The Biking Hub: Mountain

Observations » Guitar Ted

Wheel strength and axle width: Is it time to change the wheel over lock dimension standards?

The wheels on your bicycle have been going through some fundamental changes in the last five to ten years. These changes have pushed the limits of bike wheels, as far as strength and function are concerned, and the design compromises that manufacturers have made are forcing them to develop stronger materials to keep us all rolling happily off road. However, I think it may be time to make a change that would benefit all cyclists, a change that could become reality simply by increasing the length of our axles slightly.

Too much stuff, not enough space

The current rear wheel over lock dimension of 135mm is just too short. There are two things that have compromised the strength of mountain bike wheels. The first is the fact that most cassettes are being built with more cogs than they were when 135mm hubs became standard industry fare. At that time, we had six cogs on our cassettes whereas now we are dealing with nine cogs, all crammed into the same amount of space.

To make 135mm hubs work with changes in the numbers of gears on modern bikes, hub designers had to make freehub bodies that were longer, shrinking the distance between the hub flanges. The addition of disc brakes into the game forced designers to push the flanges even closer together so their designs could accommodate rotor mounts and provide adequate clearance for the brakes’ calipers.

Longer axles increase wheel strength

Wheel strength has been compromised by narrow hub flange spacing. Think of the distance between the flanges of a hub as the base of a triangle. The other two “sides” of the triangle are the spokes which meet at the rim. A narrower “base” makes the triangle less stable laterally. To compensate for this, more spoke tension is required. This is especially true for the drive side of the hub, where the spokes have almost no “lean” outwards to the drive side hub flange.

Even front disc hubs have uneven angles that the spokes make from the rim to the hub flanges. Now instead of looking at a wide based triangle with equal length sides, we now have narrow based triangles with unequal length sides. This creates stresses at the rim and on the spokes that are far greater than anything wheel designers have had to deal with before. Materials advancements have helped to keep up with the strength issues, but there is a better way.

Looking forward: Finding the numbers for a solution?

If the axle length could be lengthened, the “triangle” formed between the hub flanges and spokes could be changed to be more symmetrical with a wider base. It would also increase wheel strength exponentially. The added strength could be taken advantage of in many ways performance wise, including making current rims lighter, something that would make for better on trail performance. Strength would be as good as or better than current rim designs due to the change in over lock dimension.

The front wheel over lock dimension of 100mm could also stand to be widened a bit to accommodate disc brakes. The rotor and caliper have forced hub designers to make compromises to the front wheel that are similar compromises to those of rear wheels. The increased stresses that disc brakes cause on wheel components adds even more problems to deal with. This is usually dealt with by using heavier components in the wheels. Wider hub flange spacing would alleviate this problem.

Crossing over: moving to a new standard

The biggest obstacle facing advocates of a new standard is that the changes would require component manufacturers retool existing machinery and the adjustments many frame companies would need to make to their designs.

In my opinion, this is what has kept the over lock dimensions intact for as long as they have been. However, the compromises incurred by keeping the axle lengths the same have reached their limits. Something has to give before more progress can be made, and I think it might be time to consider changing the standards now.

This entry was posted on Tuesday, June 20th, 2006 at 12:20 am and is filed under Observations. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.