SoCalChris

Hello, I have a Trek 520 Grando. The stock crankset and bottom bracket that it came with look like they're Shimano Hollowtech II compatible, but are not made by Shimano.

The cranks are "ProWheel forged alloy, 42/28, 2-piece, 175mm length" and the bottom bracket is "Bottom Bracket Shimano type Hollowtech II Road".

Once I wear through the stock crankset, I'd like to replace it with a Shimano GRX FC-RX810-2 crankset. At that time, I'll probably change the bottom bracket out to a Shimano brand one, but am not sure which one. Would the BBR60 Ultegra BSA Bottom Bracket work, or do I need something else? I'm pretty good mechanically, but this is my first time upgrading a crankset/bb, so I'm not 100% sure on the compatibility.

Are there any major flaws in my plan? Will the crankset and BB that I chose work, or do I need something different? Thanks

tyrion

GRX cranks have the chainrings more outboard than Shimano road cranks (to allow room for fatter tires). Chain line 47mm for GRX vs. 43.5mm for Ultegra. Don't know what your current crankset's chain line is. Moving the chain line outwards might work but the chain line might not be optimal.

Iride01 

If you have a threaded shell then the SM-BBR60 should probably fit. That particular crank only comes with 48/31 rings. So if you ride less extreme conditions you'll probably like having the ever so slight higher ratios. But if you struggle in the lowest gear often, you might miss that.

The only real flaw in your plan from my perspective is in what way do you feel that you are going to wear through the current crankset? Do you ride 20,000 miles a year and have bulging quads that put out massive amounts of power at low rpm?

DaveSSS 

Shimano advertises the chainline as +2.5mm, which would be 46mm. The chainline can be moved inward with spacing washers. I use the grx 48/31 with my sram axs 12 speed drivetrain. With a 48/10 I have the same top gear as a 53/11. An ultegra or DA bottom bracket will work, but I opted for a wheels manufacturing thread together BB with angular contact bearings.

SoCalChris

Thanks for the advice, everyone.

I like to consider myself having legs like Adonis, but realistically "It wore out, I needed to replace it" is what I'll tell my wife when she asks why more bike parts were delivered

Iride01 

I just don't see the crank as improving your performance much. In my case, I would expect that before the crank and rings even wore out, I'd want another bike. But perhaps using mtn bikes for the purpose they were actually made for is more demanding on the cranks than my road biking is.

SoCalChris

I've actually got a few reasons that I want to upgrade the crankset. Whether those are good reasons or not, I'm not 100% sure, so feel free to chime in if you think I'm off base or not. I figured while I had the crankset off, I might as well swap out the bottom bracket for a slightly better one since it's a relatively cheap upgrade, hence the original question about the correct BB for me.

I don't think I'm too hard on my cranks, although I am a heavier rider (~250lbs). Most of my riding is long distance on river bed trails with minimal starting and stopping, I'll usually go on the weekends and do 60-80 miles with 1 or 2 stops. On weekdays I do more riding around my suburban home, with some moderate hills and more stopping and starting, usually about 15-20 miles per ride.

Basically my bike came with a Prowheel 42/28 tooth crank set. I have intermittent issues shifting from the small to large gear. It's not consistent, but when it usually happens it's in the middle of a long ride and gets really annoying having to keep shifting back and forth trying to get the gear to actually change. The shop I bought it from has adjusted it a few times, I've adjusted it a few times, and a separate independent shop has adjusted it a few times. Each time it works amazing on the stand and in the parking lot during a test ride, then starts acting up on an actual ride, then works again perfectly when I take it back to the shop. I absolutely love the bike, this is the only issue that I've had with it over the course of around 3,000 miles I've put on it since I got it. I've read several places that have mentioned that difficulty switching to the larger gear on the front derailleur can be often be improved with a higher quality groupset, so this is something that I'm willing to spend the money on upgrading if it will help.

Another reason I'd like to switch to the GRX-810 is the slightly higher gear ratio. For the type of riding that I do, I think that the slightly changed ratio will be beneficial to me.

Anyways, I'm relatively new to working on bikes, so if my first point about the improved shifting sounds suspicious, feel free to correct me.

Iride01 

Well that's plenty of reason for me. I have wound up with an extra set of 50/34 rings from a 105 5800 road bike group. I have put it on my bike to replace the preferred 52/36 rings when I go to Colorado to visit my son there and will be doing longer climbs than I ever could do here in the slightly rolling terrain I live in. And on those series of Shimano 105, Ultegra and DuraAce, they can be swapped out easily by just removing the ring assembly, or if you have a complete crank with rings, remove the left crank arm and slide one crank out and the other in. I don't have any experience with the GRX crank or others to know what else that can easily be done with.

As for your problem shifting and both you and the bike shop not figuring out why... well consider if you need another mechanic or something. Not necessarily the bike shops fault, but for some things, it takes a person to have an ah-ha moment to solve your issue. Maybe you just need to buddy up to the shops mechanic with some discussion for that ah-ha moment to happen.

SoCalChris

And I just saw an article on Shimano's site saying that I have to use a GRX front derailleur with that crankset, the Tiagra one that came on my bike isn't compatible. So I guess that would be one more component that would need replacing. It's not super expensive, but starting to look like I'd be going down a rabbit hole, for something that might not even really be an improvement. So this project may wind up being shelved.