Two Wheels Bad (To the Bone Edition)

"ptak appreciates old racecars" (racecarptak) 03/30/2015 at 12:52 • Filed to: two wheels real bad

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In reference to my bike exhaust post of earlier.

Pic not mine.

Context:

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I'm seventeen (sadly not a dancing queen) and I've been working on a boardtrack racer-type bike... and before you assume I have no business being near a motorcycle at all, it's more of a moped with HP in the single digits. My age no excuse for not knowing basic things, but I value my presence on Oppo (even its bike community) and I think I should try to save my dignity.

So I'd like to make it very clear that although I get funny ideas sometimes, I'd never try any of this shit without asking the experts first. Which is what I did, and I appreciate the gentlemen and lady delivering the info in a way that might get through my head.

Peace ()

So uh would specialized heat wrap work or...?

CalzoneGolem > ptak appreciates old racecars 03/30/2015 at 12:54

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RamblinRover Luxury-Yacht > ptak appreciates old racecars 03/30/2015 at 12:55

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Specialized heat wrap would change it from being torch hot to merely oven hot. It depends on how well-done you want your thighs.

ptak appreciates old racecars > RamblinRover Luxury-Yacht 03/30/2015 at 13:07

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OK, good to know. Thanks.

ptak appreciates old racecars > CalzoneGolem 03/30/2015 at 13:07

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:D

whiskeybusiness NOW A DANGER TO CROWDS NEAR YOU > ptak appreciates old racecars 03/30/2015 at 13:08

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No need to apologize, everyone is here to teach what they know and learn what they don't. I can't speak for every individual but I can almost assure you the feedback you got was with the best intentions to help you learn what can and can't be done safely. With the exception of mine, that was just for laughs :P

BoxerFanatic, troublesome iconoclast. > ptak appreciates old racecars 03/30/2015 at 13:09

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I applaud your interest in motorcycles. But please, please, please, please, please consider what you are planning to do before you do it, and read as much as you can get your hands/internet connection on.

http://www.pipeburn.com/home/2015/03/2…

People are doing what you are considering, and working with moped style bikes is becoming much more popular than it used to be.

That yellow board-tracker above is a great looking bike... with a J.A.P. (John Alfred Prestwich, J.A.Prestwich Ltd., an OLD english engineering manufacturer) style V-twin in a pre-WWII style frame, and large diameter wheels, it is a real old-school looker.

However, note how the exhaust, even being short, is away from almost everything else. Every bike that has a high or under-seat exhaust, which do look VERY cool, are engineered for it specifically, either by the manufacturer, or by an extensive custom build.

Some builders have solved the problems you are talking about, but doing it wrong can put yourself, as well as other road users at risk. A burn to your legs at an unexpected time can surprise you while you are trying to control the bike. A melted fuel line can release fuel and fumes next to a hot exhaust, and start a fire. A melted battery can release acid, or start a very severe lithium-fueled fire, depending on battery type. A melted wiring harness can short out and cause malfunctions, or another source of fire.

Heat-resistant pipe wrap is one option, but it is rated for a range of heat reduction, which may not be enough to lower the temperature enough. Double-layering with a heat-reflective metal heat shield, or even heat insulation on the inside of a heat shield, can be helpful. Other builds use ceramic coatings inside and out, but usually that is more expensive, and suited for new pipework.

And there isn't really anything inherently wrong with a properly done low-mount exhaust, either.

ptak appreciates old racecars > whiskeybusiness NOW A DANGER TO CROWDS NEAR YOU 03/30/2015 at 13:10

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I know, that was just one of those posts that seemed like a lot better idea before I posted it. I thought that was funny by the way :P

bob and john > ptak appreciates old racecars 03/30/2015 at 13:11

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everyones got to learn somehow. Rather learn by getting yelled at then by scarificing the skin on your legs or anything more important.

to answer your question: no. it wouldnt. you need to run the exhuast further away from your legs.

as an example: All of the buell guys complain of the headers warming their right leg when they ride. and look how far it is from the riders legs

whiskeybusiness NOW A DANGER TO CROWDS NEAR YOU > ptak appreciates old racecars 03/30/2015 at 13:15

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Everyone has those, at least you're thinking outside the box!

ptak appreciates old racecars > whiskeybusiness NOW A DANGER TO CROWDS NEAR YOU 03/30/2015 at 13:24

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That's one way to put it :P

ptak appreciates old racecars > BoxerFanatic, troublesome iconoclast. 03/30/2015 at 13:44

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Thanks for taking the time to type this up, I appreciate the advice. I think I'm going with the low-mount exhaust, that was my second idea so no sacrifices in aesthetics there.

I'll take advantage of your suggestion and ask what the effects are of shortening a bike's wheelbase? I've heard that 54 inches is a desirable length, and in my current plans it's hovering around 51.

ptak appreciates old racecars > bob and john 03/30/2015 at 13:50

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It's a good way to learn... not very efficient for most people though.

Wow, I wouldn't expect that to have much effect at all. (the Buell exhaust) Good to know.

bob and john > ptak appreciates old racecars 03/30/2015 at 13:52

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bike exhausts get HOT. The buells had it a little worse as it was an air-cooled motor as well

BoxerFanatic, troublesome iconoclast. > ptak appreciates old racecars 03/30/2015 at 14:09

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Bikes with shorter wheelbases tend to be sharper turning, but also less inherently stable than longer wheelbase bikes, all else being equal. better for race bikes, but tends to make them a bit more of a handful, also. longer-duration road bikes tend to have more stable, longer wheelbases, but slower turning rates, and less "sharpness".

There is a lot of other suspension context that comes into play, in addition to just wheelbase, such as rake and trail on the front suspension, and swingarm length requirements on the rear suspension.

Shaft drive has some torque-jacking effects on rear suspension design, where chain drive has some chain tension (or lack of it) effects depending on how the drive sprocket relates to the swingarm pivot, since the rear sprocket is centered on the axle at the end of the swing arm.

If the drive sprocket it concentric with the swingarm pivot, and the wheel sprocket being concentric with the wheel axle, chain tension would not change. (not many are, but the dual-chain on that yellow board tracker might be)

Usually the drive sprocket is in front of the swingarm pivot, so as the swingarm moves through it's arc, the distance between the top of the drive sprocket, and the top of the wheel sprocket change with the suspension's geometry, taking up more or less of the chain's length, adjusting it's tension.

Swingarm length can also be determined by the placement of the shock or shocks. A mono shock suspension with the shock placed between the swingarm pivot and the rear tire can take up space, and require the swingarm be longer than having a higher mount shock, or a dual side-mount shock, or other suspension layouts. Sometimes it is not a bad thing, as too short a wheelbase can result in too skittish of a bike. A short rear swingarm also requires a differently tuned shock, as the leverage is different than a longer arm. drag bikes and higher power bikes usually prefer longer swing arms, and heavy rear springs to apply the most leverage to the rear tire, and minimize the wheelie effect.

A hard-tail bike is another equation... and basically you have to view the whole bike, and the rider's weight somewhat as a lever on the front suspension... and some of that lack of flexibility and damping being absorbed by the rider's body, or seat springs if the seat is sprung.

Usually on a race bike that is designed as tight as possible, the wheelbase length is determined by the minimum safe clearance between the front suspension's design and clearance of the front tire from the frame, radiator, exhaust, and engine; and the rear suspension as mentioned, with the swingarm pivot having to fit behind the transmission's casing, accommodate the lower end of the frame, and possibly accommodate the rear shock, as well as the rear tire's rolling diameter. Having the engine and transmission between the front tire and the rear swingarm pivot makes up much of that dimensional consideration.

On road bikes, more consideration is given to rider ergonomics and placement of the foot pegs in relation to the seat, power level and application to the rear tire, suspension arrangement, and just generally being a bit more of a generous size can often contribute to a longer wheelbase, and an easier, more stable road bike.

BTW... if you are interested in motorcycle chassis and suspension engineering... a couple of interesting names to look up:

Tony Foale, and Norman Hossack. That goes WAY beyond my level of knowledge, all the way to master-class... but some of the generalized summaries of their theories may be a good place to start.

Other sites, such as http://auto.howstuffworks.com/ is another good place to get your bearings, and find concepts to further research.

BobintheMtns > bob and john 03/30/2015 at 15:17

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And that problem is not just on buells.... Many ducati riders have issues with the heat from the back cylinder only being inches from their "tender appendage." The seats tend to get a little warm after a bit of riding...... But as mentioned above, a few heat shields go a long ways. And I've got ceramic coated exhaust pipes which not only help me, but helps the engine bay stay way cooler....