Could someone please educate me on gravel bike stability and geometry?

In mountain biking, I was always told that trail (function of HTA, fork offset, and wheel/tire size)is the key measurement to look at when determining how a bike is going to behave at speed, how well it’s going to hold it’s line, etc, etc.

However (I probably have this wrong) trail isn’t given as a huge factor in gravel bike performance. For gravel I’m seeing people talk more about the BB drop (while the HTA is slightly slacker than a road bike) providing the stability and ‘planted’ feeling that you want when zooming down a rutted forest service road at 35mph.

So I’m a little confused at what to look for in a gravel bike. Is there a reason to only think about trail on an MTB and to focus on BB drop for gravel?

Thanks,
-Confused in the PNW

Check this out.

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Thanks, that’s perfect.

There is another thing:

A lot of details about geometry and how the bike behaves.

These are both excellent resources, thanks @jarsson and @zo541 for posting. A couple things that stand out to me after reading:

  1. Both trail and BB drop influence stability, but trail also affects steering.

  2. Jordan (first link above) and Tom (second link) disagree on BB drop. Jordan thinks gravel bikes should have a low BB for stability and Tom thinks they should have a high BB for “better balancing”. This tells me there is no ‘right’ answer here.

  3. Tom’s belief that ‘slow’ bikes have a higher BB and ‘fast’ bikes go lower is interesting. Neither Specialized (Shiv vs Diverge) nor Cervelo (P5 vs Aspero) follow that, but Trek (Speed Concept vs Checkpoint) does.

  4. Both agree that a few mm either way can make a big difference, both in terms of trail and BB drop.

Now for my next question: If the extra stability from a lower BB (more BB drop) comes from lowering the center of mass of the entire system, would putting a dropper post on basically negate any worries about BB drop? I mean, the differences in BB drop on these bikes is ~10mm. That pales in comparison to even a small 100mm dropper. Using a 100mm post probably doesn’t mean you’re going to drop your center of mass by the full amount, but even if you only lowered yourself 50mm, would that basically erase any differences in BB drop?

Not necessarily…first, a lower BB promotes a lower CG all the time, not just when you activate the dropper post. It is not just about descents and technical stuff, but also in turns, etc. In fact, for me, it is more about cornering stability than descending / technical stuff.

Also (and this will probably generate some discussion / debate), but it is my thought that there is an aspect of where the CG is relative to the axles. A lower BB will always allow you to have a higher portion of the weight below this line, enhancing stability. With a dropper post, you’ll still have a higher percentage of your weight above the axle line…

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One thing to keep in mind with trail:

One thing to note here in reference to the first article: larger tires will increase trail. This is important because very small differences in trail make a big difference on handling.

Especially important if you are talking about using say a cyclocross bike that is optimized for 33mm tires. I personally hate the way my CX bike rides with 700x42mm tires between the increase in BB height and increase in trail but it does make the bike more stable at speed.

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Yes great point. I was thinking about all this over the weekend while doing a gravel race. All I’ve got is my Boone. Max is 33 in the rear but I managed to get a 40mm on the front. I definitely like how it rolls, but I’ve noticed a big difference in technical climbing. I wish companies would include tire size in the geometry chart. I always assume it’s the size the bike is spec-ed with.

Hope to see you at some races this fall Hai!!

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Yep, in particular the quick dive in you get used to with a cx bike, I almost get nervous for that split second where I feel like the bike isn’t responding when running bigger tires and I’m trying to take a tight line on single track.

#crossiscoming Colin!

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I believe it was you on another thread talking about recently getting an aspero and loving the low BB? (Sorry if I have that wrong). Which fork setting are you running, 46 or 51mm offset?

Yup, that was me…running it in forward configuration (51mm). May try and flip it around at some point, but probably not until after The Rift in a few weeks.

Been very happy with it in this configuration, though…

Let me know how you like the fork flip

As I research this more and more the Aspero is looking better and better to me. With the ability to change fork offset and hence trail, you could really tailor the ride to the course without resorting to super narrow/wide tires to get your desired trail number. Have a race that’s going to be won or lost on a technical climb? Set the fork at 51mm offset, decrease the trail, and get sharper handling. Have a race with a long technical/fast descent? Go the other way, but you get the benefits of the low BB all the time. Cool.

It is a really nice bike. I own that and a Ridley Kanzo fast and aspero is better in my opinion