Touring - Understanding Bike Geometry
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01-24-05, 08:14 AM
I went to a bike shop I'd never been to last week and had a fitting done. They were the only shop in my area that does fittings. I'm looking for an all-around bike. Approximately 80% of the time will be spent fitness riding on paved and dirt roads. The remaining 20% will be fully loaded touring on paved/dirt. I had been looking at both touring and cyclocross bikes. I thought that a cyclocross bike would be a good compromise. The shop did not recommend getting a cyclocross bike because of the high bottom bracket and chainstays that are on the shorter side. (I have size 11 feet). On the other hand, they said a touring bike wouldn't feel very "sporty" for fitness riding. I can only afford 1 bike.
I take a 58.5cm frame (C-C) and 59.5 TT length. They recommended a frame with 460mm chainstays and a 78 BB drop. What effect will longer CSs and more BB drop have on handling/comfort, etc. Do these numbers look like they're in the ballpark for my intended purpose? Many manufacturers use BB height instead of drop. Is there a formula to convert back and forth so I can compare the 2?
hmmmm, I got a cyclocross for that very same reason. Hope it works. My feet are tiny but so is the bike. One prob is that my carbon fork won't handle a load (I've heard of cracks happening this way) so I'm going as ultralight as possible to stick with back panniers and handlebarbag. It sure is sporty though!
Seems like it is going to have a long wheel base. My rear to center (not chain stay length) in this drawing 513 mm which is long, I decided to shorten it by 50 mm, but I still have clearance for 180 mm cranks and panniers. Not much can be done with the front end, or the top tube angle but still this is going to be a long bike. Bottom bracket height I tweaked as much as I could, for ground clearance, I raised it a bit.
Did they say anything about stem length? Reach? Front to center.
For me the hard part about finding a ride I like is hand position, which effects body position. The seat can go up and down a bit, to make pedaling comfortable, and forward and backwards to effectively change the seat tube angle. My solution to hand position is a Look Ergo Stem, if it is not comfortable I can change it while riding. Of course after stopping.
I also lowered the top tube a bit, to stiffen the front up and give more stand over clearance. I did not change the geometry of the "fit" by doing any of these thing. The basic design of this frame came from Tim Paterek's software program. It is DOS based runs on windows (in dos mode), is free, here is the link http://www.timpaterek.com/software.htm. His book is great even if you never intend to build a bike. Chapter 2 of his book explains alot about frames and fit.
Trail on the bike as drawn ( not yet built) should be about 65 mm which is supposedly ideal for a touring bike. The steering is not twitchy. I based my design on actual diameter of the tires (not the rims) I intend to use on the bike.
With any luck I should have the frame built in about a month.
Hit the wrong buttom and posted by mistake.
Longer chainstays change where weight is centered on bike. May or may not effect how stiff the rear is, depends on stiffness of the stays themselves. Flexi long ones might make the ride a little softer and more comfortable. Longer flexi ones might make the bike fell well say sloppy or loose. Long or short stiff ones make for a rough bouncy ride, but snappy and quick.
Bottom bracket height is not much of a comfort issue, when the seat post can move up and down, to change how much or how little your leg extends. Its a ground clearance issue.
Handling and steering are more functions of the front end geometry. The thing to know is "trail." Most bike makers include this info in thier specs. 65 mm is supposedly the ideal for a touring bike, as it makes for a stable front end. Shorter trail and the bike handles "quicker."
Me if I could only have one bike and intended to do any touring at all, I would get a bike for touring. I would not get a "sporty or quick " one as it would not handle well loaded, it might not even have braze ons for mounting racks. A touring bike with decent tires (something like a conti top touring 2000 700 x 37mm) should handle well on paved roads as well as dirt ones. Those tires are wide enough to run at lower pressure for rough roads, and higher pressure for smooth ones.
Best thing as everyone has said before, test ride it before you buy it.
Next best thing in my opinion, have the bike custom built.
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