Best material for sand tires?

How to design, build, and race a sculpture
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DrDiabolical
Posts: 10
Joined: 27 Dec 2011, 1:01 pm

Best material for sand tires?

Post by DrDiabolical » 02 Mar 2012, 12:28 pm

OK,

So I've got 5' diameter wheels on my new kraft. And I've wrapped them with 2.10" wide mountain bike tires with an aggressive tread. That seems to work great on pavement and dirt. But what about sand? From experience, I know that a kraft with similar wheels weighing A LOT less than mine bogged down in our soft sand last year and was nearly impossible to push out.

I have seen Flatmo, Funguy, and a lot of other CA racers wrap their wheels with some sort of thick white material for the sand course. My question: what are they using? Anyone else have something that works well and can be attached and removed from a tall narrow wheel in just a few minutes? Unfortunately our race course is very short (2 hours and its over) and all transitions are done on the clock. I only need to cross a few hundred feet of sand. But I want to absolutely dominate my competition without resorting to brute force or just carrying the kraft over. That's what they do and I'm looking for a more "kinetic" solution.

Thanx,
Dr. Diabolical
evil genius

Elliot
Posts: 49
Joined: 10 Jan 2011, 2:31 pm

Re: Best material for sand tires?

Post by Elliot » 17 Mar 2012, 4:15 pm

DrDiabolical wrote:...some sort of thick white material...
Ethafoam. ("Effin foam." :shock: ) Probably polyethylene foam, but I'm no chemist.
I'm told it is used for packaging. The above mentioned guys in Arcata get it from Wing Inflatables, which is a manufacturer of inflatable boats and pontoons, located right there in Arcata and themselves active in KSR. But I don't know if the foam the guys use is left-over packing material or actual product material.

Ethafoam deteriorates from outdoor storage, but is otherwise close to indestructible. Easy to carve with a knife and does not crumble.

If you have difficulty identifying the stuff locally, I can mail you a sample.

DrDiabolical
Posts: 10
Joined: 27 Dec 2011, 1:01 pm

Re: Best material for sand tires?

Post by DrDiabolical » 02 Apr 2012, 11:33 pm

Thanx. I'll start looking for a local source. I worked for years in shipping, so I think I have an idea of what the stuff is.

BTW: I had the first "pedal test" of my new machine on 3/30. In low gear, which is less than 12 gear-inches, I can move the machine on asphalt by cranking the pedals with one hand. There's something you may want add to your tutorial. And that is "put a bearing right next to the sprocket" when driving anything by chain. When geared down, chains can transfer tension loads in the thousands of pounds. They can and will bend even a 1" solid axle like it was a pipe cleaner.

At this point, I've got California-quality torque output from my "compact transmission" assembly. The problem is how to manage that torque without turning parts into pretzels. Are you familiar with FUNGUY's freewheel jackshafts made from rear bike hubs? I know how to build them. And also how to twist them into spirals.

Elliot
Posts: 49
Joined: 10 Jan 2011, 2:31 pm

Re: Best material for sand tires?

Post by Elliot » 03 Apr 2012, 11:46 pm

Oh yes. I've torn #40 chain in half.

But 12 Gear-Inches isn't all that low. I've used somewhere around 7 -- and we detonated the rear differential on the Bluesmobile in the Port Townsend Dismal Bog. Allen Brown in Corvallis had 1 G-I on his famous Rex. They were spinning as fast as legs can move, and I had to really stare at the tire to detect that it was moving. Unstoppable, with adequate traction.

That ethafoam has a smooth glossy surface that feels almost oily-- at least the pieces I have. But store it out of sunlight -- it will turn to powder eventually.

Elliot
Posts: 49
Joined: 10 Jan 2011, 2:31 pm

Re: Best material for sand tires?

Post by Elliot » 04 Apr 2012, 2:10 pm

Hmmm.... My notes show I had a low gear of 5,7 G-I on the Chopper at the 2009 Grand Championship -- and it was not low enough.

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