All-road/Gravel bike custom geometry help

[snguyen268]

Hi all,

I am designing a custom all-road/gravel Ti bike and would love some of your opinions on a couple things.

1. Which of the 2 attached geometries is more in-line with what I need? My intention for the design is to have it optimized for road/rough road/maintained dirt road, do well on rough gravel/fire roads, and somewhat capable of getting me through some single-track. In other words, toward a road bike with wide tire as opposed to a drop-bar MTB. What I am trying to achieve here is a geometry that will not feel sluggish on road, especially on climbs while still give me confidence off-road. My question is wheelbase length or steering more important for "stability", especially for off-road? I understand stability is achieved by a combination of those factors and more but I am just trying to decouple their effect. Also, is there a sweet spot of trail figure I should aim for? Currently thinking 59-65mm. I am planning to run 35-38mm on a regular basis (currently trail figures are calculated based on 38mm) but do plan to go up as terrain requires (frame is designed to clear 700c x 45mm or 650b x 53).

2. T47 vs. BSA BB? I am running Di2, which precludes the ability to use 30mm BSA BBs (not enough room for cable routing according to my understanding). Should I go for T47 to have maximum versatility? Any other downsides besides slightly heavier BB and not as common as BSA? I am getting a new crankset for this build as well (my fitter recommends going down to 165mm) and there seems too be more options for 30mm cranks so maybe it's a good idea to not limit myself to 24mm spindle? Are 30mm cranks like Rotor ALDHU worth the extra $$$ over 24mm cranks like Ultegra 8100?

Thanks all for taking your time!

[Hilltopperny]

I think the first geometry chart is closer to a road bike than the second, but the second looks to have more toe clearance and slightly slacker head tube angle.

I personally like a more neutral feel for my off road bikes. I have had aggressive geometry fat tire road bikes that did pretty well over most of the terrain you are trying to cover, but they were not as good over gnarlier terrain or single track.

My latest build has a 71.5 HT angle and 74 ST angle and a dropped stay to fit a fat tire while maintaining a 43cm chain stay and fitting a 2.1" 650b knobby with good clearance. I did The Black Fly Challenge on this set up and it handled everything pretty well, but I am not much of a climber at 220lbs. My neck hurt coming down the single-track at the end, but the chunky tires and bike held their line through the mud and tracked better then my more road oriented bikes in that section. It also did better in the sand and in general over the bigger rock strewn sections.

My Kirk Terraplane is a road bike with decent clearances. With 32mm tires I can do everything described above with the exception of really sandy and bigger rocks. It does everything really well, but struggles on the actual spots where a more mtb oriented gravel bike would get you through. Still a very capable bike if you are comfortable riding 32ish tire. The bike just eats up washboard and stays planted! If it had room for more tire it would likely track well over the sandy and larger rock strewn stuff as well. Geometry is attached.



Sent from my SM-S127DL using Tapatalk

[tomato coupe]

I’d go with the second design— it’s always nice to have a third water bottle cage.

[CAAD]

I would go with the first one. Also, the third bottle cage is just a bottle launcher and will get caked in dust and dirt. For my purpose, I don't see the need for super slack gravel geometry. My bike handles just fine and it's essentially a road bike with clearance for larger tires. I have taken that thing down some nasty dirt/gravel roads at speeds 30+ and technical chunky gravel.

[NHAero]

I don't think the two designs are much different, and I doubt you could tell which is which blindfolded. Both make sense to me for an all road, not too slack and not too steep.

I have the 3rd bottle cage on my Bingham and am very glad of it. I needed that much water on my 4 day VT trip last year. The under-DT bottle gets dirty and I just poured the water from it into one of the upper bottles when it got empty.

[tomato coupe]

Originally Posted by NHAero

I don't think the two designs are much different, and I doubt you could tell which is which blindfolded. Both make sense to me for an all road, not too slack and not too steep.

Yep.

I have the 3rd bottle cage on my Bingham and am very glad of it. I needed that much water on my 4 day VT trip last year. The under-DT bottle gets dirty and I just poured the water from it into one of the upper bottles when it got empty.

Yes, again. (And, you don’t lose bottles if you use good cages.)

[xterra_dan]

Personal preference, but when you are off-road you will appreciate the slightly longer front center with 1cm shorter stem of the 2nd design. Don’t think you will notice a difference in handling/steering on road, either.

[NHAero]

Originally Posted by tomato coupe

Yep.


Yes, again. (And, you don’t lose bottles if you use good cages.)

I was actually amazed at how well the King Iris cages held onto the Specialized Purist water bottles on my VT dirt road trip and at D2R2. Not the slightest hint of ejection.

[mstateglfr]

Just a couple of thoughts from seeing those drawings...

- bb drop is the same and is a good number for what I like on a gravel bike.
- chainstay length is a nice compromise- sweet spot for gravel tire size and comfort vs cutting sharp corner turns.
- the reach for both frames is crazy long compared to the stack height. 403mm reach and 542mm stack? That is a lot of reach, at least it seems to be a lot for an AR/gravel bike which you also want to rock on some singletrack.


This is a custom bike so I assume the stack and reach are set for you to fit it properly. I just wanted to point the extreme number out, in case that was an oversight or if it was just based on you moving other frame numbers around and didnt realize the resulting reach. A 542mm stack AR/gravel bike would have something closer to like 380mm reach give or take...I think. I am admittedly not confident in that as I only focus on the largest size frames since thats what I would ride, but again just mentioning it so you for sure see that and know its correct for you.

Trail is something everyone is different with. My gravel bike has 60mm of trail and I love it. I think it is nimble while not being twitchy. I think it is stable while not being sluggish.
I am not sure I would want less trail for a gravel bike- having some trail helps keep things stable on loose bumpy surfaces. But too much trail and the bike may feel pretty annoying on paved roads and especially climbing.
Not sure what tire size you plan to use, but make sure that tire size is referenced when looking at trail geometry since if you run trail numbers with a 35mm tire and use a 43mm tire, the actual trail will be different from what you expected during planning.

[robt57]

My take away from those is that one is a size up, the other a size down.

I'll say if it will spend more time off pavement than on, the longer WB be my choice.

I have a rule on trail, not over 60CM [50-52 road, 65-6 off]. But like similar comment above, I ride a 400/600 reach/stac, from a bigger riders eye is how I see geom.

I also have a HTA rule of 72.5 min. But that is in line @ 72 with smaller frame than I ride. I also tend to like faster steering front ends, being 10 years of MotoCX racing when younger has a well developed gyroscope in my balance center/brain. Which thankfully @ 66 yrs old hasn't diminished that I can notice.

Think Pidcok descending or Sagan bunny hopping of fallen riders and staying up. You either have that or or don't the way I see it.

[John H.]

I would go with the 2nd one. In fact, my all-road type gravel bike is very close to that one.
1st reason is head angle. 71.7 will play better off-road but will not be troublesome on the road.
2nd reason is front center. If you have decent sized feet, design 1 is likely to have toe overlap. Can come into play more often than on the road. Think uphill switchbacks. Not a deal breaker, but can be annoying.

My suggestion for Shimano is BSA. Lots of good BB's exist. Sounds like you are going for internal hose routing? My best suggestion for this is to have the builder use 2 holes in the stay, have the hose exit near the bb, pass externally under the bb shell and then back into the frame one the lower side of the downtube. In practice this is perfectly clean and you will never see it. DI2 wires will easily find their way around a BB. Fully internal with the hose passing through the BB shell is a pain. Less so with 30mm, but still a pain.

[robt57]

Originally Posted by John H.

Sounds like you are going for internal hose routing? My best suggestion for this is to have the builder use 2 holes in the stay, have the hose exit near the bb, pass externally under the bb shell and then back into the frame one the lower side of the downtube. In practice this is perfectly clean and you will never see it. DI2 wires will easily find their way around a BB. Fully internal with the hose passing through the BB shell is a pain. Less so with 30mm, but still a pain.

This make me remember a Strong frame that was resold here earlier this year due to locations of these holes and business under BB not being OK with PL user who had the frame made.

Di2 certainly will simplify this under BB with only the hydro line in play [under BB] on Di2 bike.

Just a data point from my own PL observations.

[snguyen268]

Thanks everyone for your generosity in sharing your opinions. Those are super helpful.

Originally Posted by mstateglfr

Just a couple of thoughts from seeing those drawings...

- bb drop is the same and is a good number for what I like on a gravel bike.
- chainstay length is a nice compromise- sweet spot for gravel tire size and comfort vs cutting sharp corner turns.
- the reach for both frames is crazy long compared to the stack height. 403mm reach and 542mm stack? That is a lot of reach, at least it seems to be a lot for an AR/gravel bike which you also want to rock on some singletrack.


This is a custom bike so I assume the stack and reach are set for you to fit it properly. I just wanted to point the extreme number out, in case that was an oversight or if it was just based on you moving other frame numbers around and didnt realize the resulting reach. A 542mm stack AR/gravel bike would have something closer to like 380mm reach give or take...I think. I am admittedly not confident in that as I only focus on the largest size frames since thats what I would ride, but again just mentioning it so you for sure see that and know its correct for you.

Trail is something everyone is different with. My gravel bike has 60mm of trail and I love it. I think it is nimble while not being twitchy. I think it is stable while not being sluggish.
I am not sure I would want less trail for a gravel bike- having some trail helps keep things stable on loose bumpy surfaces. But too much trail and the bike may feel pretty annoying on paved roads and especially climbing.
Not sure what tire size you plan to use, but make sure that tire size is referenced when looking at trail geometry since if you run trail numbers with a 35mm tire and use a 43mm tire, the actual trail will be different from what you expected during planning.

Re:trail figure. That makes sense. I am planning to run 35-38mm on regular basis and might go above 40mm when terrain requires. Hence, I want to take into account the whole range of wheel/tire combos I will ride in designing HT angle + fork rake combo. For now, I think I am around 61mm-65mm for 700x35-45mm with the "slacker" geometry. So hopefully I am striking a decent balance? I don't mind the steering be a tad "slower" on road if it means I get a bit more stability off-road (my road bike currently is at 66mm trail with 28mm tire so anything below that will probably still feel nimble for me).

Thanks so much for pointing out about the stack/reach. It definitely seems extreme to me as well (based on my research comparing these designs with stock-geometry race gravel bikes). Those numbers came from my current fit on my road bike where reach is 378mm (running 120mm stem) and stack is 532mm (stem is slammed on 15mm top cap). According to my fitter, I have relatively long arms, can rotate my pelvis forward and keeping my back flat so he thought longer reach/lower stack works very well for me. Do you think it's a good idea to reduce these numbers even though they feel comfortable for me on the road currently?

Originally Posted by John H.

I would go with the 2nd one. In fact, my all-road type gravel bike is very close to that one.
1st reason is head angle. 71.7 will play better off-road but will not be troublesome on the road.
2nd reason is front center. If you have decent sized feet, design 1 is likely to have toe overlap. Can come into play more often than on the road. Think uphill switchbacks. Not a deal breaker, but can be annoying.

My suggestion for Shimano is BSA. Lots of good BB's exist. Sounds like you are going for internal hose routing? My best suggestion for this is to have the builder use 2 holes in the stay, have the hose exit near the bb, pass externally under the bb shell and then back into the frame one the lower side of the downtube. In practice this is perfectly clean and you will never see it. DI2 wires will easily find their way around a BB. Fully internal with the hose passing through the BB shell is a pain. Less so with 30mm, but still a pain.

Awesome!!! Thanks so much for your input. How do you like your all-road bike on road/off-road? Anything you wish you would do differently?

So my frame will have the cable routing strategy like you described where hydraulic cable exits before BB and goes external along chainstay. However, according to my frame builder, BSA will not allow enough room for di2 cable + 30mm axle crank?

Originally Posted by robt57

My take away from those is that one is a size up, the other a size down.

I'll say if it will spend more time off pavement than on, the longer WB be my choice.

I have a rule on trail, not over 60CM [50-52 road, 65-6 off]. But like similar comment above, I ride a 400/600 reach/stac, from a bigger riders eye is how I see geom.

I also have a HTA rule of 72.5 min. But that is in line @ 72 with smaller frame than I ride. I also tend to like faster steering front ends, being 10 years of MotoCX racing when younger has a well developed gyroscope in my balance center/brain. Which thankfully @ 66 yrs old hasn't diminished that I can notice.

Think Pidcok descending or Sagan bunny hopping of fallen riders and staying up. You either have that or or don't the way I see it.

Thanks so much for your insights! Having MotoCX experience is definitely a huge plus. Though I am confident in my bike handling skills, I definitely need a bit more help from the bike when I go off-road as I don't have any MTB or cyclocross background. Do you think trail in the range of 61-65mm is reasonable for an all-road/gravel bike?

Originally Posted by CAAD

I would go with the first one. Also, the third bottle cage is just a bottle launcher and will get caked in dust and dirt. For my purpose, I don't see the need for super slack gravel geometry. My bike handles just fine and it's essentially a road bike with clearance for larger tires. I have taken that thing down some nasty dirt/gravel roads at speeds 30+ and technical chunky gravel.

Thanks so much! Have you found a terrain where you feel a bit "underbike"? What tire width do you often runs on your bike? I plan to do 35-38mm for most of my riding.

[robt57]

Originally Posted by snguyen268

Thanks everyone for your generosity in sharing your opinions. Those are super helpful.


Do you think trail in the range of 61-65mm is reasonable for an all-road/gravel bike?

I do. I probably lean different than most on front ends. For me, I'll take lower turn in stability over flop. But some pilots don't notice any of this. It just take a bit more intuitive CG positioning on the bike with lower trail when you hit soft stuff, especially into soft turns IMO.

In more extreme terminology, I call 70+ trail choppers. When I got a disc CX I use on gravel and as Allroad, I went Boone over Crux. Crux more choppery to me. Better or off road maybe, not for on IMO.
But as I said, my GyroScope plays a part. So my comments should be taken as subjective.

Off road, IMO if you are going to stay seated or can't move around a lot to keep VG centered comfortably, I'd say the more choppery/lax front end = stability.

[fredd]

As for the crankset, I'd go with Ultegra over the Rotor. Yes the Rotor is lighter and arguably looks nicer, but Shimano rings shift significantly better than Rotor ones IME. And Rotor's 4 bolt spider won't really take Shimano rings, which is deeply annoying.