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Friction
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Posted by: Steve_Rotolone

07/24/2003, 07:37:56

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Tribology books say that friction is independant of the contact surface. If this is true than why do drag cars get more traction on take off with wider tires?






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Re: Friction
Re: Friction -- Steve_Rotolone Post Reply Top of thread Forum
Posted by: ReubenAG

07/27/2003, 14:46:28

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Friction is generally independent of area, for two smooth surfaces of similar surface hardness.But in the case of rubber tyres on tar, the softer rubber protrudes into the small valleys in the surface of the harder tar. At the limit of traction (grip), these rubber protrusions start to shear off - laying rubber down on the surface. As there is some degree of positive drive from these asperities (most of the grip is still friction), the sum of the area rubber asperities will influence the amount of grip available. Obviously for a wider tyre, the contact patch is bigger, increasing the area of rubber which could protrude into the tar surface. As force is shear stress (which is constant for a given rubber at a given temperature) times area of the asperities, a wider tyre will give more grip, AT THE LIMIT OF TRACTION - this relationship only becomes valid when the rubber tyres start shearing.

I'm open to correction on this - I think this is also why Formula 1/Indy cars run lower tyre pressures than normal road cars - 125-140 kPa in F1 as opposed to the 200-240 kPa in most road cars. The lower pressures result in a bigger contact patch, allowing more grip. It also increases rolling resistance, which is a function of wheel speed. So the extra grip available at the start and through corners will compromise the top speed some-what. I'm not sure how big a component of the total drag rolling resistance is - I'm sure that the aerodynamic resistance is the biggest component.

 







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friction / grip
Re: Friction -- Steve_Rotolone Post Reply Top of thread Forum
Posted by: RKimball

07/27/2003, 01:38:17

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Actually, the track surface has texture, you want more soft rubber to mold to and grip that texture in order to transfer work force against that textured surface (a bit like tread in mud). Rubber surface is sacrificed in the exchange as it is both ripped loose from the other rubber mass by the texture and is lost in heat reversion (the breaking down of rubber used in molding of rubber shapes, reversion is the re-melting [simply and sort of]). Thus rubber is molded/melted to the track, and lost in ash and small particles/chunks into the surrounding area. The next car to race gets some advantage from this surfacing of the track, which again is pealed away from the textured pockets. The cycle continues.
-randy-



** The worst suggestion of your lifetime may be the catalyst to the grandest idea of the century, don't fail to listen to suggestions. -randy-


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Re: Friction
Re: Friction -- Steve_Rotolone Post Reply Top of thread Forum
Posted by: spade

07/27/2003, 00:03:38

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Friction is the force that keeps an object from moving or once moving trys to stop it and is independent of contact area size. It also acts in the opposite direction of movement.The amount of friction acting against a set of narrow tires would be the same for a set of wide tires on the same surface. The reason dragsters use wide tires is because the wide tires give more area to push against the asphalt and this is a work concept.






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Re: Friction
Re: Friction -- Steve_Rotolone Post Reply Top of thread Forum
Posted by: spade

07/27/2003, 00:01:51

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Friction is the force that keeps an object from moving or once moving trys to stop it and is independent of contact area size. It also acts in the opposite direction of movement.The amount of friction acting against a set of narrow tires would be the same for a set of wide tires on the same surface. The reason dragsters use wide tires is because the wide tires give more area to push against the asphalt and this is a work concept.






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