All-road/Gravel bike custom geometry help

[Spoker]

I'm in the lower BB drop category.
Since you're running 165 mm cranks you can already go lower by 5 mm.
Design for the road tire, and when you go to rough terrain the bigger tire will raise it.

[.RJ]

I dont see T47 going away and will future proof against just about any crankset that you'd ever want to use. If you want to run a rear brake cable internally you will need a T47 BB to get the clearance you need.

BSA may restrict your choices in the future. How? Who knows. I went T47 on a custom steel road frame, and I wish I'd done BSA - the internal routing has not proved really necessary and aesthetically the BSA would have looked better and I didnt consider that when making my choices.

[Wunder]

Originally Posted by snguyen268

Getting closer to getting all the parts for my build and trying to make up my mind on some decisions. Hope you guys have some insights to share

1. On drive train selection, I am pretty set on doing a SRAM 1x mullet with 46T + 10-52T using the SRAM Transmission. The rationale here is I calculated gear ratio of my current setup on my road bike to figure out what are my most heavily used gear ratios for specific terrains and found that 46T + 10-52T combo will offer all of my favorite gear ratios and some more on the low end, which is exactly what I am looking for. Is there anything else I should consider? Anyone has experience with using SRAM Transmission yet on a gravel/road bike? If so, what are your thoughts? If 1x MTB mullet is a bad idea, what alternative system I should look into?

2. I am still back and forth about whether it is a good or bad idea to go with T47 BB (over BSA)? T47 seems to gain popularity recently so maybe it's here to stay for a long time? Since BB is not a changeable part of the frame, I just hope to go with one that is relatively "futureproof"

Thanks a bunch!

46/10 is a HUGE gear, are you sure you need that much? My gravel bike runs SRAM 11 speed with a 42T and 10-42, that yields 27-114 gear inches on 35mm tires (even higher if using 42 or 50mm). I've considered dropping the ring to a 38 or 36 if I was riding a very hilly course.

I personally like BSA, it's been around a long time and works well. Tons of crank options.

[.RJ]

If you dont mind doing cable shift, you can easily get a 10-46 with Sram 11 speed, too.

[snguyen268]

Originally Posted by Spoker

I'm in the lower BB drop category.
Since you're running 165 mm cranks you can already go lower by 5 mm.
Design for the road tire, and when you go to rough terrain the bigger tire will raise it.

That makes sense! I will definitely consider that.

Originally Posted by .RJ

I dont see T47 going away and will future proof against just about any crankset that you'd ever want to use. If you want to run a rear brake cable internally you will need a T47 BB to get the clearance you need.

BSA may restrict your choices in the future. How? Who knows. I went T47 on a custom steel road frame, and I wish I'd done BSA - the internal routing has not proved really necessary and aesthetically the BSA would have looked better and I didnt consider that when making my choices.

I actually will not be running hydraulic hose through BB but around it externally. One thing I am safeguarding against is the ability to run di2 + 30mm crank, which seems to be too tight with BSA and makes me leaning toward T47. Of course, shimano might go completely wireless in the future and I am going to run SRAM for now, so that might be a moot point and I am just overthinking?

Originally Posted by Wunder

46/10 is a HUGE gear, are you sure you need that much? My gravel bike runs SRAM 11 speed with a 42T and 10-42, that yields 27-114 gear inches on 35mm tires (even higher if using 42 or 50mm). I've considered dropping the ring to a 38 or 36 if I was riding a very hilly course.

I personally like BSA, it's been around a long time and works well. Tons of crank options.

Thanks for the insights! I agree with you as I don't need that 46/10 combo. Reason I am thinking 46 is not really because of the 46/10 combo but because I am still going to be doing a lot of road riding and 46 gets me the closest to the 3 heavily used gear ratios on my current setup( which is 50/34 + 11-34). If I end up needing lower gears, it should be simple enough to get a smaller chainring and swap out (at least that is my current thinking). Not sure if that rationale makese sense? Or I should be thinking about it differently?

[.RJ]

Originally Posted by snguyen268

One thing I am safeguarding against is the ability to run di2 + 30mm crank, which seems to be too tight with BSA and makes me leaning toward T47. Of course, shimano might go completely wireless in the future and I am going to run SRAM for now, so that might be a moot point and I am just overthinking?

I wouldnt count on Shimano going full wireless in the future. With 12 speed just released it will be at least 5 years before we see a significant groupset redesign and they seem to really value long battery life which means battery inside the frame somewhere.

But if you're looking at Sram transmission, this is a non issue. I would base designs around broader bike standards rather than what one brand might do far off in to the future - i.e. I wouldnt build a drop bar bike with post mount brakes in 2024.

[NHAero]

Answering an earlier question:
I'd keep your wheels and spend the limited $ on components. The thinking here is that wheels are such an easy swap to make and there are always PL deals to be had

I think your rationale for the 46T ring choice to get the gears you know you ride in most makes sense. That's how I set up my gearing as well. It's more important to have close ratios in the middle of the range (for me) than at the high end of the range.

I chose BSA on my Bingham but was clear on not wanting to run the rear brake hose or cable housing in the BB. It pops out at the bottom of the DT and runs under the BB back to the caliper. I like not having to mess with a BB if I want to change cabling etc., just as I wouldn't want routing through a headset. Yes, I wear my Retrogrouch medallion proudly.

I would guess that T47 is more flexible in the future though - who knows what is next?

[snguyen268]

Originally Posted by .RJ

I wouldnt count on Shimano going full wireless in the future. With 12 speed just released it will be at least 5 years before we see a significant groupset redesign and they seem to really value long battery life which means battery inside the frame somewhere.

But if you're looking at Sram transmission, this is a non issue. I would base designs around broader bike standards rather than what one brand might do far off in to the future - i.e. I wouldnt build a drop bar bike with post mount brakes in 2024.

Totally fair. That's my rationale currently for leaning toward T47 given that seems to be the general standards on all-road bikes these days? Of course caveat being a lot of them are carbon frames with full internal routing while here I am doing a metal frame with external routing over BB.

[snguyen268]

Originally Posted by NHAero

Answering an earlier question:
I'd keep your wheels and spend the limited $ on components. The thinking here is that wheels are such an easy swap to make and there are always PL deals to be had

I think your rationale for the 46T ring choice to get the gears you know you ride in most makes sense. That's how I set up my gearing as well. It's more important to have close ratios in the middle of the range (for me) than at the high end of the range.

I chose BSA on my Bingham but was clear on not wanting to run the rear brake hose or cable housing in the BB. It pops out at the bottom of the DT and runs under the BB back to the caliper. I like not having to mess with a BB if I want to change cabling etc., just as I wouldn't want routing through a headset. Yes, I wear my Retrogrouch medallion proudly.

I would guess that T47 is more flexible in the future though - who knows what is next?

Yah, that's my current thought as well. Gonna keep my wheels and ride them out for the next couple years then get a new pair later.

Your Bingham's routing scheme is similar to what I have in mind so it does make sense to go BSA. But I might the bullet and go T47 based on the comments here so far, might backfire (or not) in the future but who can really predict it I guess?

[.RJ]

Originally Posted by snguyen268

Totally fair. That's my rationale currently for leaning toward T47 given that seems to be the general standards on all-road bikes these days? Of course caveat being a lot of them are carbon frames with full internal routing while here I am doing a metal frame with external routing over BB.

My $0.02 on this is, if its steel, stick with BSA. Sram seems locked in on Dub and there's a BSA Dub BB.

[NHAero]

Originally Posted by snguyen268

Yah, that's my current thought as well. Gonna keep my wheels and ride them out for the next couple years then get a new pair later.

Your Bingham's routing scheme is similar to what I have in mind so it does make sense to go BSA. But I might the bullet and go T47 based on the comments here so far, might backfire (or not) in the future but who can really predict it I guess?

I'm not sure which "advances" have made for difficulties as frames get older. Certainly we see "wireless only" as a filter that knocks some potential buyers out. When thru axles came in, disc brake bikes with a QR interface still worked (I have one) but became less saleable in the used market. And getting new DB wheelsets that can be QR still is not a problem - three of mine are Bitex or DT Swiss, with easy end cap swaps.

[snguyen268]

Another thing that I have recently been thinking about is the "chainstay/max chainring constraint".

I see that modern gravel/all-road carbon bikes nowadays allow max chainring of 52T or even 54T (1x wide) while permitting 45mm tire clearance (the new Cervelo Aspero, for example). However, for this Ti frame I am waiting on, I originally thought 50T clearance should be feasible but it seems like with a 425mm chainstay/45mm tire clearance, the max is 46T and maybe 48T using 1x wide crankset and a chainstay yoke. My understanding is that I will have to compromise either chainstay length or max tire clearance in order to fit a >48T chainring. Now, I am not a pro racer or anything so 48T is plenty and I am not thinking about this question from a practicality perspective but more from a design perspective.

What is the biggest factor that limits the max chainring size, especially on metal tubing bikes? Is it chainstay length? Bottom bracket shell width? Or chainstay shape/thickness? How is this constraint often dealt with?

[.RJ]

Originally Posted by snguyen268

What is the biggest factor that limits the max chainring size, especially on metal tubing bikes? Is it chainstay length? Bottom bracket shell width? Or chainstay shape/thickness? How is this constraint often dealt with?

Mostly the chainstays (length, shape, yoke or not) and tire clearance.

Keep in mind that these 'wide' 1x groups have chainlines spaced out further and wider q-factors, so there is a compromise there too.

IMO, a 48T chainring would be pretty bonkers to have on a gravel bike, especially for a 1x. My Crux has a 42/11-40 and its fine for the local gravel (regular roads, rolling and no huge climbs) and my custom has a 36/10-42 and I'm about to switch it up to a 38/10-46. The gearing on either of those two bikes will get me almost 30mph @ 90rpm.

I would not compromise tire clearance or CS length for a bigger chainring beyond a 42-44T.

[Alistair]

Originally Posted by .RJ

IMO, a 48T chainring would be pretty bonkers to have on a gravel bike, especially for a 1x. My Crux has a 42/11-40 and its fine for the local gravel (regular roads, rolling and no huge climbs) and my custom has a 36/10-42 and I'm about to switch it up to a 38/10-46. The gearing on either of those two bikes will get me almost 30mph @ 90rpm.

I would not compromise tire clearance or CS length for a bigger chainring beyond a 42-44T.

If the bike is running a 10-52 cassette, a larger chainring makes more sense. But, even then, not sure you'd ever need more than a 46t.

46x10 is a hair taller/faster than a 50x11. And 46x52 is lower/slower than a 34x36.

But, as you say, for a 10-42 or 10-44 "wide road" cassette, a 40 or 42t ring makes sense and will be fast enough in the 10t cog for all but the fastest descents (at least for mortals/amateurs).

[snguyen268]

Thanks guys for the insights!

Completely agree that 48T is bonker. My default setup will be 46T + 10-52T mullet, which is good for 99% of my riding. My wish for a 48T is more for the occasional pure flat road rides where I prefer some of the gear ratios that it offers. In other words, my thinking of chainring size is more to fine tune the gear ratios in the middle rather than to achieve a higher max/min gear ratio, if that makes sense.