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Old 09-30-2020, 09:59 AM
lightweenie lightweenie is offline
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Join Date: Jul 2017
Posts: 7
Help me understand Bike Geometry: trail along sizes. Pegoretti vs. Specialized

I was under the impression, that trail is one of the major factors affecting bike handling, and that a desirable handling for a road bike is around the 57-58mm range with slight deviations depending on the exact purpose of the bike -- but independently of the frame size.

Looking at the pegoretti geometry chart seems to verify this: the trail ranges between 55 and 59mm with 55 probably being a sort of compromise at the sizes 52 and 53 given that the fork comes in only two different rakes (I assume).

However looking at the specialized sl7 geometry which is a bike very often praised for its handling characteristics and the trail is all over the place ranging from 52 to 71mm (it also comes with a fork of two different rakes).

Could someone explain to me why this is? Does the SL7 compensate for the extreme trail figure for some sizes in some other ways so that it still is able to offer great handling? Or is there something else going on?
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Old 09-30-2020, 05:58 PM
Peter P. Peter P. is offline
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Join Date: Mar 2006
Location: Meriden CT
Posts: 5,385
The smaller size Tarmacs have shorter top tubes, as do most bikes in smaller sizes.

If you do nothing else when building frames but shorten the top tube, eventually the front wheel will hit the down tube.

So framebuilders, or in this case Specialized engineers, compromise and slacken the head angle. This enables them to use shorter top tubes, to fit shorter riders. Unfortunately, they have to increase the fork rake to get some semblance of normal steering, and they apparently found a limit to fork rake before something went awry in handling, resulting in that 47mm fork rake/71mm of trail.

Sadly, short riders never really get to enjoy handling which is the preferred "normal" when it comes to rake, head angle, and trail.

One solution is to build the bike around 650c wheels or as Georgeana (sp?) Terry did, offer a 650c wheel in the front and 700c in the rear.

Last edited by Peter P.; 09-30-2020 at 06:02 PM.
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