Evolving thoughts about tire size for gravel
 

In Central Pennsylvania, most of the good gravel is either going up or going down a big hill. The 2.25" tires on my MTB definitely feel a lot more secure on downhills, and don't noticeably slow me on uphills. I was thinking about going with 44mm tires because it would be easier to get crank clearance and tire clearance. I'm using a lugged bb, so that's another constraint.

I'm currently using 38mm tires on my gravel bike, and I'm fairly happy with them, but I wouldn't mind more security on downhills. Then, to add to my analysis paralysis, Jan published this blog post. https://www.renehersecycles.com/blog...eid=4c0ea92129

Yes, I posted this in gravel and had second thoughts and moved it.

From what I have found you have to be willing to use MTB cranks. The bike pictured below is a drop bar gravel bike ( it has a 44mm head tube with reworked lugs) and has a Continental Race King 2.2 tire installed. Its a lugged frame with paragon disc mounts. As you can see there is plenty of clearance between the crank/chainstay and tire/chainstay. If I remember it takes some massaging of the BB CS angle to get them wide enough. It has a 39 tooth big ring on it in the pic, you can get up to a 42 tooth on the stock Sram cranks, there are aftermarket rings that go bigger, I have not found a Shimano that goes past 40 teeth. Raceface makes a mtb crank, 30mm spindle, that accepts an Easton Cinch chainring set 46/30. The Raceface crank is about 7mm wider than a standard mtb crank and allows for shimming so it might fit.

I am currently working on another frame and plan to use a GRX 46/30 crank, not sure what the max tire width will be but for this one i am looking for somewhere around 50mm between the stays and doing a mock up it looks ok. I'll know in the next week or so.

https://cimg5.ibsrv.net/gimg/bikefor...1e564ec685.jpg
https://cimg6.ibsrv.net/gimg/bikefor...c7d21a3d2b.jpg
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https://cimg8.ibsrv.net/gimg/bikefor...a55e60ccbc.jpg

700x 42 is about the max that I feel comfortable with being able to fit a road crankset with a reasonable 430mm chainstay length. My personal Gravel bike has 650 x 47 tires with a 48/34 road compact and 425mm chainstays. I'm very familiar with the area that you are riding and the Terrene Elwood 650 x 47 tires are great for that terrain. My crazy friend creates routes that include the snowmobile trails near Williamsport and I'm glad to have the extra cushion for those rocky trails and even some of the huge chunks of rock embedded in the gravel roads.

What stays are those on the white bike? I would really like to go with the GRX crank, so maybe I'll stick with 44mm tires. Pretty sure I can get that all to fit together.

Already showed the bb shell who's boss just to get the back wide enough to fit a hub in there. The decision is really which end to cut. Cut from the front and that limits the tire. Cut from the back and it limits the crank. I wish I could bend the stays around a little, but my attempts at that didn't work out.

The chain stays are CX s bend with about an inch cut from the BB end the rest from the dropout end

https://www.bikefabsupply.com/chains...-chainstay-107

I used these to attach the stay to the dropout arm.

https://www.paragonmachineworks.com/...pout-plug.html

You could always run a smaller tire in back but use the 700x55 up front. Even Lael says she'll do it for races with lots of mud for better rear clearance.

Design it as a mullet and you don't need to make any compromises.

In the framebuilding section on the herse site, Jan has a BB shell with the CS sockets splayed slightly more than typical road BB. Not sure how the other angles line up to your needs but I've been wanting to try this shell as it seems like it would solve problems - or at least save blacksmithing time.

I'm getting pretty good at bending the sockets on oval bb shells. The first one took forever because I was being gentle with it. They really bend pretty easily with a heavy-duty chain stay.

If I were going with round chain stays, I probably would go with the RH shell. And also their pre-bent stays.

I was playing with bikecad and it left me dissatisfied. Looked like I would have to go with longer chain stays than I want for 700c 2.2" and I'm not going with 650b because I have plenty of nice 700c wheels already. But the bikecad stays might be bigger than the ones I'm going to use. I guess I have to go back to the mock up.

That's right, they have round ports. It's been a while since I looked at them but they struck me as a good idea given the trend to wider tires on all bikes.

The RH oversize bb might have worked out, but I'm pretty sure I want a dropper on this bike and I don't like the idea of a 27.2 dropper.

What BB are you using to get a larger seatpost?

Thanks

Nova has a 2OS BB shell. Pretty sure I have LLewellyn lugs

This bike makes me think that I'm overthinking it: https://www.blackcatbicycles.com/hello-monsta

That bike makes it look easy because it has boost spacing.

Okay, I missed that but boost is only 6mm wider, so not a lot. I was wondering about the crank they have on there. Must have a longer axle? Otherwise it looks a lot like a road crank. on edit: looks like it's an M30 mountain crank, so the bb is wider too. http://www.whiteind.com/crankm30

With boost, the BB shell is wider, thus the spindle, so you avoid the crank conflicts with the stays. This with the wider rear end allows for additional tire clearance.

I've been in the same decision loop that you are in about a future build. I'd like a mixed terrain tourer but the tire/crank clearance issue kept coming up. I currently only know how to build with lugs. The best way through this that I've come up with is to use disk brakes and two different wheel sets. One 650B with a 2.1 and a 700c with 38's. Both are close in overall diameter (702 mm vs 706 mm). I know this does't meet your need to use your existing wheels but it's where I've landed.

As far as I know, which may not be very far, the BB maybe wider but the outside dimension of the BB with bearings is the same, I think around 95mm.

You can get what you are looking for using lugs and a prefabbed disc caliper mount/dropout with a mtb/gravel crankset, wider q-factor and can use a small BCD allowing for smaller chainrings, see picks above of my white frame, I am in the process of finishing a frame that will fit GRX 46/30 and up to a 2.2 inch 29er tire. Before the gravel boom, the fit issue for the cranks has been finding a non-square taper 2x crank that will fit a 46/30 as MTBs all went to 1x. I don't know if the latest frame will fit a 48 tooth or not without crimping the chainstay.

There are other options beside GRX but to me the price gets prohibitive. The White crankset on the linked bike is $500+ GRX is under a $100.

Still mullets are pretty cool. :)

GRX has a slightly wider chainline than road. My current gravel bike has a cheap mountain crank on it, so the Q factor isn't great. I would like to get the new bike to use a GRX.

Thanks for pointing out that it is boost so I looked at it a little more, I was feeling inadequate. But I don't really want to go with tires as wide as the BCC uses either.

well, the columbus gravel chainstays are back in stock at framebuilder supply. I stared at that drawing for 15 minutes and I don't understand how the brake mounting area could possibly be parallel to the disc. I saw a picture of them on facebook, but I haven't found it yet.

If I did direct mount, I'm not sure what dropouts I would use.

I ordered the chainstays and the brake mount. Have to figure out what dropouts to use. I think the drawings must not be to scale?

I received a pair of the road stays a couple days ago. Bought them from Ceeway, I didn't see them on Framebuilders, It took a while to figure out from the drawing that the view is from the bottom, which made it hard to picture. Also the drawing only shows the rear 2/3 of the stay as the rest is straight. I am using 1 inch round through axle dropouts from paragon
https://www.paragonmachineworks.com/...dth-12-mm.html

I have done a mock up and the brake mount does go parallel to the disc and should fit upto a 42ish mm tire. I ordered an Incepi flat mount jig and am waiting for it to arrive to assemble the rear end.

Thanks. The road drawing makes even less sense to me than the gravel drawing. As we have established that you are tall, your dropout choices are easier. I'm worried about the seat stay clearing the brake caliper. I might have to figure out how to bend seat stays

When I was looking at the drawings of the chainstays I thought the 3 CX stay would be more like a 3 bend 29er stay and the road stay more like the regular CX stay. I didn't account for the diagram not showing the 100mm or so that was straight. Being a beginner framebuilder and just lucky I guess, I have not thought about seat stay to caliper clearance. Have you looked at the dropouts linked below.

https://www.bikefabsupply.com/rear-d...00mm-thru-axle

https://www.bikefabsupply.com/rear-d...-axle-m12-x-15

Attached is a pic of the road flat mount stays and a pic of the caliper clearance using a 1 1/2 inch round dropout the CS is 1 inch dia. The seatstay attaches at 52cm above the BB


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Thanks, forgot about Bike Fab supply. I ordered the C66 dropouts, hopefully they are tall enough. Otherwise I'll have to bend some stays
https://www.bikefabsupply.com/rear-d...-x-142-m12x-15

Thanks for the pictures of your stays. Not sure why columbus doesn't have a scale drawing of these. Doesn't matter for a lot of tubes/stays, but in this case it's important.

Boost, at the least the 148 version, has nothing to do with bottom bracket width. It simply moves the chainring out 3mm to match the 3mm wider chainline at the rear. That extra 3mm does allow for wider tires, but its not because the bottom bracket is wider, You can buy a boost chainring to fit your standard crankset. It just has less offset than a standard chainring.

If you want to build with a lugged bottom bracket and fit wide tires, you will have to have long chainstays. Welding and fillet brazing opens up a whole new world of bending and shaping the stays to fit your needs.

I looked at those Columbus flat mount stays and decided to stay away from them. They basically lock you into a single spot that your dropout can attach and be the proper distance from the brake mount. At that point, you have to cut from the other end of the stay to get your desired length and hope that it lands in the right spot on the bottom bracket with proper tire clearance.