# The Naked Scientists Forum

### Author Topic: When is wheel-spinning helpful for control?  (Read 2827 times)

#### thedoc

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##### When is wheel-spinning helpful for control?
« on: 29/04/2012 11:34:02 »
Steven Wasmer  asked the Naked Scientists:

Hey Naked Scientists,

I just returned from a Hummer rock crawling tour in Moab UT. Beingout "rock crawling"on the petrifiedsand dunes I got my wheels turning (that pun is for you chris) about the nature of traction. When rock crawling the idea is a slow and steady pace inorder toprevent a break in traction. Wheel spinis abigno no. We see this every day it rains and we floor the gas pedal from a complete stop. The wheels overcomethe friction of the road, break free, andyou go nowhere fast. However when watching motor sports inloosematerial such as sand or dirtwheel spin is part of the technique. When dirt bikes launch off the line the ones throwing the largest rooster tails shoot out in front.In tractor pullsthe vehicles use large amounts of wheelspin and its only when the wheel spin stopsthatthey come to a halt. Conversely, if you get your car stuck in the mud or snow and you spin the wheels all you do is dig a deeper hole. The difference in technique must lie in the surface in question but im not sure of the physics involved.

Can you help?

Steven
USA

What do you think?
« Last Edit: 29/04/2012 11:34:02 by _system »

#### David Cooper

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##### Re: When is wheel-spinning helpful for control?
« Reply #1 on: 29/04/2012 20:22:32 »
I'm guessing that the amount of traction initially reduces when the wheel starts to slip because it doesn't dig in so well (on the very small scale), but as you rotate it faster it begins to apply a force at some point that may be greater than the force that can be applied without losing traction. Clearly it's only going to work if there's enough weight pressing down on the tyre, and it's also going to wear out your tyres more quickly than an ordinary motorist would be keen to do. Rally driving often looks as if it has a lot in common with driving a hovercraft.

#### s_wasmer

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##### Re: When is wheel-spinning helpful for control?
« Reply #2 on: 01/05/2012 16:51:17 »
I spent quite a while thinking about this some more and think it may be the same physics which applies when you strike water quickly.  Strike water with an oar quickly and you get a slap but the oar has difficultly penetrating.  Doing the same motion slowly gives the water a chance to "get out of the way".  Could this be what is happening with the tires in the sand?

#### grizelda

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##### Re: When is wheel-spinning helpful for control?
« Reply #3 on: 02/05/2012 04:30:39 »
Sort of. I think the wheel is throwing the earth and incurring an opposite reaction, sort of like a rocket engine. this allows the bike to be steered by "body english" as the normal effects of friction with the ground are negligible, so lateral motions which could not be done using friction can be done using reaction.

#### CliffordK

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##### Re: When is wheel-spinning helpful for control?
« Reply #4 on: 02/05/2012 07:19:23 »
Perhaps it depends on the surface.

On mud, spinning the wheels can be really bad.
For one thing, one often ends up with a hole where there wheel is...  and one doesn't go anywhere.

Often one can do better with a light touch on the accelerator.

One also has to think about the differential, weight of the vehicle, and etc.  With a typical car and open differential, one spinning wheel and one is stuck.  A 4x4 with locking or limited slip differentials may be a different story.

#### CZARCAR

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##### Re: When is wheel-spinning helpful for control?
« Reply #5 on: 02/05/2012 18:04:53 »
ice is a solid, its the friction of the spintire that melts & wheel spins................sand is a hybrid? [solid/liquid]

#### Geezer

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##### Re: When is wheel-spinning helpful for control?
« Reply #6 on: 02/05/2012 18:31:11 »
In some situations, if you are able to propel enough material (sand, gravel, etc.), it will produce a fair amount of forward thrust. A good example of this is the jet boat.  Sand/gravel/mud is denser that water, so that will help a bit too.

There will also be some amount of friction between the tire and the substrate, even on ice, and that also contributes to the forward force produced.

Really soft surfaces like snow and sand create an additional problem because, not only is there a lack of traction, but the vehicle is continually having to climb up a slope, even when it's on the level, if you see what I mean. I believe four-wheel drive makes such a difference in those situations because all the wheels are pulling their way "up the slope".

#### The Naked Scientists Forum

##### Re: When is wheel-spinning helpful for control?
« Reply #6 on: 02/05/2012 18:31:11 »