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Shorts From the Summer '00 Ask the Mechanic Column

By Andy Wallen

Andy hits the tire iron on the head with these two excerpts from the current Ask the Mechanic.

Bent Wheel Blues

Dear Andy,

My bike has a bent rim and I would like to try to repair it myself. Could you tell me how to straighten out the bent rim and what tools I need to do so.

Thank you,
Bart

Bart,

Bent is a relative term. Rims that are truly bent can't be satisfactorily straightened. Steel rims that are only marginally out of true in any direction are usually a lost cause. If your aluminum rim is not true, it can usually be straightened; however, there are times when even the best of rims is ready for the recycle bin, and the average Joe probably can't make this call--in other words, some people work for hours on something that cannot be repaired, because they don't know better.

At the very least, you'll need some sort ot truing jig and a spoke wrench. As for how to proceed, the principle is simple, but the act requires experience and knowledge which I really can't impart here. You basically either tighten spokes on the opposite side of the spot or spots where the rim contacts the indicator of your jig, or you loosen the spokes on the same side of that spot or spots. You must also keep in mind that excessive tightening of the same few spokes in order to achieve lateral trueness will create flat spots (out of round, or not radially true) and will probably screw up the rim centering (wheel dish), so you must constantly be aware of all 3 components of wheel truing, even if your objective only involves one.

Most folks should just take it to a shop. There, usually for no charge, they can tell you to scrap it or they can fix it for about $10-$12.

Andy

Tire Sealants R Us

Andy,

I am doing a report on bike tire sealants. I would be glad if you were to write me on your opinion on this. Thank you for your help.

Jazzz

Jazzz,

I'm not sure what specific information you require, but in general, Slime seems to be the product of choice, and for the most part, it works. We use a similar product (MK 30, with kevlar) with overall good results. Some time ago, there were some environmental concerns with these products, and I'm not sure whether the products were re-formulated, or that the controversy just died down.

The advantages are that sealants coagulate when there is a small puncture, so quickly that air loss is minimal, usually not noticed. They work on the most common punctures, usually up to 1/8 " in size. Flats are usually caused by thorns, tacks, pieces of wire, or small shards of glass, which are exactly what sealants seal. I know people who have ridden in desert areas where a large thorn called a "goat head" literally destroys untreated inner tubes, but don't flat Slime filled tubes.

The biggest disadvantages of these products are added rotational weight and difficult installation. I have put Slime into a presta tube, and it is not recommended. If you have presta valves, it is best to buy the pre-treated tube. We've had difficulty getting it into certain brands of shrader tubes as well. Also, this stuff does not work after the fact; it must be installed before the tube gets punctured. There are products that claim to seal an existing puncture, and I have yet to see satisfactory evidence that they actually work.

Andy

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