Monday, June 18, 2007

How To - Why Wheels Go Out of True and How to Prevent That from Happening

Is your wheel out of true?

Should be a simple fix, right? Just take it to the shop, they turn a few spokes, and presto, you have a straight wheel again.

That might be the case, but you might be getting back a wheel weaker than the one you dropped off. It really depends on exactly what caused the wheel to go out of true.

Spoked wheels (the only kind of wheel I'll refer to in this article) are inherently very weak laterally (left-right) and strong vertically (up-down). The wheel is an incredible complex system capable of supporting something like 300 times its own weight vertically. Laterally though, it's a lot less impressive. Loaded laterally at just 40 or 50 times its own weight the wheel will start to fail.

Spokes are actually long threaded rods - the spoke nipple is a funny shaped nut. You tighten the nut (spoke nipple) to pull the rim towards that spoke (really the spoke base - where it goes into the hub). You loosen it to let the rim move away a bit. The wheel is strong because of the tension in the spokes.

The best way to have a wheel go out of true is to have a spoke lose some of its tension (i.e. the spoke nipple loosened up). It stops pulling the rim into place, letting the rim drift away to the other side. For example, if a right side spoke loosens up, the rim will wobble to the left just a bit. If you re-tension the spoke, the wheel should be true again.

Unfortunately that's rarely the case.

One warning sign is the "Loose Spoke" on an otherwise straight-ish wheel. A loose spoke shouts out that the rim is bent. A single loose spoke indicates a lateral bend (it could be more than one spoke but they'd be on the same side of the wheel with one opposing spoke between them). A pair of loose spokes right next to each other indicates a vertical bend ("flat spot"). More than two loose spokes could indicate a combination of those two symptoms.

If you hit something hard enough with your wheel, you can bend the rim. This means you actually alter the permanent shape of the rim from one of a flat circle to a not-so-flat circle (lateral bend), a flat but no-longer-circular hoop (vertical bend, i.e. "flat spot"), or a not-so-flat no-longer-circular (i.e. your rim is toast).

A flat spotted rim (has a flat spot - like a flat tire has a flat spot) can be bent outwards using a special tool that resembles an 18th century surgical tool, both in appearance and precision. It's very rare that a flat spotted rim gets bent back round but it's theoretically possible. It's usually quicker to lace over a new rim. Flat spotted rims are hard to ride - the brakes grab at that flat part, the brake pads may rub the tire there too, the wheel thumps, and the bike bounces up and down as you roll down the street. Not a pleasant thing - something you endure until you can get it fixed.

A laterally bent rim can be forced into true by violating all the rules of spoke tension. The problem with forcing the rim into place is that the rim will immediately try and return to its natural (bent) state.

There are three options for a laterally bent rim:
1. Let the rim stay in its natural bent state if it's not too bad (say 5mm or less out of true). I've ridden/raced on wheels like this, sometimes for more than a year or two on particular wheels.

2. Replace the rim. when doing so, replace all the spokes. Spokes are like rubber bands, they lose their elasticity after a while, and if you're paying for a rim and a build, get new spokes too. It is false economy to bypass this step.

3. "Bend" the rim back by using equal and opposite force. This typically involves completely loosening the spokes in the area (after marking where the single loose spoke is located), hitting the rim pretty hard on a flat, solid surface (bench, ground, rock, etc) so that you hit the
rim away from the loose spoke. If you are lucky, the rim will be perfect. If not you're buying a new rim. On a wheel you're about to trash, this is a nice, no-loss situation. I've salvaged maybe 3 really nice race wheels like this (and didn't salvage maybe 30 or 40). This only works for
aluminum rims. Please don't come crying when you bash your carbon wheels against a handy-dandy curb trying to "true" them.

To be honest though I think we all agree that it's better if your wheels don't get bent in the first place. While riding on even on the roughest streets, proper riding technique will preserve even the flimsiest of wheels/tires. You pay the price only if you mess up. I've trained on city streets (NYC, Chicago, more) on light wheels shod with light tires with no problems.

There are some basic techniques to use:

1. Bunny hops, i.e. when you "jump" the bike. Even if you don't gain altitude, at least you don't drop into the potholes. Smooth bunny hops put virtually no impact on your wheels when you land. Most of the shock is absorbed by you, the rider.

2. No extreme out of saddle action when going over bumps. In other words, no radical sprinting side to side while going over those really big potholes - you really raise the chance that you'll knock your wheel laterally out of true.

3. In normal road traffic, go around holes by asserting yourself in your lane. In a peloton of riders you have to resort to the bunny hop as swerving around potholes while in a group is the number one way to announce to everyone that you have no clue how to ride a bike. I say that in bold italics because even as a Cat 3 I see other Cat 3 racers do this all the time. It's really frustrating because those guys cause crashes and near crashes, all for no real reason at all. They simply don't know how to ride a bike properly. Anyway you have to be aware of where those potholes are (i.e. eyes way up the road) and plan accordingly.

4. Smooth pedaling reduces the stresses you put into your bike - it typically means doing lower rpms in bigger gears. This stabilizes your body so it puts less movement into the bike. It also lets you loosely hold the bars so that you allow the bike to bounce around a bit. This is recommended for dirt roads etc.

5. If you get forced to ride over bumps, relax your grip on your bars, let the bike do what it needs to do, and don't brake unless you have to. I ride on dirt roads with the occasional rock the size of a baseball but rarely flat. It might be dumb luck but the riders I am with flat more often than I do and I haven't won the lottery yet. At the Nutmeg Classic I ended up riding on semi-soft sand at 38 mph. Other than distracting me from the sprint, nothing happened.

These techniques have helped me preserve my wheels. Hopefully you can use them to help preserve yours.

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