Friday, October 13, 2006

Front End Geometry for 29"ers: Two Different Paths

Lately throughout the testing and research on 29"er rigid front forks I have noticed that the solution to the fork trail issue is being handled in two different ways. First of all, it's important to know just what the "problem" is and why there needs to be a solution in the first place.

When mountain bike front end geometry got "figured out" in the late 80's/ early 90's a certain set of geometry figures were arrived upon that were generally accepted to be the best balance between stability and quickness for a 26 inch wheeled off road bike. Dubbed "Norba Geometry" by the magazine hacks of the day, this Norba, racer influenced geometry soon became the "standard" by which all other XC mountainbikes were judged by. Important to our discussion today are the offset and head angle numbers generated by this trend which were 38mm and 71 degrees respectively.

When 29"ers were being developed throughout the 90's, it became apparent that the "standard" Norba geometry wasn't effective in the big wheeled size. Using a 71 degree head angle with a 38mm off set was slower feeling than with the 26 inch wheels. This was due to the fact that by having the front axle higher off the ground with the 29"er, you automatically increased the trail of the fork. This made the bikes feel slower to steer around tight trails and in slower technical situations. Something had to be done to get back that quick 26 inch wheeled handling in a big wheeled package.

This was done by going to a steeper head angle, which effectively reduced the fork trail figure. Problem was that if you got on the smaller end of the size range in terms of fitting a frame to a person, there were serious toe overlap problems. So, the head angle was seen to be limited to about 72 degrees to keep toe overlap in check. These early geometry problems were part of the reason that the idea of "29"ers are only for big people" got started. It was due to the difficulty that early designers had in getting a quick handling package that could be executed in a smaller sized bike with 29 inch wheels. Remember, the rigid fork offset was thought to be "sacred" because of the suspension fork manufacturers who had all settled on a 38mm offset for the fork crowns. ( More or less- some forks varied a millimeter or two from 38mm) Changing tooling to accomodate a super small niche market was quite out of the question.

So, another approach was developed in which the rigid forks offset was changed and a 72 degree head angle was kept. The increase in fork offset reduced trail to a point where a designer could attain 26"er like quickness and still be able to produce a bike in smaller sizes without fear of toe overlap. Some designers have gone from the 38mm offset to as much as 47mm of offset in an effort to increase the quickness of the handling on 29"ers.

This brings us to today, when 29"ers are poised to become part of mainstream mountain biking. Now, with the influence of Gary Fisher, even 26 inch mountain bike suspension fork crown off set is being changed. When I spoke with Gary Fisher a couple of weeks ago, I asked him if he thought the suspension fork crown offset would be increased for 29"er suspension forks in the future. He assured me it was going to happen. So, designers will have two choices soon for resolving the handling issues with29"ers with suspension forks.

This is still a very hot topic for debate amongst designers of 29"ers. Jeff Steber of Intense Cycles said that he believed the best way to achieve the 26 inch bike handling he loves in a 29"er was to increase the head angle to an unprecedented 73 degrees. His company produced several proto type 29"ers using different head angles with 38mm offset crowned Rock Shox Reba suspension forks. These protos were thrashed by several test riders all over the west coast and the 73 degree head angle was universally liked better. Intense isn't the only company jumping on the steeper head angle bandwagon. Ellsworth and several other smaller builders have gone with a similar approach.

So, where does all of this lead me in my fork testing? Well, depending on which fork offset your carbon rigid fork happens to have, you could end up being disappointed in your bikes handling. Then again, you might be delighted. It all depends upon what camp your frame designer believes in. If the frame has a steeper head angle than 72 degrees, your 38mm offset crown might work better for you. If you have the 72 degree or less head angle, you might want to seek out the 47mm fork offset to get quicker handling. Then agian, you could always go the other way if lots of stability is your cup of tea. It all depends on the trail figure your after and how that affects handling. For my testing purposes, it almost dictates that I have a "proper" frame for every fork, which gets to be a pain!

So much for being a low budget researcher!

Check out my first impressions of the On One Carbon Superlight fork.

Also, the Trek Factory Gnome resigns his posistion and returns to moto-pimping! I am aghast!

Ride yer bike! Then ride it some more! Have a great weekend!

No comments: