Solid Rubber Wheels - rating?

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gidge348
Posts: 129
Joined: Sat Mar 07, 2009 12:50 am

Re: spin, spin, spin!

Post by gidge348 » Wed Aug 11, 2010 8:09 pm

David Leikvold wrote:Ian, your comments about the forces involved at 135mph got me thinking. A road racing kart like the 125cc that turned up at Speedweek 2009 and ran 122mph or thereabouts runs a 5" diameter rim with a tyre that is about 9.5" diameter. The 250cc International two cylinder karts have double the horsepower of the 125cc and run a lot faster and would easily exceed 140mph at places like Phillip Island or Eastern Creek. They can do that all day with a rubber tyre that, brand new, weighs only 1.28kg on a rim that weighs 612g with bearings and valve stem (I have a 5kg electronic scale for weighing coins if anyone was wondering). So the forces acting on that tyre would be significantly higher because of the much smaller overall diameter but are obviously manageable. Without chopping up a tyre to look it seems to me that the bead wires couldn't be much thicker than clothes line wire (but obviously better quality!) and the thinnest part of the sidewall is only about 4mm thick and probably only 2 ply. But there's no denying they do the job.

Unfortunately they don't have a marked speed rating so they probably wouldn't get through scrutineering on anything but a kart. Despite that, Jack Costella uses them on his small blue Nebulous Theorem 5 lakester chasing some tiny bike engine records. I'm NOT proposing a rule change here but I think if someone was building a really small lakester or streamliner that was unlikely to ever exceed 150mph then I think road race kart tyres could be useful. Having said that, it's probably easier and less controversial to use a solid aluminium front wheel/tyre of the type Jack runs on Neb 2, 3 and 7.

Cheers
Dave :D

Sorry this is no use to you Paul.
:

Hello David,
I probably oversimplified the position in regards to the forces by saying that the force would be 7 tons at 200 mph.
This is correct, but this force is distributed over the whole tyre assembly. What we are really interested in is a small part of the tyre that may let go and the local centrifugal forces on that part ONLY. That is, if you took a 1 square centimetre of the tread that weighed say 40 grams and ran at 200 mph there would be a local force on that small piece of tread of 1cm would be in the region of 12kg trying to tear it off the carcass of the tyre. So if your tyres will handle that, all well and good, if not .....well things could get untidy.

With regards your comment about kart tyres that..... “The forces acting on that tyre would be significantly higher because of the much smaller overall diameter” this is not strictly correct, the local forces on the tyre are identical regardless of diameter; the determining factors are mass of the part, and the ground speed it is running at. Take as an example 1 brick on a 1 meter string, spin it around at a given RPM and you have a given force. Use a 2 meter string and spin at the SAME RPM and the force will be doubled. Use a 1 meter string and 2 bricks and the force doubles again. The actual forces on the tread of the tyre (per gram of tyre mass) be it a go kart or a tractor tyre “travelling at the same road speed” will be identical. One may be further from the centre but travelling at a lower rpm, these balance each other out.

Just to throw something else in mix, the main problem with solid tyres is that they do not deform like a pneumatic tyres so the contact patch is next to zero and steel has a higher kinematic viscosity than rubber (ie not as sticky), so breaks free very easily in turning and traction. LSR tyres usually run at high PSI so have small contact patches relative to their size anyway so solid wheels are a possible on salt lakes.(But I wouldn’t like to drive on solid wheels around a race circuit though.)

Polyurethane or rubber bonded to metal wheels do not adhere well enough, so even at modest speeds they will “throw off” of the base metal.

How about a wheel and tyre in reverse.... think of a flat metal tread with holes in the circumference. On the INSIDE of the rim is vulcanised either polyurethane or rubber with some type of reinforcing belts in it. As speed increases the Urethane/Rubber will be forced out of the holes by centrifugal force, then the weight of the vehicle would push up against the projections to give you the tread deformation needed for traction and steering. They would probably act like solid wheels at slow speeds but normal tyres at high speeds and centrifugal force will be acting to push the tread ONTO the rim not tear it off.

All the rubber and urethane compounds have known amount of deformation and destruction limits given the forces applied. These could easily be calculated, given the speed, mass, size of holes, amount of tyre deformation required etc.


Let’s hear ideas to shoot this down..... :) :) :) :)
Last edited by 2 on gidge348, edited 0 times in total.

momec
Posts: 667
Joined: Sun Sep 11, 2005 8:15 pm

Post by momec » Wed Aug 11, 2010 9:49 pm

My only idea is the rule book says must be pneumatic tyre.
Chris
Acme Racing #251

teamleader
Posts: 31
Joined: Thu Jan 31, 2008 9:17 am
Location: Basso

Post by teamleader » Thu Aug 12, 2010 9:12 am

Chris both the DLRA and 2010 SCTA rule books say that approval for any non pneumatic wheel/tyre shall be sought in writing from the contest board.

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