Frame geometry - BB height, trail opinions

23 replies · 1,440 views

pushstart
181 posts
#1Nov 1, 2020, 8:28 PMedited
Hi folks -- I'm working on updating my titanium gravel frame (designing a new frame) and wanted to poll folks here to get some differing perspectives on a couple of geometry measurements that I find intriguing.

I specified my current frame to be pretty similar to the Habanero "Cyclocross/Touring" frame that it replaced. This was really at the beginning of the gravel craze, but lo-and-behold that frame geometry lent itself perfectly to this new trend. Relevant handling numbers include 430mm chain stays, a 78mm BB drop, and a 72.5º HTA.

The trail calculator suggests I'm running between 64mm and 66mm of trail with my Niner fork with 45mm offset on 38-44mm tires. (And I notice the difference; maybe there's some threshold right there, but the 64mm is noticeably more neutral than the 66mm.)

For this next frame, I'm updating a few things that have since become standards or more widely available (e.g. switch to flat-mount brakes, T47 BB, full internal cable routing), but I'm also changing the attitude a bit -- targeting larger tires (clearance for 700x50 and plan to run at least 45mm), a more off-road capable machine. E.g. internal dropper post routing and I might do the ratchet mod for 1x12 SRAM Eagle mechanical build. So not the sort of bike I'd use to try to hang on a road group ride (whenever we can do those again), but recognize that a lot of gravel is pretty road-like so ultimately it should be a bike that can behave well on pavement.

I've read a lot of opinions and looked at geometry for lots of frames; the vast majority out there seem to run quite high bottom brackets. 70-75mm of BB drop seems to be the standard. I'm concluding that they must be doing this because most of these frames also seem to be targeting interchanging 700c and 650b wheels. And, indeed, more than 75mm of BB drop on a 650b wheel (with 48mm-53mm tires) is getting low. But if I want to run 700x40mm+ tires on a gravel bike, I'd need a 27.5x2.4" tire to match that ride height ... and I don't even know of a 395-400mm carbon fork that can clear that. So 650B is not a consideration here. And seems like it would always be a weird compromise for anything that wasn't a road+ bike.

So my instinct was to do 80mm of drop. Or even more. But I'm wondering if anyone that has a frame with more drop (and runs larger-diameter wheels/tires) has any opinions one way or another on this. E.g. is pedal strike a significant problem when venturing off-road (as much off-road as you'd want to do on a drop-bar bike)? I know the Diverge has 85mm(!) BB drop and anecdotally I don't hear of people complaining they can't take it off road. On the other hand, the Gorilla Monsoon (which *does* clear those big 27.5x2.4" tires) still has a high BB with only 71.5mm BB drop. Also, I'm not entirely sure it matters (or, rather, one gets used to it and either high or low BB ends up being just fine). Maybe that's why this isn't something that seems to come up in discussion much.

I certainly have to be aware of pedal strike on my bike with 78mm drop with 38mm tires, but I wouldn't say I find it a problem.

And, finally, the next frame is built around a fork with 50mm offset Slackening the HT to 72º gives me the same trail range with larger (46-50mm) tires. I'm debating slackening the head tube to 71.5º as I imagine that could be nice when pointing the bike downhill (especially through anything technical), but haven't ridden a drop-bar bike with larger trail numbers and not sure I really want to mess with that. My modern hard-tail MTB has quite slack HT and must have massive trail numbers, but that's a very different application and I wouldn't want my gravel bike to wander so much when climbing.

Anyway, figured there'd be others on here that have given this a lot of thought and reached their own conclusions that they'd want to share. Having only room for a few bikes of my own [the damned kids!], I could benefit from the experiences of others.
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Volsung
878 posts
#2Nov 1, 2020, 8:36 PM
You dont have to design it. Sounds like you're describing a Nordest Albarda.
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pushstart
181 posts
#3Nov 2, 2020, 5:43 AM
Interesting, not sure I saw that frame. So that seems much slacker HT than I'd want and shorter CS (I want keep tube chain stays for internal routing, and I like the longer rear end for stability). I also need a bigger frame than their large.

But, yes, that does have a big BB drop and does have the clearance I'm looking for. Thanks for mentioning that.
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shiggy
404 posts
#4Nov 2, 2020, 8:16 AMedited
I ride dropbars on everything, everywhere.

Most important is the weight distribution, especially on the front, but also the ability to easily make weight shifts as needed. Many years ago I managed to figure out what that is for me. It is far from the current fashions in geometry. My position behind the front axle is the same for all my bikes, road, unpaved, MTB, but the reach and bar height varies. Generally, the frame reach ends up longer than common on production road and gravel bikes, and shorter on my MTBs.
For trail, I like 65-75mm off-road, unloaded riding.
On road and for front loads I prefer low trail geometry, 35-45mm. This keeps the steering crisp with the weight and is still stable.
Also remember that larger/heavier tires and wheels slow and stabilize the steering for a given trail.

IME BB drop/ht is most important for pedal clearance. My MTBs have very little drop. My 26” wheel hardtail has ZERO drop. My 29” bikes 30-45mm of drop.
I built my road/unpaved bike with 80mm drop and 180mm cranks, designed to use 650Bx48 tires with fenders. Have used up to 27.5x2.25 without fenders on trails. Yes, clearance is an issue, but doable.
Mostly I run 650Bx38 or 42 tires on the road. Can’t pedal through ANY turn. Advantage: I can put a foot on the ground without getting off the saddle. That’s it. I notice no difference in stability or cornering ability over a high BB.

No one number makes or breaks a bike. It whole package is what matters. It has to work together, and work for the rider.
My frame designs all start with my hand position, which then sets the front axle. Set my target for trail. My stem length (usually 100-120mm).
Then every other dimension is what it takes to connect the dots.

Chainstay length: I never go for “as short as possible”. I want clearance for tire/crankset/FD, a good chain angle, and a bit more stability at speed. “Long” stays make little difference for climbing or responsiveness for me, especially for taller riders.
2
smoothmoose
210 posts
#5Nov 2, 2020, 2:25 PMedited
pushstart wrote:
And, finally, the next frame is built around a fork with 50mm offset Slackening the HT to 72º gives me the same trail range with larger (46-50mm) tires. I'm debating slackening the head tube to 71.5º as I imagine that could be nice when pointing the bike downhill (especially through anything technical), but haven't ridden a drop-bar bike with larger trail numbers and not sure I really want to mess with that. My modern hard-tail MTB has quite slack HT and must have massive trail numbers, but that's a very different application and I wouldn't want my gravel bike to wander so much when climbing.

Also toe overlap if you are running 700x50+. I'm small size and get toe overlap with 27.5x2.2. Slacken the HTA will help. If you want to keep snappier steering you can go for more fork offset maybe 55mm to reduce the trail measurement and that will also help with toe overlap.
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JWB475
154 posts
#6Nov 2, 2020, 8:12 PM
My Ti bike has 70mm BB drop (Why Cycles R+), and I occasionally take it on singletrack with 27.5" x 2.1" tires. Normally I ride 700 x 44mm tires on my gravel rides. The 27.5" tires are almost the same diameter, but not quite and lower my BB by ~5mm..

On singletrack I have to be pretty cautious and choose my line carefully to avoid pedal strikes. I've gotten much better at reading the terrain and knowing when I can and cannot pedal. But early on I speared the ground and almost got bucked off my bike.

Since 650B isn't a consideration, you can probably get by with more BB Drop, but if you intend to ride singletrack I wouldn't go much lower than 70...
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pushstart
181 posts
#7Nov 2, 2020, 10:10 PM
Thanks to everyone. This is great stuff and just the sort of perspective I was hoping for. Good call @smoothmoose on checking the pedal clearance measurements. I do have a little overlap when I run fenders w/44mm tires on my current bike, so I should make sure that I won't have an issue with that.

That's a good data point, @shiggy, that you don't find the lower BB more stable. I have been testing some gravel bikes with higher BB and I notice it when I first start riding, but you're right that once I get used to the fact that I have to get off the saddle at a light, I don't know that I find the lower bike more confident either. Also, with the fact that I plan to fit a dropper post, the 5-10mm of drop is probably far less significant. I'd definitely use the dropper.

I do like taking my bike off-road, so probably sticking with a higher BB makes sense.
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chas
1,305 posts
#8Nov 5, 2020, 7:27 AM
I agree with shigging on the BB being over rated. Personally (on pavement) I love being able to power out of a turn before anyone else does, and I really, really hate pedal strike (because it leads to nasty crashes). I echo his comments about mountain bikes having high BB drop.

Also - going to 650b is typically going to lower your BB height, but I never hear anyone saying - OMG, the bike is so much more stable with 650b (or less stable with 700c).
1
boots2000
26 posts
#9Nov 5, 2020, 8:37 AM
Are you working with a builder? Or doing a DIY frame? Often times the frame builder has lots of good input on these things.
Question- What is the general size of the bike? Small or large?

Depending on where you live you might want an even slacker head angle. I have been riding bikes with a 71 degree head angle and 50-55mm of fork rake and the ride great.
My bikes are similar to about a 55cm frame.

One of the things I have done on my custom is a longer toptube. By going longer I get more front center and this reduces toe clip overlap when running 700c wheels.
I go 1cm longer in the tt, 1cm less in the stem.
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chas
1,305 posts
#10Nov 5, 2020, 2:34 PM

One of the things I have done on my custom is a longer toptube. By going longer I get more front center and this reduces toe clip overlap when running 700c wheels.
I go 1cm longer in the tt, 1cm less in the stem.
.

Canyon did that with my Inflite. Never had a drop bar with a 70mm stem before, but they figured if it worked for mountain bikes, it could add more stability to a gravel bike.

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pushstart
181 posts
#11Nov 5, 2020, 9:27 PMedited
I'm not working with a builder; I'm working with a fabricator, specifically Waltly. I've worked with them on a bunch of different frames now and enjoy this part of the process a lot.

So my frame size is 59cm ETT, or somewhere around 410mm reach. So a size-large-ish frame.

After some back and forth, I've tamed the bike a bit and opted to keep the supported tire-clearance numbers more moderate (700x45, probably fitting 700x48 -- but not the 2.0" originally envisioned) so I can keep the cable routing internal through the T47 BB -- avoiding extra holes and zip-tie mounts for RD housing under plate-type DS chainstay. (This also makes Di2 a more elegant option.)

My geometry right now is looking like the image below. This is mirroring my current bike fit and ensuring no toe overlap and neutral trail numbers with larger 700x44 tires. A little higher BB than current bike given comments above though kept it low enough to touch ground with my toes. No intention of 650B here, so I think this is a good drop number to support gravel-bike-level off-road.

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chas
1,305 posts
#12Nov 6, 2020, 7:27 AM
Re: BB drop in your original post: I don't think this will change your numbers, but keep in mind that bb drop of 75mm on a road bike (designed for 23mm tires) and a gravel bike (designed for 44mm tires) is going to have a significantly difference BB height (even with the same "drop"). Food for thought, but looks like you are fine.

When I road the original Diverge, I thought the BB drop made the bike feel a little sluggish, and put the pedals too close to the road when I turned (but that's just me - I ended up with 68mm drop and no pedal strike).

What length stem are you using? What is your cockpit length (stem to seat tube)? To me, the cockpit length is the most critical for my fit & comfort (many other measurements are adjustable).

How small a tire will you run? I find I can change my trail by about 10mm by changing tires, so a neutral trail with 30mm tires gives me additional stability when I get closer to 50mm. Sometimes I run 10mm larger tires up front, to give me more trail, more flotation, better gravel steering, a smoother ride, and 'cause I can.

I find that a road bike trail is wonderful on pavement as the bike can dive into a turn and carve pretty hard, but off road I want the bike to be smooth and relaxed. I did dive into a turn once on gravel and it did not end well. ;-)
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shiggy
404 posts
#13Nov 6, 2020, 7:59 AMedited
For a given STA and chainstay length (horizontal), increasing the BB drop decreases tire clearance at the seat tube.
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pushstart
181 posts
#14Nov 6, 2020, 1:14 PM
@chas, I love thinking about this, so appreciate you brining up these points. Yeah, the smallest tire I'd put on here is 700x38, but really focused on 700x40-45 area, so BB drop was with that in mind. I think will be pretty comfortable. I have more drop on my current gravel frame (78mm) and don't have any real issues w/ riding singletrack or gravel races.

I'm using a 100mm stem. I like that right now, though I could be convinced to go to a 90mm. I don't have any toe overlap issues with this as designed around 59cm, though. I run a shorter frame on my road bike (575mm ETT) w/ 120mm stem. Stem to seat tube is 590mm if I'm understanding you right (i.e. ETT). The "quick fit" saddle tip to bars (angled) measurement is 568mm. STA is 73.5 due to my short femurs (my body likes knee over pedal spindle).

@shiggy, yeah, that makes perfect sense. I lengthened the (non-horiz) stays to 435 to make sure plenty of room there. Though comparing to my current gravel bike, even with 430mm stays (stay length -- non-horiz), 78mm drop, and 44mm tires, I've got a good 10mm of clearance to seat tube. So this should be plenty of room. Probably will make a full fender more practical on larger tires.

I appreciate all the feedback and look forward to building this up and cross-comparing it.
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shiggy
404 posts
#15Nov 6, 2020, 4:33 PM
10mm clearance is tight if you want to use fenders
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pushstart
181 posts
#16Nov 6, 2020, 8:54 PM
shiggy wrote:10mm clearance is tight if you want to use fenders

Yeah, that was just example on my current frame. New frame has longer stays and higher BB so probably more like 2cm. (I will run smaller tires if I want to use full fenders.)
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rosey4
3 posts
#17Dec 5, 2020, 12:57 PM

I had a similar thought process to you, I think- designing a Ti frame to be built by Waltly and wanted to try several features. I ride a 61cm Hakka MX and love it, but always think when I'm riding that I wish it was a little longer, lower, slacker- I'm 6'3' and inflexible, so there's 30mm spacer under the stem, which I don't like. So I made the mutt by taking the Hakka geo and stretching it out just a bit in every direction. Also made the BB drop at 80 to lower overall center of mass. Not sure if the image will show up, but it's 60cm tt, 74st, 71ht, 435rc and 671fc with the 80mm BB drop. With a 50mm offset fork trail will be about 68mm on a 45 tire. For the janky logging roads and smooth singletrack I ride here in Oregon it should handle pretty good and still be OK on the pavement. I ordered it in October so I expect it by the end of the year. Waltly's been fine to deal with so far, but I have yet to see product. I'm not sure if the cumulative effects of my design will result in something wacky, but what the heck- it's a fun experiment.
Have you ordered yours, and what is the final design? It's interesting that your design is almost exactly like the 61cm Hakka- just from that datapoint alone it should be a really fun riding bike. Good luck with your project-

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stu
646 posts
#18Dec 5, 2020, 3:04 PM
I think if you're planning to spend the money for a titanium frame you may as well get the geometry customized for your needs. A few years ago I looked around and decided the Lynskey Backroad was the frame closest to my ideal. However since it was designed for long distance touring it was taller than I find comfortable. So I ordered one size smaller than recommended with the top tube made 5 cm longer. The ordering process was straightforward and the result suits me perfectly. I happen to love the stiffness and stability of this frame but others might not like the extra weight or reduced responsiveness.
0
pushstart
181 posts
#19Dec 6, 2020, 8:39 PM
rosey4 wrote:
I had a similar thought process to you, I think- designing a Ti frame to be built by Waltly and wanted to try several features. I ride a 61cm Hakka MX and love it, but always think when I'm riding that I wish it was a little longer, lower, slacker- I'm 6'3' and inflexible, so there's 30mm spacer under the stem, which I don't like. So I made the mutt by taking the Hakka geo and stretching it out just a bit in every direction. Also made the BB drop at 80 to lower overall center of mass. Not sure if the image will show up, but it's 60cm tt, 74st, 71ht, 435rc and 671fc with the 80mm BB drop. With a 50mm offset fork trail will be about 68mm on a 45 tire.

This is fantastic info / feedback and exactly the sort of thing I was hoping this thread would lead to 🙂 So I ended up compromising a bit and decreasing tire clearance in the end (will probably fit 700x48 at the very limits, but more comfortable with 700x43-45). Because I was targeting smaller tires but still wanted to focus on off-road, I did actually lower the drop to 75mm (i.e. raise the BB vs. what I was originally thinking). Trying to tease out the character for this bike, but given that it'll have internal dropper routing I didn't want to preclude it working well on rougher stuff.

I think the 80mm drop you spec'd is probably the right number. But I also suspect it this number won't make or break the bike.

Thanks for pointing out the Hakka. I have a couple of friends with this bike, but for some reason when I think "gravel bike geometry" this is never on the list of bikes to pull up to reference. It does indeed look like what I ended up with was very close to the geometry of the Hakka (a little lower BB, a little longer chain stay), but otherwise very close.

I'll update with the results of the now-underway build. I decided to try out a wider T47 shell (86.5mm) with hopes/expectation that this will add a little extra room for the internal routing in the chain stays. The tire clearance was a compromise to stick with tube chain stays (vs. switching to a plate on the drive side) so I can keep RD housing (or Di2 wiring) in the frame.

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Involuntarysoul
76 posts
#20Dec 6, 2020, 8:50 PM
stu wrote:I think if you're planning to spend the money for a titanium frame you may as well get the geometry customized for your needs. A few years ago I looked around and decided the Lynskey Backroad was the frame closest to my ideal. However since it was designed for long distance touring it was taller than I find comfortable. So I ordered one size smaller than recommended with the top tube made 5 cm longer. The ordering process was straightforward and the result suits me perfectly. I happen to love the stiffness and stability of this frame but others might not like the extra weight or reduced responsiveness.

off the shelf Lynskey are build like touring bikes because they cannot control who are buying the frame and have to assume the rider weight 300lb+ therefore they have to use strongest tubing everywhere, the bike in the end feels dead and super heavy
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