Bike Geometry

[li10up]

Can those of you who know about this stuff explain it to the rest of us. This could be a thread that has all the info in one place in simple terms that newbies need to know.

Please keep it in very simple terms. I'm looking for things like how the slope or length or whatever affects this or that. A stiff frame does this. What makes a stiff frame, etc. etc.

Please keep it simple.

Thanks in advance.

[Kathi]

This does not explain geometry but it does explain where and how the rider stresses the tubes.

http://www.serotta.com/pages/characteristics.html

A relaxed geometry has a slacker seat tube angle and a higher head tube (higher handlebars). A racing geometry will have a lower head tube (lower handlebars).

Seat tube (the tube your saddle is on) angle determines the setback of your saddle. A seat tube angle of 74-75 degrees puts you very close to the bottom bracket or center of the bike. A 72 degree sta will put you farther away from the center. Seat posts have setback or no setback (zero degrees). So for someone like me with a short femur I have a 74.5 degree seat tube angle and a seat post with zero set back. Even though my bike is custom my saddle is still pushed as forward as it will go.

If you have a long femur a steep seat post could cause problems. That's why I said Lisa may have felt to scrunched when making comparasions between her Rivendell and the Specialized.

One example of a relaxed geometry is the Trek Pilot. For most people this bikes puts their handlebars in a higher riding position which many riders like.

I also find it interesting that I could have used a 76 degree sta but Serotta chose not to do it.

[Kathi]

I don't think slope affects the ride as much as what tubing is used and the stiffness of the bottom bracket. WSD bikes are sloped, especially on very small frames because the company is trying to get the head tube higher for shorter arms. Because the ht has to be so high they have to slope the tt for standover height.

That's why small frames on 700c wheels are so sloped. My frame is sloped 1 degree, looking at it you can't tell. I was really surprised because I thought it was going to need more slope.

[li10up]

A relaxed geometry has a slacker seat tube angle and a higher head tube (higher handlebars). A racing geometry will have a lower head tube (lower handlebars).

Seat tube (the tube your saddle is on) angle determines the setback of your saddle. A seat tube angle of 74-75 degrees puts you very close to the bottom bracket or center of the bike. A 72 degree sta will put you farther away from the center. Seat posts have setback or no setback (zero degrees). So for someone like me with a short femur I have a 74.5 degree seat tube angle and a seat post with zero set back. Even though my bike is custom my saddle is still pushed as forward as it will go.

What does slacker seat tube angle mean? Does that mean it tilts more to the rear?

Can you exlain setback? "Seat posts have setback or no setback" I can't figure out what this means.

[Kathi]

Yes, to the sta, men's bikes have a slacker sta. than women's frames. But some manufacturers are going the opposite direction with this because they claim that women have longer femurs. Not sure what happened to me!

Post with zero degree setback

http://www.universalcycles.com/shopp...5&category=765

Post with setback

http://www.coloradocyclist.com/commo...629&TextMode=0

Weight distribution (too far forward or back) also affects how a bike handles thats why fitters look at foot to pedal, butt to saddle, and hands to handlebar. Besides, to far back your to stretched out and to forward your to scrunched.

[li10up]

Ahhhhh, a picture is worth a thousand words.

Thanks Kathi.

So how do you know if you want/need a slacker seat post angle? How do you know if your femers are shorter than normal or your arms are shorter, etc? I guess I'm just average or I would know.

[Kathi]

Mine was intuitive, I'd slide forward to get over my pedals but then I was on the saddle nose so I'd slide back. I knew something was wrong but couldn't figure it out. When I bought my mtn bike I had a professional fit. It was my
1st one by a Serotta fitter. He kept moving my saddle and checking my fit, finally he said, "your femur is so short that I can't get you centered properly over the pedals" so he put on a zero degree seatpost. He was happy.

When riding the new mtn bike I quit moving back and forth, my saddle was flat and didn't hurt, my shoulders didn't hurt and my hands didn't go numb like they did on my old bike.

So off to the fitter with my road bike, I got a zero degree seatpost, raised my handlebars 4cm and voila! a very good fitting bike.

The fitters worked with those three areas, foot, knee, hands. I'm don't remember them being concerned about tt length or standover height.

Your body will tell you if you aren't fitted well. For me it was my saddle, had to be nose down, shoulders hurt and the constant moving back and forth.

So if you're not having any issues or don't feel you're to far behind center your probably ok. Most bicycle companies give the geometry for each model they sell. You can look at a specific model to determine the seat tube angles.

I knew I needed a 75 sta but didn't know why until I had a good fitting.