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  Last updated: 9/21/2018

Tires and Tubes

Tire Size and Optimum Pressure

Maximizing both speed and comfort on a ride is a function of three inter related variables - tire size, sidewall construction, and inflation pressure.

Traditional teaching was that a 23 mm tire rolled faster than a narrower 18 mm version with a wider tire contacting the road in a curved "D" shape compared to a flat "U" shape of a narrower one. And as the size of the contact area with the road was dependent on inflation pressure (running the same pressure in a fat or skinny tire provided the same contact areas), it was the shape of the contact area that made the difference with a D shape (23 mm) providing a performance advantage.

Maximizing the inflation pressure was thought to gave a smoother ride with less rolling resistance, and a greater resistance to impact, pinch-type, punctures.

Cornering differences were thought to depend on the rubber compound in the tread rather than the width of the tire. But rubber compound being equal, wider tires had a better grip for cornering as the D shape provided a greater contact surface during banking on tight alpine bends.

The net result was the consistent recommendation that performance was maximized with a 23 mm tire, kept at maximum (for that tire) inflation pressure.

It was not unusual to see chat room discussions such as this:

Q. Why does it feel harder on the turbo trainer with a narrow tire than a wide one?

A. Might I suggest it is simple mechanics. The air pressure in a narrow tyre must be greater to resist deformation at the area in contact. A wider tyre requires less pressure. Narrow tyres must wrap themselves around the roller of the drive wheel until equilibrium has been achieved, using F=PxA, where F is Force, P is pressure and A is area.. A wider bag will give more area therefore less deformation. Go for a 23 mm bag and it will give a better ride both on the road and on the roller than a 18 mm or 20 mm bag. Of course if you can get a narrow tyre at 200+psi the equation balances OK as well, but that requires a big effort to do that. Settle for 120-130psi.

This article summarizes current data on bicycle tire dynamics and refutes the previous assumption that higher tire pressures decrease a tires' rolling resistance. (Additional background can be found here.

From dynamic testing we know:

How do you pull this all together? To quote: "On smooth roads, 25s are about as fast you get - our research indicates that 28s and 32s aren't slower, but neither are they any faster (that includes air resistance at speeds of about 18 mph)....if your bike can handle wider tires, you can get more comfort and better cornering with wider tires, without losing any speed.

On the average back road, wider tires make your cycling much more enjoyable: the significant additional air volume they allow makes for a more comfortable ride, and they better handle the bumps and related vibrations, in effect smoothing out the ride. Additional good news is that when they are made right, these wider tires aren't any slower than narrower ones."

And if you are wondering how much better? "At moderately high speeds of 18-20 mph, a supple tire can make you 8-10% faster than a stiffer, but otherwise similar tire. That is far more than the difference a set of aero wheels makes (1-2%)."

For those of you interested, here is a link to Compass Bicycles, the company championing supple, low pressure tires. And whose blog got me thinking about the topic.

Tubes versus tubeless

A nice discussion of the pros and cons of tubes.

The issues are rolling resistance (it appears that tubeless - with sealant - have a higher rolling resistance) and pinch flats (wider tires are less prone to them). Plus any benefit of fewer flats with a tubeless tire may be outweighed by weight and maintenance issues. My conclusion is that a tubed wider tire (28 to 30) at a lower pressure is faster and more comfortable, and the risk of a pinch flat is within acceptable limits. But if you are riding on gravel or in the desert, then a tubeless is a reasonable option to consider.

Changing a Flat

As you put the tire around the rim, squeeze it so the beads (the open edges) go into the rim's center, which is the deepest part. With the beads in this depression, the effective diameter of the rim is reduced. Then when you get to the last several inches of unmounted tire, you can pull it up and slip it into place.

It helps to have a thin rim strip and a skinny tube that takes up minimal space inside the tire. And make sure that no part of the tube gets trapped between the beads and the rim. (This would increase the effective rim diameter and makes it even more difficult to mount the tire by hand.) Inflating the tube just enough to remove wrinkles prevents it from getting floppy and finding its way beneath the beads. But remember to deflate it before rolling the last tough section of tire onto the rim. Dusting the tube and inside of the tire with talcum powder may make a difference. The theory is that it helps the last bit of bead slide easily over the rim. (an added benefit - talc may reduce flats by preventing abrasion between the tube and tire.)

Here's a step by step:

Patching a tube

Glueless patches? They work well. The hard part is peeling the backing away, especially with cold hands. Either use sandpaper provided or wipe the punctured area with a clean rag to remove any tire talc. Remember to clean an area bigger than the patch.

Traditional patches are a bit messier because of the glue. For these, use the sandpaper to gently rough up an area slightly bigger than the patch. Put on glue sparingly in an area slightly bigger than the patch. Wait for the glue to dry. Peel off the foil and apply the patch. Removing the cellophane is optional although the directions say you should.

How many patches? That is a personal choice, more patches mean more chance there will be a slow leak. Is the next flat a leaky patch, a new hole, or all of the above? The best approach is to avoid patching a flat during a ride. To do this, carry a couple of new tubes in your seat bag and if you puncture, find the culprit in the tire, remove it and put in a fresh tube. Then you can patch the bad one when you get home (don't forget) and put it in your bag as a spare.

Questions on content or suggestions to improve this page are appreciated.

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