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so, why does geometry matter if...(7 posts)

so, why does geometry matter if...mlbd
Jul 12, 2002 3:25 AM
i've been road riding about a year now. i'm looking to get a new ride this summer, and, of course, geometry has been a consideration. your body contacts the bike in three places: pedals, seat, bar. if you always set the seat so that you have the proper KNOP, and always get a stem to give you whatever reach is comfortable, why does geometry matter, especially in terms of efficiency and power? I have a Lemond, which has a larger seat angle than some bikes. But I still set up the saddle with the proper KNOP so does the seat anger make a difference? I'm definitely confused.

for efficiency and power, assuming that you can fit into thebill
Jul 12, 2002 3:51 AM
cockpit comfortably, I don't think that geometry does matter. As far as I can determine, geometry matters to the size of the cockpit, which, as you point out, can be adjusted within reason, to the wheelbase, which matters to handling and stability, to the chain stay length, which I've heard matters to power transfer, but I'm not convinced, and to the headtube angle, which can matter to handling fairly dramatically. I guess if you're fighting the bike to remain stable, that's not very efficient, but I think that it's mostly a handling thing.
"but I think that it's mostly a handling thing"Ray Sachs
Jul 12, 2002 4:31 AM
Yeah, but it matters a lot over a long ride. Bikes that you're not well balanced on and which don't handle predictably and stablely (word?) will wear your ass out over time. I've had a few bikes that I could get an identical "contact point" fit on, but some of them felt like an extension of my body and others felt like a strange and unwelcome extra appendage. I could ride all of 'em for 20-30 miles, but the center of gravity / balance differences become a real issue somewhere around mile 50-60.

Well, you can technically make ANY bike fit you, but...Gator
Jul 12, 2002 6:37 AM
it doesn't mean that it will necessarily perform well. Depending upon what you want to do with the bike, geometry can be very important. Let's use your arguement for a second. You buy a bike and the seat height and stem length are perfect; it's comfortable.

Let's then say you want to race the local crit circuit. If it turns out your properly fit bike has a 71 headtube angle, long chainstays and a low bottom bracket height, you're probably not gonna have much luck. The slow steering will make it very tough to make the tight turns, and when you do lean into the turns, your pedals are going to hit the pavement because of the low bottom bracket -- it's a disasterous combo. Combine that with poor acceleration/power transfer because of long, flexy chainstays, and you have a comfortable bike that fits you correctly rendered utterly useless because of its geometry.

So while geometry isn't the ONLY thing to consider, it is a VERY important thing to be aware of when choosing your ride.
it matters...C-40
Jul 12, 2002 6:44 AM
Geometry does matter, but the variations between brands are often not as great as people seem to think. Take a look at a new Lemond geometry chart for instance. The laid-back seat tube angles are gone. Lemond frame geometry is pretty standard stuff now.

The most important items to consider are the seat tube angle and top tube length in the size that is vertically correct for your inseam. The STA should work with the seatpost that you have selected to place the seat rail clamp fairly close to the middle for maximum adjustability. A change in seat post brand can sometimes be helpful. The TT length should allow the use of a stem that is not excessively long or short.

If you compare frames with what appears to be significantly different geometry you must know how to make an appropriate comparison. My often-posted example is the 55cm Colnago compared to a 55cm Litespeed Vortex. These two frames are vertically as identical as you can get; virtually identical standover height and head tube length (including headset). The Litespeed has a 73 STA with a 55.5cm TT, while the Colnago has a 74 STA with a 54.3cm TT. If you don't account for the difference in STA, the Colnago would appear to have a shorter TT. What is often ignored, is the fact that the saddle must be pushed back an additional 1.2cm on the Colnago to produce the same KOP as the Litespeed. When this is done, the TT length on these two frames is effectively the same. The only difference is the position of the saddle on the seatpost. If you need a lot of setback to attain your desired KOP, you may run out of adjustment with a frame that has a 74 STA. Changing seatposts can fix this problem.

The those who like math, the formula for difference in "effective" TT length (or total reach) is : saddle height x (cosA-cosB). The value obtained is added to the TT length of the frame with the numerically larger (steeper) STA.

Now some will scream that there are other differences between these frames. This is true, but they affect the performance of the bike, not it's fit. For example, the chainstay length, which is 1% shorter on the Colnago and the wheelbase, which is also not more than 1% different, will make the weight balance differ by a whopping 1%. The head tube angle is also steeper on the Vortex, but changing the fork rake can make the difference in trail quite small. The Colnago has 68mm of trail, while the Litespeed with a 40mm rake would have 64mm of trail.
Picture a triangle.dzrider
Jul 12, 2002 6:50 AM
It's points are at the center of your bottom bracket, the center of your handle bars and the center of your seat. This triangle can be rotated with the point at the bottom bracket remaining in place. The dimensions of the triangle remain the same, but the rider is in a different place and different posture depending on how the triangle is rotated. The placement of the triangle effects the posture and balance of the rider. This is how frame geometry changes the ride quality of a bike.
Picture a triangle, two points and a vector.Ping_Pong
Jul 12, 2002 8:20 AM
The relationships to be considered are those between the triangle you mention (bars, bb, seat), the centre points of the two wheels, and the vector representing steerer tube (with its impact on steering quickness and stability).