you are correct. The rears locked caused it to just drag the rear wheels. Locked front tires caused it to swap ends and send the rear end swinging around. That makes more sense.

Either way, there was some physics lesson we learned behind it all. Something to do with friction? I don't remember and I can't really get in touch with my professor anymore to ask him about it.

David suggested this text to me many years ago now. It was good advice.

http://www.edccorp.com/library/TechRefPdfs/EDC-1110.pdf

Mechanics of
Vehicles
By Jaroslav J. Taborek
1957

The link doesn't work for me...

Corrected

I am guessing that Benz used a technology similar to a freewheel ratchet on a bicycle. Might get real exciting on certain road surfaces. It doesn't look like chordal action would be too much of a concern.