N=1 Geometry question

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robeambro
Posts: 1828
Joined: Sat Jul 07, 2018 6:21 pm

by robeambro

Hi all,

I think I have set my eyes on a gravel bike to serve as a N=1 (mostly fast road rides, with occasional not-too-gnarly gravel, mostly typical UK gravel that can be tamed with 40-42c tyres). Ideally the bike will be raced on both tarmac and gravel.

In terms of stack & reach, I fall between two sizes, one a bit too low and quite a bit too short, and one a bit too tall.

The advantage of the size S is that I can "make it taller", whilst the same cannot be said of the size M (even with a -12deg stem, it's roughly 10mm taller than I'd like). Ideally I'd get the size S and add some 15mm spacers, but I am worried about potentially HTA / stem length and handling implications.

From what I am estimating, the choice is between a size S, with an HTA 70deg, and a 110mm long stem, or a size M, with an HTA of 71deg and a 90mm long stem.

Frankly, I have always ridden pure road frames with ~ 73deg HTA and 90mm stems so I don't really know how big of a change will it be and/or what to expect when moving to something like a size S as described above.

Can someone enlighten me and give me their 2c on what I could expect from the change and whether there is a clear "best choice" between these two sizes based on the information I've given?

Size S M
Seat tube 510 mm 550 mm
Top tube 525 mm 545 mm 564 mm 583 mm
Head tube 137 mm 157 mm
Head angle 70.0° 71.0°
Seat angle 73.5° 73.5°
Chainstays 420 mm 420 mm
BB Drop 75 mm 75 mm
Wheelbase 1,002 mm 1,012 mm
Reach 364 mm 376 mm
Stack 545 mm 568 mm

OnTheRivet
Posts: 732
Joined: Sat Jul 24, 2010 9:41 pm

by OnTheRivet

90mm stems are atypical on road bikes, you generally see them used by people with shorter torsos (and or flexibility issues) and can make a bike handle a bit differntly than intended for the geometry. I'd definitley look at the small with a 110 stem. That being said if you have to run more than 3cm of spacer you may need to look at a different frame all together.

by Weenie


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robeambro
Posts: 1828
Joined: Sat Jul 07, 2018 6:21 pm

by robeambro

OnTheRivet wrote:
Tue Nov 29, 2022 3:21 am
90mm stems are atypical on road bikes, you generally see them used by people with shorter torsos (and or flexibility issues) and can make a bike handle a bit differntly than intended for the geometry. I'd definitley look at the small with a 110 stem. That being said if you have to run more than 3cm of spacer you may need to look at a different frame all together.
I am indeed all arms and legs and no torso, I run a very high saddle to bar drop but can't deal with excessive reach.

My fit is fine, and I know exactly where I want to be in terms of X and Y coordinates vs the bottom bracket. As mentioned in the initial post, I am only concerned about handling especially on the Small size, due to the very slack HTA.

Alolympic
Posts: 12
Joined: Tue Jun 19, 2018 11:23 pm

by Alolympic

I am not 'an expert' but from my trial and error experience.....

A slacker head tube will typically give you more confidence at speed, especially down hill and keep its line more easily. But a slacker H angle will feel slightly less nimble. The increased trail - relative to a constant fork offset - will increase wheel flop. Not a problem, but switching from a bike with a steepr road geometry will take some steering adaption.

I have recently fitted an Slack R integrated headset on my gravel bike - changing HTA from 71.5 to 70.0.
I only notice positives tbh, especially as someone who wants to build confidence descending.
Another impact is a longer front centre, with potential for going bigger (assuming fork clearance) on tyres without toe overlap.

Casagrande
Posts: 12
Joined: Fri Mar 19, 2021 6:01 pm

by Casagrande

My 2c on the geometry: my gravelbike has a HTA of 70.75. On road tyres it feels very stable in a straight line, but on faster descents (60+ km/h), it needs be be 'pushed' into sharp corners. For my next gravelbike, I will be looking for something more road oriented with a steeper HTA around 72-73 degrees, for example Time ADHX or Canyon Endurace CF. I usually ride smooth and fast gravel lanes, not a lot of single tracks / technical stuff
Carrera SL730 6.0kg
Bianchi Oltre XR4 7.5kg

stevesbike
Posts: 324
Joined: Thu Oct 24, 2019 5:33 pm

by stevesbike

OnTheRivet wrote:
Tue Nov 29, 2022 3:21 am
90mm stems are atypical on road bikes, you generally see them used by people with shorter torsos (and or flexibility issues) and can make a bike handle a bit differntly than intended for the geometry. I'd definitley look at the small with a 110 stem. That being said if you have to run more than 3cm of spacer you may need to look at a different frame all together.
90mm stems are not that uncommon on gravel bikes - some like the aspero and crux have particularly long top tubes that are purposely paired with a short stem to give the benefit of a long front center/wheelbase for stability with a short stem for nimbleness re steering. Extra stack is also OK for a gravel bike, since many people choose a more upright position than their road bike. And a smaller size could mean toe overlap issues.

robeambro
Posts: 1828
Joined: Sat Jul 07, 2018 6:21 pm

by robeambro

stevesbike wrote:
Thu Dec 01, 2022 11:24 pm
OnTheRivet wrote:
Tue Nov 29, 2022 3:21 am
90mm stems are atypical on road bikes, you generally see them used by people with shorter torsos (and or flexibility issues) and can make a bike handle a bit differntly than intended for the geometry. I'd definitley look at the small with a 110 stem. That being said if you have to run more than 3cm of spacer you may need to look at a different frame all together.
90mm stems are not that uncommon on gravel bikes - some like the aspero and crux have particularly long top tubes that are purposely paired with a short stem to give the benefit of a long front center/wheelbase for stability with a short stem for nimbleness re steering. Extra stack is also OK for a gravel bike, since many people choose a more upright position than their road bike. And a smaller size could mean toe overlap issues.
Considering that wheelbase is some 24mm longer than my current road bike, would toe overlap still be potentially an issue when choosing what effectively is an undersized bike? Of course the front tyre would be fatter but I wouldn't expect that to offset the increase in wheelbase entirely..

by Weenie


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satanas
Posts: 315
Joined: Sat Jul 04, 2020 5:45 pm

by satanas

Re dimensions and TCO: We don't know the dimensions of your current bike, and wheelbase isn't particularly helpful in any case. What you should do is measure the current front centre, and the amount of clearance (or overlap) that you have now, then figure out what the front centres are of the potential new frames; this requires only Pythagoras' Theorem if you know the wheelbase, chainstay length and BB drop. Once you know that and the largest tyre size likely it shouldnt be hard to figure out whether there'll be a problem. As for stem length and whether a longer or shorter wheelbase is desirable, that boils down to personal preference, so I'd try to test ride as many bikes as possible, on different terrain if you can.

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