Bike Frame Sizes, Geometry, Angles and All That!

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Your frame has to fit, that is the most important thing. Frame manufacturers all have there own ideas as to what the geometries should be for their bikes, but there are a few dimensions that you cant mess around with too much, here we take a look at frame design, just to please a certain Mr. Greene!

Bennett Greene has written to let us know his disappointment that we did not go more in depth with our article on frames. Here is what he had to say: “I was hoping that the article in the previous mailing about frames would have been more in depth. It mentions nothing about geometry, stiffness or compliance. It just says that good top frames are similar in quality, and the buyer should choose based on “color”.

Isn’t that insulting to the kind of bike rider who cares about learning these things? We all know that a Trek Madone frame and an Orbea Orca frame are different. Why not discuss those differences, or the difference between Orbea Opal and Orbea Orca?”

Well, Bennett, just for you here is a more in depth investigation into frame angles, tube lengths and overall frame geometry. We do think that there are more important issues than the color, but we still stand by our opinion that in the end your choice might come down to whether or not you like how the frame looks, remember you have to look at that frame for hopefully quite a few years.

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Frame Dimensions

Basic Geometry

There are some dimensions that you can not mess about with too much, these are the length of the chain stays, the distance between the back wheel axle and the bottom bracket axle, the other is the distance between the bottom bracket and the front wheel, the chain stays of the bike must be around (F) 42 cm.

This is the basis of all bike designs and if you change anything here it will affect the handling of the bike, if you change these dimensions then you could have a dangerous bike that will not corner or descend safely or a bike that is so close at the front end that you will catch your foot in the front wheel.

The total wheel base for a normal road bike with 700c wheels should be around (G) 100 cm, this can vary from around 98 cm to 104 cm depending on the overall frame size, this length can be changed, but if it the frame is too short it will be too “lively” and handling will be unpredictable and could be dangerous.

If the bike is too long this will give a comfortable, soft ride, which is great for touring, but not for racing as it will feel like pushing a garden wheel barrow, slow to respond and a very sluggish performance.

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Slopping Top Tube

The Other Dimensions

The length of the seat tube (A) used to be dependent on the length of the riders leg, but since Mr. Burrows invented the compact frame design for Giant bikes this hasn’t been the case and the length of the top tube (D) has been more important to the fit of the rider as you use a long seat pin to make sure the saddle is at the correct height.

With a slopping top tube you need to draw a horizontal straight line from the head tube (E) towards the top of the seat tube (A) or actually where the top tube would be, if it was a standard frame, like D in the diagram. You then compare this to your present set up to find your top tube length. H is the stand over height from the tube to the ground.

The size of E depends on the size of the other tubes and has to be adjusted to suit the overall frame as do the seat stays which depend on the length of the seat tube and its angle.

Frame Angles

On factory made frames the seat angle will depend on the size of the frame, the smaller the frame the steeper the angle of the seat tube (C). Smaller frames are for smaller riders who have shorter thighs so they have to sit further forwards to get over the pedals. Taller riders need a more laid back angle because their thigh is longer.

A more “relaxed” angle give a slightly more comfortable ride, but the important thing is that the riders knee has to be over the pedal otherwise you won’t get the correct leverage and this is what regulates the seat angle (C). The angle of the head (B) has to be more laid back on a smaller frame so that the wheel base is nearer the correct length as the top tube is shorter.

Then its more up-right for the longer frame to bring the front wheel closer, this can also be adjusted by the rake of the fork, but this affects the steering and handling of the bike when cornering. So with a smaller compact frame with a slopping top tube you would find that the seat tube would be around 75 degrees and the head tube angle would be around 71 degrees. With a large frame you would find a seat angle of around 72 degrees and a head angle of around the same.

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Horizontal Top Tube

That is all the science involved with frame building, the frame should fit the rider, but within reason, it must stay between those parameters so that the frame will perform perfectly, cornering, climbing, descending etc. Now when frames were made of steel the builder could tailor the frame to suit the shape of the rider, now with frames made of carbon it’s either not possible or too expensive, so “off the peg” frames are a necessity.

When you want to buy a new frame then check out the geometry on the frame makes web-site and compare to your present bike to get the correct size. Years ago you had to choose from many frame sizes in half inch or centimeter differences, now with compact frame they normally come in five sizes, making things a lot simpler.

I hope this all helps to explain the tube lengths and frame angles, but in the end, as I said before, it might come down to the nicest color, what do you think Mr. Greene? Next time we will discuss the differences between some of the more available frames on the market.


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