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.

[li10up]

What makes a bike stiff? What are the pros and cons of a stiff bike?

[Kathi]

I think I have some information on bike stiffness bookmarked on my computer at home, I'm at my ski condo until Thursday but I will try to find it for you when I get back home.

Stiffness has to do with the tubing and how it is used in a frame. I was very concerned about stiffness when I built my new frame. I had a Cannondale 1995 vintage aluminum frame and whenever I hit chip and seal type roads I had to decrease my speed. Many days I felt like I rode a jackhammer all day. I was told the frame was too stiff for me, a 100 lb person and I wasn't heavy enough to flex the frame. Then I went to an Aegis Swift. It was carbon and I think pretty stiff but I didn't feel the jarring of the aluminum frame. The bike has a "snappiness" that I hadn't experienced on any bike. Whenever I stepped on the pedals it just "gets up and goes".

When I was researching bikes and materials for my custom frame I was told that the snappiness that I loved on my Aegis is due to a stiff bottom bracket.

I demoed a steel frame, ti frame and carbon/ ti frame. The carbon /ti frame was the similar to my carbon frame. But I was worried that ti would be to stiff. Serotta was able to build me a ti/carbon frame with a stiff bottom bracket for snap but with enough flex in the tubing that I have a comfortable ride. Actually, quite a wonderful ride.

From what I remember from my research a too stiff frame can affect handling on corners and downhills.

Also, wheels can affect the ride. Most wheels are built for men but if you're not in their weight range they might be to stiff for you.

For me building a custom frame was a big issue. I wanted a ride like my cf frame, custom cf is hard to find in my size and I was afraid I'd end up with a too stiff frame. Fortunately for me, I found a company that could come up with the right combination of tubing, with the right flex and build me the ride that I wanted.

Here's an article on frame flex,

http://www.kirkframeworks.com/Flex.htm

[li10up]

So is it a true statement to say that the more your top tube is slopped the slacker your geometry is?

And the slacker the geometry the slower the bike? Is this correct? And if so, why is this?

[Eden]

So is it a true statement to say that the more your top tube is slopped the slacker your geometry is?

And the slacker the geometry the slower the bike? Is this correct? And if so, why is this?

On the first point I'm pretty sure the exact opposite is true - a more traditional frame bike usually has a more slack seat tube and racing bikes, especially compact frames (sloping top tube) have steeper seat tubes. Though it is true that slack seat tubes are usually found on bikes that are built more for comfort than speed.

Everything you wanted to know and more at the Wiki...
http://en.wikipedia.org/wiki/Seat_tube