Salt Surface Traction and Other Questions

[MarkV]

Is there a known coefficient of friction that's generally accepted between the salt surface and a rolling wheel?  I realize that the composition of the wheel in contact with the salt surface is part of the equation (rubber compound or solid metal), but as the aero forces increase, you're essentially pushing against the wall, and tire slip could occur if the power applied exceeds traction.  Also, does the salt surface shear, meaning that as you apply more power, is there a tendency for the surface to slip over underlying layers?  How about this - is there a rule-of thumb concerning the unit loading of the tire/wheel combination?  If you had a very narrow or crowned wheel, then your vehicle's weight would be concentrated in a very small area.  Is it possible to overload a wheel so that it creates furrows in the surface, and in doing so, causes the rolling resistance to actually increase?    I understand the fact that traction isn't a straight line graph with vertical loading, but I wouldn't want to make the wheel thickness excessively thin, either.  Maybe the rules have minimum width or unit loading requirements and all of this is a moot point!

Thanks in advance to any replies! 

[DallasV]

I'm not sure about the scientific part of your question, but creating furrows in the salt is usually followed by an official asking you to put it on the trailor, as some that have run narrow aluminum wheels will attest. I'm in a roadster so it doesn't effect us to much, but the furrows raise hell on bikes. 200+ on 2 narrow tires trying to stay out of a furrow does not sound like a good time to me. But those bike guys (& gals) are crazy anyway so maybe they have a different opinion.

[V8Pinto]

Yeah...my Busa was doing that at 190 last year!  I told my crew chief (Grandpa) that it felt like riding a hound dog on a scent hahahaha.

[Sumner]

If you are going to be running over 200 there are only a few tire choices that meet the rules, so you will have to use one of those.  If you choose to make aluminum wheels then so some searches on that on the site.  You will have to get them approved though like Dallas mentioned.

I see figures of .4 to .6 for the coefficient of traction for the salt.

You might want to read some of this..........

http://purplesagetradingpost.com/sumner/bvillecar/bville%20-%20LSR%20Thoughts-Index.html

.... it might help and answer some of your questions and then again it might not.

c ya,

Sum

[Jonny Hotnuts]

There is a misconception about aluminum wheels salt. Aluminum does not mean that it will furrow and rubber does not mean that it won’t.

If you take 2 wheels with the same contact patch surface area and same cross sectional profile on the same vehicle with the same weight, the tires, rubber and aluminum will make the same mark in the salt. The problem comes in that the aluminum does not deform when turning and wants to skip. The way to solve this is to have thinner aluminum wheels with "sharper" sides and this is where the problem starts. This basically is giving the aluminum wheels the grip in the same manor as ice skates. Maybe if someone showed up with a special “Salt Zamboni” everyone could run solid wheels!

I think if someone had the time and money they could come up with an aluminum wheel that’s profile allowed adequate traction and was wide enough that its footprint would not differ from a rubber tire.
I had thought something on the lines of the profile of a 170 MC tire could possibly work.
I will likely never know.


Good luck!

[Harold Bettes]

RE: Coefficient of Adhesion on salt

Howdy All,

I think that planning on anything greater than a coefficient of .5 is fairly wishful thinking. IF it turns out to be slightly better than .5 then you will be in good shape if you planned on more slippery coefficients.

Hard packed is somewhat better than loose packed, but both exist on the courses at the same time. The divots that exist from whatever the cause do not have as good a coefficient of adhesion as does the hard packed stuff.

Regards to All,
HB2

[Stainless1]

Most of us are happy if our "slip" is 7% or less on the salt.  12-18% will usually blister a tire.  Car or bike, same deal.

[Glen]

There have been many try with alum. tires over the years. Very few have had good luck with them. The designs very as size etc. The salt is so fragile that any non rubber tire is really looked at by inspectors.
If they seem acceptable they are allowed a trial run and the course is checked right after the run, usually a SCTA chase vehicle will follow them down.

[dwarner]

Do yourself a favor and do not restrict your vehicle design to a solid wheel/tire combo. The salt varies greatly from year to year, 1 mile to the next in a single run. IF you do not have a rubber tire option you are on the trailer.

DW

[Richard Thomason]

In the 80's when there essentially no tires, Bruce Crower built a set of drive wheels out of aluminum that were covered with gilmore belts. Only liner I have ever seen that actually spun out at speed and didn't roll. Maybe it was superior chasis design and not the wheels, but he didn't run it again.

[Ratliff]

There is a misconception about aluminum wheels salt. Aluminum does not mean that it will furrow and rubber does not mean that it won’t.

If you take 2 wheels with the same contact patch surface area and same cross sectional profile on the same vehicle with the same weight, the tires, rubber and aluminum will make the same mark in the salt. The problem comes in that the aluminum does not deform when turning and wants to skip. The way to solve this is to have thinner aluminum wheels with "sharper" sides and this is where the problem starts. This basically is giving the aluminum wheels the grip in the same manor as ice skates. Maybe if someone showed up with a special “Salt Zamboni” everyone could run solid wheels!

I think if someone had the time and money they could come up with an aluminum wheel that’s profile allowed adequate traction and was wide enough that its footprint would not differ from a rubber tire.
I had thought something on the lines of the profile of a 170 MC tire could possibly work.
I will likely never know.


Good luck!

Whether you're running on dirt or salt with tireless wheels, you want wheels that plane on top of the surface instead of plowing through it. Plowing through the surface is how you get inconsistent and unpredictable steering response as well as no steering feel. A radiused tread surface with peripheral grooves cut into the tread has been found to work well for the front wheels.

A tireless wheel on dirt has about a .3 traction coefficient.

[Ratliff]

RE: Coefficient of Adhesion on salt

Howdy All,

I think that planning on anything greater than a coefficient of .5 is fairly wishful thinking. IF it turns out to be slightly better than .5 then you will be in good shape if you planned on more slippery coefficients.

Hard packed is somewhat better than loose packed, but both exist on the courses at the same time. The divots that exist from whatever the cause do not have as good a coefficient of adhesion as does the hard packed stuff.

Regards to All,
HB2

.5 is what Reid Railton used in his calculations.