Gravel geometry

Can anyone explain gravel geometry to me? There seems to be no consensus as to what makes for good gravel geo. Some people claim long chainstays, others short. Some claim slacker headtubes, others steeper. Even in the same model, different sizes get different geo.

I assume they go with slacker headtubes on smaller bike sizes to make room for the front wheel so it doesnt clip toes. I cant figure out why smaller sizes get steeper seatubes.

Can anyone add some clarity for me?

“Gravel” encompasses everything from road bike with fatter tyres to mountain bike with hook bars, so there is no one answer to your question.

On most bike designs the seat tube gets steeper relative to the head tube as the bike gets smaller because the top tube length changes more than the front centre.

This is because the front centre is usually around 60% of the wheelbase, the other 40% being the rear centre which is determined by chainstay length and BB drop. Moving too far from this 60 : 40 ratio will spoil the way the bike handles.

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@Lyrebird_Cycles builds frames and has likely forgotten more about frame design than I know, so he’s the authority here.

What is striking to an amateur is how much longer effective tts and reach get, along with slacker head tube angles. Presumably this is to reduce toe overlap. To a degree, chainstay length simply has to get longer to allow greater clearance.

But a Mark says, gravel is a broad church. You could (and I have) ride something like a Factor LS with 30c slicks on pavement and (if you didn’t know and weren’t told) not really notice it wasn’t an endurance road bike with big tyre clearance. No one would make that mistake with a BMC URS…

Well there is, but there is no consensus on what a gravel bike is. :crazy_face:

Think of it this way….if the spectrum of bikes goes from road bikes on one end to MTB on the other, a gravel bike roughly sits in the middle. But if you slide it a bit towards the road side of the spectrum, your geometry requirements shift one direction (quicker handling, etc). If you go to the MTB side, the requirement shift another direction…more slack angles, etc.

A bike like the Cervelo Aspero (basically a road bike for gravel) has very different geometry than a bike like the Evil Chamois Haggar (almost a hard tail MTB w/ drop bars). Both are designed for very different types of riding….even if they both fall under the “gravel” umbrella.

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All true. Although i dont think the chamois hagar is good gravel bike geometry.
I think there probably is a sweet spot for the bulk of gravel bikes. Most seem to keep the headtube angle steeper than 70, which i think is to keep weight on the front wheel. Mtb geo goes slacker but this is assuming most of your turns will be in a standing pisition with your weight forwards whereas a lot of gravel cornering is seated with a slack sta. As far as chainstay length and sta, i figure sta is mostly about getting into a preferred pedalling poaition and chainstays are about tire clearance and balance. Has anyone here found a balance they thought was the sweet spot?

I don’t think so either….at least for my type of riding. If I was riding more single track, rowdier style gravel, it could be great (but even then I am skeptical and would probably choose a MTB).

These things interact, which is why I was talking about front centre / rear centre balance above.

Lengthening the FC reduces the weight on the front wheel, especially when seated.

Lengthening the RC reduces weight on the rear wheel, especially when standing.

Somewhere around 60 / 40 is usually a pretty good compromise.

Edit: I just looked up the Chamois Hagar and in my size it is 63.5 / 36.5. I don’t think I would enjoy riding this.

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This is exactly true , and also gets to the heart of the issue.

“Gravel” is way too broad a term to provide a specific informed answer. Imagine if you replaced the term “gravel geometry” with “sneaker design.” Sneakers can be worn for everything from causal walking, to tennis courts, to restaurant work, medical fields, basketball courts, turf fields and everything everywhere all at once. But the design is dependent on the surface conditions.

Same should be applied to “gravel bikes.” MTB has already implemented the practice, and it’s high time “gravel” did as well. CT posted an informative primer on “grades” of gravel and tire choice, and it would help if they also followed that up with geometry and grades of gravel.

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As others have rightly pointed out there’s a lot of variation in gravel. There has been a definite trend away from road bike geo and close to MTB inspired geo in recent years - probably due to people not just riding their ‘gravel’ bikes on hard pack roads.

The Scott Addict and BMC URS are probably the best examples of this more progressive geometry - longer TT, shorter stem. I’ve got an Addict currently and it’s very very capable on and off-road geo, the MTB-inspired geo definitely doesn’t hold you back much on the road but it’s much more capable off-road than roadie gravel geo.

I prefer the very small and relatively smooth type of gravel. The really big, round kind is tough to ride, and the small sharp stuff is just nasty.

Not only there is a different cursor set between road and mtb, but there are also other parameters which put the cursor on other axes like is it made for touring and how much weight can it carry or should it be light and playful. All these parameters end up having an impact on geometry. Also the type of handlebars involved.

Another thing that’s important in addition to all the handling geometry are the respective stack and reach figures, and how upright vs low they place you. Here in Indiana, I’ve tended to look for gravel bikes that allow me to get into closer to a road geometry (in terms of body position) to get down in headwind sections while also having a slack-ish steerer angle and trail figure for stability and handling on twisty loose bits. The 56cm Knolly Cache has turned out to be pretty perfect for me, though I run it with a slightly longer stem than it was designed for.

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I have a “bike geometry classifier” project that might help with this question. There are some links to explanations of mechanics of geometry of gravel bikes frames.

As I state in the intro - the site is very much a work in progress and the organization largely reflects some thoughts as I’ve moved throught it. Maybe go to the reclassifier section if you want the tl;dr. The project will be cleaned, organized, and hopefully, much more interactive but that will take some time.

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Interesting project, and I have much more to digest. But I have one observation / suggestion.

You have a few charts with Reach & Stack.

  • Considering that Reach is a Horizontal measurement for the geometry, I’d suggest setting that as the X-Axis of the chart so it’s horizontal (not the vertical that it is now).

  • Similarly, Stack is a Vertical measurement for the geometry, so it would seem that the vertical Y-Axis would be more logical to match on the chart.

  • Might not make much real difference, but having more direct correlation to the actual physical dimensions and reference chart seems like it would make more sense.

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These kinds of considerations are super important for communication so yeh, why didn’t I think of that!

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and…done

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Personally, I would try to get stem lenght in the equasion. Especially when it comes to reach.
And rather than head angle, I would consider trail. Trail influences steering (the most), and is the result of a given head angle in combination with a given fork offset (and other factors like tire width, which also translates into height).

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  • And Handlebar Reach may well be relevant too. There can be 20mm difference in final hood position between bars (70-90mm reach range on many). All part of trying to get to a an reach value to where the rider is actually positioned.

Whoa, this is amazing.

On the OT I would say generally relative to a road bike: slacker + taller headtubes, longer wheelbase and FC, lower BB, shorter stem (wider bars to compensate).

If I had $ to play with I’d love to try a Cervelo Aspero and also a Ritchey Outback as from reading reviews they seem like quite different but fun bikes in their own way. As it is, I have about 9k kms on a Salsa Warbird (second gen) and before that about 8k on an old Kona Jake which was decidedly racier. I think for my riding style I actually prefer the “CX” geometry for gravel (I’d rather ride my hardtail if the terrain gets really nasty.) But then I haven’t ridden each side by side on the same terrain to really compare - and I use and enjoy 40mm tires now which would not clear on older CX bikes.

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Actually, I think the logic is the other way around: a wider bar means more reach, hence a shorter stem. (But you can only use a short stem if you have enough trail for the type of riding, and also for the stem lenght.)