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Hi All, 

Can anybody explain in simple terms, how to interpret the numbers in a Geometry chart? I know what the numbers are physically measuring, but what does this mean in terms of bike fit and ride? What should I be looking for?

For example, from the marketing guff I assumed that the Kinesis RTD and GTD were largely the same bike, built for the same purpose (long fast rides), just giving the option of Titanium or Alloy. However, the geometry charts are different and the GTD actually seems closer to the 4S disc (an all year/winter race bike)... 

I bought a Btwin 500 in XL but it's way too big for me - the bars feel about 4cm too far away... The size chart suggests I should have bought an L but the top tube is only 2cm smaller... I know a bike fit will tell be what numbers fit me best, but before dropping a healthy amount of cash on a new bike, I figured I should learn what they actually mean in terms of how the bike feels/rides as well as fits....

Thanks in advance!

7 comments

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matthewn5 [1337 posts] 2 months ago
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The two key measurements are Stack and Reach.

Reach gives you the distance from the bottom bracket to the steerer. If it's too big you'll be stretched out awkwardly and to compensate will have to run a short stem and push the seat forward. You probably won't be able to get comfortable and pedalling won't be efficient.

Stack is how high the top of the steerer is above the bottom bracket. If that's too small you'll be bent over in with the bars in a possibly uncomfortable 'too low' position. To compensate you'll need a naff stack of spacers under the stem or an inverted (riser) stem.

I'd suggest measuring a bike you're comfortable riding using the same criteria as the bike you're interested in, put those figures in a spreadsheet, and compare that other bike with the one you know. Then you can be sure that you can set up the new bike to be comfortable.

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Freetime101 [52 posts] 2 months ago
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Thanks, seems straight forward!

What do the other numbers effect? I.e. chain stay length, seat tube angle etc? I know what they mean physically, but how do they effect the ride?

Cheers!

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Canyon48 [1139 posts] 2 months ago
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Freetime101 wrote:

Thanks, seems straight forward!

What do the other numbers effect? I.e. chain stay length, seat tube angle etc? I know what they mean physically, but how do they effect the ride?

Cheers!

Ditto this, stack and reach are the important two.

If you start looking into tube angles, it gets horrendously complicated (I use CAD models of bike frames all overlaid each other to decide how I'll set up a bike).

Broadly, steeper angles = more aggressive and slacker angles = less aggressive.

The size guides and charts companies produce are there for a reason and they do work.

 

The long answer about what geometry actually means...

Chainstay length: short chainstays = shorter wheelbase = more lively & less stable

Tube angles: steep angles = less rake = more lively and less stable

Seat tube length: doesn't really matter (though, generally, short seat tube = longer seatpost = more comfort)

Effective top tube length: (Personally, this is how I get an estimate of what size bike I want), more or less determines how long the bike will feel

Wheels base: same effect as chainstay length really i.e. long wheelbase = more stable

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Freetime101 [52 posts] 2 months ago
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Thanks for the detailed info! This links into my other thread (kinesis gf_ti vs whyte wessex) - I've got some thinking to do!

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Mathemagician [61 posts] 2 months ago
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You might find this handy for comparing 2 bikes directly. You put the relevant numbers into the spreadsheet and it shows you schematics of the bikes overlaid on one another.

http://gearinches.com/blog/misc/bike-geometry-comparator

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dottigirl [863 posts] 2 months ago
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Kinesis annoy me as they have no old model archive. I have two 48cm T2s with different length head tubes so they must have slightly different geometry. However, I can't confirm what the difference is.

I've been entering all the figures I have found for various brands onto a spreadsheet,  but so many charts only have a few measurements.

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Freetime101 [52 posts] 2 months ago
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Mathemagician wrote:

You might find this handy for comparing 2 bikes directly. You put the relevant numbers into the spreadsheet and it shows you schematics of the bikes overlaid on one another. http://gearinches.com/blog/misc/bike-geometry-comparator

Awesome thanks  - this'll keep me busy for a few hours!