The Biking Hub: Mountain

MTB 101 - Gear » Guitar Ted

Demystifying the 29″er: Geometry class

Editor’s note: This entry is the third in Guitar Ted’s introductory series on the 29 inch wheeled bicycle. The first entry is a brief introduction to the twenty niner, and the second discusses some of the main benefits that a 29″ wheel bestows on riders.

Using a 29 inch wheel in a trail bicycle application is a little more complicated than it might seem at first glance. There is much more to it than taking an existing frame and simply increasing the length of the frame’s chainstays and the fork’s legs! Whenever you change one aspect of geometry on a bicycle, it affects the geometry of the whole bicycle and it is no different when you change wheel sizes.

This writeup looks at the ways 29″er designers address several things in their designs that help replicate the handling of a 26 inch trail bike.

Making a 29”er handle correctly

Trail. One of the most important things was the geometry of the front end. When adding a larger wheel size, the original 29”er pioneers realized that the trail of the fork was affected negatively. Trail is the description of the relationship of the tire contact patch to axis of your steer tube. Ideally the axis of your steer tube, where it intersects the ground, would be in front of the tire contact patch with the ground. This gives the bicycle stability in the front end.

Think about a shopping cart wheel as an example. When a 29 inch wheel is accompanied by a standard 71 degree head tube angle, the trail figure is too large to keep the front end steering in a quick manner. This can be remedied in two ways. The fork offset can be changed, or the head tube angle can be increased. Of course, a combination of both could also be employed. With the fork offset of most suspension forks set at approximately 38mm, 29”er designers choose to increase the head angle. This shortens the trail figure to give the 29”er a quicker handling front end.

Center of gravity. Another effect of the larger wheel size is the elevation of the axles in relationship to the bottom bracket. The axles of 29″ wheels are higher than 26″ wheels making the bottom bracket drop measurement a larger figure for 29”ers than for 26″ bicycles. This has the effect of lowering the center of gravity of the rider in relationship to the wheel axles, yielding a bicycle that is tougher to endo and more stable in corners.

Wheelbase. The wheelbase of a 29”er would seem to have to be longer than a 26 inch trail bike, but designers have addressed this concern as well. That’s why you might notice many 29 inch designs with severely sloping top tubes. This is done in an effort to tighten up the wheelbase and allow enough exposed seat post to get proper stand over and comfort. Some designers have even employed bent top tube designs to accommodate riders even further.

If a frame designer takes these specific 29 inch related issues into consideration, a great handling trail bicycle will result. In my next installment, I will cover a few riding tips that will open up some of the specific 29 inch wheel benefits that you can use to get the most out of your ride.

This entry was posted on Wednesday, March 22nd, 2006 at 12:01 am and is filed under MTB 101 - Gear. 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.