Static Friction v. Sliding Friction

(I intended to post the following on this forum earlier today but "slipped" and put it on the Farmall board.)

For decades, when driving on a slippery surface with a stick shift vehicle, I have been aware that often the traction improves after shifting to a higher gear. Knowing that static friction is greater than sliding friction, only yesterday, did I connect with an idea that I think is the possible reason for the better traction in the higher gear. Maybe, I am the last person in the English speaking world to make the connection and maybe I am wrong in my analysis, but here it is: When changed to the higher gear, the torque requirement on the input shaft to the transmission is increased. Thus, more pressure is applied to the compression springs connecting and absorbing the “rotational” shock between the clutch drive and the rotor on the input shaft to the transmission. So rotational shock absorbed in the clutch springs, allows for an increase in the static friction between the tires and the surface, they drive on. It is possible that spring loaded rotational devices could be used to improve power transmission in other places that are now using solid or fluid coupling.

Regardless of what gear you have the vehicle in, the friction coefficient between the tire and slick surface remains the same.

A 4 million horsepower engine will hold the slick surface in a lower gear exactly the same as a 200 horsepower engine in a high gear, provided they are the same weight.

It's all in how you touch that gas pedal.

I think you're making this overly complicated...
I'd say that it boils down to an engine's torque curve. When you gear up you simply have less torque available at lower engine speeds, thus the wheels don't break traction.

Rod

GEEZ lower gears put more power to drive wheels. I thought that everyone new that much.

If you use less power to the drive wheels then there is less chance that they will brake free and spin.

If on Ice then I doubt that you can find a high enough gear to help.
Walt

thread replies remind me of an old car I had, 1953 Plymouth - fluid drive. Early version of an automatic. It had great traction cause it didn't have enough power to spin the wheels.

You're on the right track, but it's simpler than that: Your engine at idle produces a fixed torque. Shifting to a higher gear reduces the torque applied to the wheels. If the torque at your wheels is great enough to overcome the static coefficient of friction, then the tires slip, so less torque is better.

Of course, the clutch does come into play. Since the engine wants to stall if you start out in a higher gear, you naturally have to let it slip a bit as you're starting out, which further reduces torque at the wheels.

The "Cruise-O-Matic" transmissions in the Fords of the mid-sixties had a shift position specifically for starting out in slippery conditions. All it did was lock the transmission out of first gear.

I guess I've always looked at driving in snow a little different. In rear wheel drive vehicles I always run a gear lower on slippery conditions. Engine speed has no affect on how much traction the wheels have, it's just a little harder to 'feel' when they slip. The advantage comes when the wheels do start to slide. Letting off the throttle, slows the rear wheels allowing them to pull the back of the vehicle out of a fish-tail. If the engine is already idling, you don't get this option. I wouldn't recommend trying this with a front wheel drive car... it doesn't work as well.

Power is the same.
For the identical HP. Lower rpm has higher torque. Higher rpm has less torque

Good 'ole fluid drive. My 1950 Dodge had it too. She took a while to pull away from a stop light, but conveniently had a 3 speed manual on the column if you were in a hurry.

You know, I use a lower gear too most of the time. Seems like there's more feel. In high gear, when the wheels do spin they REALLY spin. A good example happened just yesterday when I was driving the S-10 on an icy road in 5th gear, or overdrive. I happened to glance down at the speedometer and it read 70 when I was really doing 45. I didn't feel any 'vibes' telling me the wheels were spinning faster. If I was in a lower gear the engine would have been a howlin. Jim

Yessir, I learned to drive in my Dad's '49 Dodge with fluid drive. The greatest thing in the world to learn on, you couldn't kill the engine. I've driven "stick shifts" every since, and currently drive an '05 F-250 with V-10 and 6 speed. I cannot for the life of me figure out these younger drivers that drive along fine till they come to a slick spot, then jamb the brakes on. Sometimes letting off my accelerator isn't enough to stay a safe distance behind.

The advantage offered to braking with ABS, is that the wheel rotation is pulsed between rotation and non-rotation, and in that process, passes through the point that offers the most static friction and traction for stopping. The point of my original post was that a similar pass-through, is offered to the drive traction, when the compression springs in the clutch plate work, (because of being in the higher gear), i.e. compress and expand, varying rotational speed as well as the static friction component where the rubber meets the whatever. The drive system might be improved if the drive wheels had individual rotation compressible devices on each drive wheel instead of just the one on the engine's clutch.

I guess I'm confused now. I could be wrong here, but as I understand it, once a vehicle is moving and the clutch pedal is released the engine and transmission are locked. If there is any slipping after this point, the clutch needs to be replaced or adjusted. The springs are only relevant while the clutch is engaging regardless of what gear you are using. Even if they do abosrb some shock from the drive train, they have have a relatively small amount of motion (under 1"), which is probably irrelevant to an engine that is turning at over 1000rpm.

Your tires are using 'static' friction unless they are spinning. Many new cars have a traction control system which will either decrease the engine speed or apply braking force to the wheel that is spinning.

Thanks for the insight !!! Helps explain why my clutch flew apart when I took the bolts out. All that static friction energy stored up in there is very dangerous. OSHA should be made aware of this.

Mathias,

Try this, it works with front wheel drive, rear wheel drive and even four wheel drive. When you are in a situation where you're likely to lose traction, push in the clutch or throw the transmission out of gear. For example, if you're making a tight turn or stopping on ice. You will be amazed at how much more control you will have if the tires can free-wheel. The reason is simple: A spinning or sliding tire lacks traction in any direction, including laterally. Once the tire is turning at vehicle speed, it "grabs" the road due to static friction being greater than dynamic friction.

Letting your rear wheels "drag" you out of a skid doesn't really work. The sliding rear tires lack lateral traction and if there are any lateral forces on the vehicle (due to turning forces or sideways slope) the rear end will slide in the direction of the force.

An interesting corollary to this is what happens when you lock up the rear wheel of a motorcycle. Experienced riders will tell you to NEVER release a locked-up rear brake when you're on pavement. If you lock it up you must bring the bike to a full stop. If you release the rear brake, the rear tire will suddenly gain traction and cause the motorcycle to instantly change direction to wherever the rear wheel is pointed. This usually results in a deadly "high-side" crash.

None of the above is correct.
The static or dynamic friction never changes. It is a function of weif=ght and the surface only.
The reason for less spinning is that you have less torque available at the wheels in a higher gear.
The clutch has nothing to do with it.
Power is the same throughout the geartrain at a particular engine rpm, except for efficiency loss. Torque and rpm change with gearing.