Wednesday, December 06, 2006

MTB's and the Land Speed Record

yep I made it out again. Same layers, shorter ride, and no drafting school buses this time.

I was making some good speed at the beginning of the ride but started to falter as the route I picked goes uphill till my turnaround point. Then on the way back the bike was undergeared and I didn't feel like spinning like a madman so I didn't go that fast - in fact, my time for going out was the same as coming back in.

The position on the MTB is one that optimizes control over the bike. It sacrifices aerodynamics but retains the hunched over stance necessary to recruit the glutes (your butt). The aero sacrifice is never as obvious as when you're trying descend quickly on an MTB.

It's something else too.

It's the same position that John Howard used in setting the current Land Speed Record back in the 80's - 152 mph. He was drafting a modified 337 mph LSR car so aerodynamics was not a concern. His concern was controlling the bike at 150 mph.

And as a rider more able to do short efforts, say, under a couple minutes long, the LSR would be an interesting proposition for moi. When JH broke the record, the previous speed, 138 mph, was held for a while. And it's been 20 years since JH broke the record and no one has even attempted it. It takes a while for a challenger to step forward and say "I'm going to break the LSR".


I'm not sure of the physics of doing a sustained sprint behind the equivalent of a 200 mph semi, but I know from personal experience that drafting trucks is limiting by gearing and how far you want to go. (And by cops who pull you over, but that's a different story.)

From a gearing point of view, the only way to break the record would be to have a "reduction" gearing system. This means the primary cranks (which the rider pedals) would be turning a gear which in turn would turn another crank. The gearing is increased exponentially.

To calculate relative gear ratios, take your standard 53x12. 53/12 x wheel diameter (27" for simplicity) equals 119". At 100 rpms, such a gear would go about 35 mph. Fast, but not that impressive. But, if that 12T was in turn driving another 53T chainring, and that was turning a 12T, you'd have ((53/12)*53/12)*27" or 526". This gear would travel 137 feet for every revolution, or, at 100 revolutions, go 156 mph.

That is fast.

And it is what JH did, albeit with a slightly smaller gear and smaller tires. Such a setup, with two sets of cranks, is called a "double reduction gear".

Based on calculations with Z rated (149+ mph) motorcycle tires which are much smaller than a bicycle tire, a triple reduction gear would work only if using 59T chainrings and 11 or 12T cogs.

Since 11's are not optimal (they aren't round), I started thinking of quad and whatever five would be. Quad reduction gearing would allow for a mix of 56T and 59T chainrings. The 56T chainrings would allow the different "bottom bracket" axles to be situated closer together. The caveat - it would work only if using 11T cogs. 12T cogs would limit the top speed to 131 mph or so (160 rpm).

The Fiver would allow use of a standard 55T chainring with 14T cogs. The initial drive would be a "normal" setup with a crankset driving a multiple gear cassette. Using a 12-19T setup on the Fiver, top speed would be an optimistic 225mph at 160 rpm or a more reasonable 197 mph at 140 rpm.

To put the goal out of reach of most people (and give the record another 20 years to rest), the record would have to be pushed up to 200 mph. Otherwise everyone will come out with a faired turbo Supra and go 160 or 161 or whatever it takes to beat the "new" record.

My thought is that once up to speed, rolling resistance will be the primary limiting factor. Aerodynamics should be negligible or even negative, if a properly designed draft fairing is built. A taller tire would be preferable as would be one that is narrower than the four inches I managed to find. But finding tires rated over 150 mph which aren't 300mm wide isn't that easy.

The parts I don't understand are related to going 200mph, since I've never gone that fast in a car. Like effect of tire diameter on balance, frame geometry needs, control issues (steering damper needed?), and what sort of vehicle (and power) would be required to go 200 mph while hindered by a drafting box. And who has one that would be willing to put a box on it.

The rest of it would be pretty straight forward, like the bike itself. High performance motorcycle wheels, disc brakes for the same, a possible fairing in case I drop out of the draft, triple clamp forks, and a very stiff aluminum or carbon frame set. A lot of body armor for the rider, a full face helmet, some communication gear, and a whole lotta speed.

Anyone has a CAD program for simulating air flow around an object?

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