Bike Geometry Advice

by pirnie   January 30, 2013  


For a long time now I've had my eye on a Wilier Gran Turismo, and a couple of days ago I finally got around to talking to my LBS about it. However, there's a bit of a problem in that they can't get hold of one in my size for me to test Crying

They can get hold of an Izoard XP for me (for a small fee) which I could ride. Thing is the geometries on the Izoard and the Gran Turismo aren't exacly the same (links below)

I'm looking at the XXL (lanky git!) and most of the dimensions are the same but there's a couple of differences.

My question for those with more experience of bike fitting than me is; given the difference in geometry do you think riding the Izoard will give me a good idea of whether the GT will suit me, or will I be wasting my time?

Thanks in advance!

4 user comments

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Not sure, the Izoard appears to have an integrated seatmast making comparisons a little more difficult. The top tube is the same length but there is 2cm difference in the centre-to-centre distance. Not sure why this is, does the Izoard have less ground clearance for the BB? The seat tube is also slightly steeper on the GT.

For test ride purposes, I guess you could mount the saddle slightly forwards, but the difference c-2-c might mean that once comfortable on the Izoard, you would be looking at raising the saddle height on the GT (to achieve the same level of knee bend given that the bottom bracket is closer to the saddle), but that in turn would increase your body's 'drop' to the bars. Also, you might like the handling of the Izoard, then find that the GT's slightly higher centre of gravity doesn't suit you.

Then again, I know nothing about this stuff. Someone with more knowledge will be along imminently, I'm sure!

Last night I would have considered trading a very loud baby for a really nice bike.

posted by notfastenough [3071 posts]
30th January 2013 - 15:19

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Yeah, I'm a bit confused about the integrated seat mast thing. On the geometry diagram it seems to have one, but none of the pics seem to show it.

All good points thanks, I'm not sure the centre of gravity thing will be an issue, based on my current bike I'm pretty sure the GT will be an improvement. As I'm planning on buying a frame and building it up over a few months I just don't want to end up spending months of work and a couple of thousand pounds building a bike that ends up not fitting me!

Anyone else's opinions on this would be much appreciated Smile

posted by pirnie [184 posts]
30th January 2013 - 19:47

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The difference in C/C means that GT has a bit more sloping top tube which will give a bit more standover clearance and can be ignored really for fitting purposes. I also doubt there is any difference in BB drop, but even if there is it will measure a couple mm rather than cm. So while the frames are of different shapes, the position of the saddle relative to the handlebars relative to the pedals are pretty much the same (provided the stem, amount of spacers and saddle set up are the same). So i'd say that you'll get a very good insight into how a GT would feel after riding an Izoard.


just found reach and stack number for those bikes on competitive cyclist's website (reach is horizontal from bb to the top of the head tube, stack is vertical measurement between the same points) and they are exactly the same meaning that bb heights will be the same as well.

posted by mhtt [42 posts]
30th January 2013 - 21:24

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Great, thanks! That was kind of what I was thinking as well, but it was nice to have it confirmed! (I found the competitive cyclist numbers too eventually) Okay, think I'll give the Izoard a test ride sooner or later and see how it fits me! Smile

posted by pirnie [184 posts]
31st January 2013 - 17:15

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