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Frame Geometry

1061 Views 3 Replies 4 Participants Last post by  Guitar Ted
This is a question to all you folks out there that get to ride alot of bikes. I don't, so my experience with different frame geometies is limited. I only know this. Up until last year when I got my SuperCaliber, the 26" bikes I rode (about 5 of them) were all very twichy. By that I mean when desending at any speed in rough terrain (ie. rocks), my front end always wanted to go left or right and "bounce" me off the the desired line I was trying to pick. With my SuperCaliber, I am amazed at how well it tracks straight through rough terrain. It will bounce up and down, but will always stay on the line that I choose. It took me a long time riding it to develop the confidence that it would do this. It really never does anything really bad. This is great in races when you can stay calm, and have confidence in your bike, especially toward the end of a race when you get tired. I just did a race yesterday that was very rocky and loose on the desencts. Some of my buddies commented on how smooth and fast I seemed to be able desend these rought sections, and granted, this is on an aluminum hardtail!

I love this about my bike. The things I could see that give me this quality are:
1. Head angle
2. Longer wheelbase
3. Longer chainstays
4. 29" wheels
5. Lower pressure (less "bounce")
6. Long Top Tube (25.5" on XL SuperCaliber) and Short Stem (105mm).
7. Some magic combination of any or all of the above.

Now, I hear alot of talk in this forum about people wanting bikes with quicker handling, ie. shorter wheelbase, chainstay, toptube, ect. I can understand that if you ride smooth singletrack that has alot of very thight turns, this might be desirable. But I really have very little trouble getting my bike to make hairpin turns. In fact, with lower tire pressure and better traction, it turns better than any bike I have ever owned. In a race, when you are blasting down a rough desent, I want the bike to hold a line and keep me out of trouble.

Maybe the best thing I can say about my bike is that if I were to go buy a new bike today, I would be afraid to change one thing about it for fear of losing this characteristic.

So I guess what I'm looking for are your thoughts from people that have ridden a Gary Fisher geometry bike, and have ridden others. Do your other bikes with better tight handling sacrifice the stability that I so much like about my bike? If I were to buy another bike or advise a friend that wants to buy one, what do I need to look for that gives me this ride?

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add Trail and lower BB to your list

I'll work with your numbers. If I need correcting, I'm sure it will happen.

1. Head angle - this is typically steeper by a degree or so in 29"ers to offset increased trail caused by 29" wheels. IMO the steeper angle doesn't contribute to the stability factor. A steeper angle will place more weight on the front wheel causing better traction in corners. Usually the trail of a 29er ends up somewhere between typical 26" xc numbers and DH numbers. This adds to stability.

2. Longer wheelbase - is a factor of longer chainstays and increased fork length. This definately contributes to stability. IMO, those looking for a quicker bike can add a little BB height to add some twitchyness.

3. Longer chainstays - this actually makes sense for taller riders, regardless of wheelsize.

4. 29" wheels - yup, rolls over objects easier.

5. Lower pressure (less "bounce") - more traction??

6. Long Top Tube (25.5" on XL SuperCaliber) and Short Stem (105mm). Don't know about this personally.

7. Some magic combination of any or all of the above.

add 8. BB height - a lower BB (fishers have low BB right??) results in lower center of gravity and greater stability.
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It's all subjective - but here are some answers.

I do this stuff for a living now, so I should be able to give you a definitive answer. The problem is, nobody can every seem to agree on *exactly* what creates a bike that handles _x_ way. Some of the trouble comes from the fact that the terms ("twitchy", "flexy", "stiff", "stable" are subjective to rider, terrain, etc). So essentially there's no real agreement on *precisely* what factors will result in a great bike. There are, however, some useful generalizations that can be made.

1. The frame needs to fit the rider. A lot of the handling characteristics of the bike come from the position of the rider, and from the riders ability to CHANGE position (get out of the saddle, get over the back of the saddle, get forward to slide the rear wheel around a switchback, etc). If you're on a bike that doesn't fit, it will compromise handling in almost all situations (so don't buy a bike that's too small or too big because you think it'll be "faster turning" or "more stable" or what have you - you're giving up more than you get).

2. Head angle/trail. There was a huge thread on this a while ago that resulted in no consensus. I built some experimental forks and played around with head angle/ride height/trail and found some interesting but certainly not definitive answers.
-Handling seemed most "neutral" at 55-65mm of trail (regardless of how this figure was achieved - head angle changes, rake changes, or some combination). Stable on the straight stuff, stable in the turns.
-At greater trail, the bike was extremely "stable" (meaning, resisted both steering and terrain inputs while going straight) but had a frightening tendency to dive into turns in a very "unstable" manner. Very nice on low speed drops, however.
-At significantly less trail, the bike was almost unrideable on straight sections (all terrain and rider input resulted in serious deflection from the intended line, most of the time) and felt as if it would endo on steep drops. Turn initiation was easier, but once in the turn, the bike stayed "twitchy" to the point of being quite frightening.

3. BB drop. Ever try riding a kid's scooter with a couple of bricks pile on the deck to stand on (so that you're above the level of the axles)? It's not very fun. What matters for mountain bikes is where your center of gravity is relative to the axles - on a 29er, average BB drop is 50-75mm, whereas on a 26" bike, 20-35mm is common. The extra drop translates to extra stability.

Those are the 3 biggies, IMO. There are a lot of ways to get a bike that handles nicely, but you probably don't want to mess around with 1) good fit, 2) reasonable amount of trail (accomplished any number of ways), and 3) low BB relative to axles (but you don't want to hit your pedals on the rocks, either).

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More info

I think Walt is right on, but for another veiw, you might try reading Keith Bontragers article in the newest edition of Dirt Rag. It explains things like trail, and why a bike corners well or not. Also; there is a link in the article to a tech-geek article on the same subject. Happy Reading!
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