Mrbilll

Wanted to replace the no name crankset on my 21 Sirrus x 4.0.
Was thinking of a GRX 810 42t, does that sound doable ?

bboy314

Should be doable. Looks like the stock crankset has a 24mm spindle, so the bottom bracket should work but might need spacers. There’s also the chance the current BB cups are too wide, in which case you’d need a new BB.

Also, looks like the bike is stocked with a 40 tooth chainring, so you may need a longer chain with the 42.

I wouldn’t expect the new crankset to make any significant difference, but nothing wrong with upgrading just because IMO.

RiceAWay

There are three standard sizes of crank shafts. Most of them are Shimano compatible and include all of the Shimano cranks and the EXO series of FSA cranks. Replacing the Shimano bottom bracket with Campagnolo cups allows you to place the 25 mm BB with the 25 mm Campagnolo cranksets. FSA also makes an upgraded crankset with a 30 mm shaft in the EVO series of cranks. This will not fit into some bikes.


Now if you have an old style of crank attachment, there are the square drives which come in a different number of widths (double, triple and Campagnolo which has a different taper and is NOT compatible with anything but square mount Campagnolo BB's. There is also the ISIS which appears to be a non-tapered shaft with splines and is usually hollow and lighter than the tapered type.


The GRX crank is a 24 mm shaft and if you have a sealed bearing modern crank on your bike now all you need is the proper tools to install it. This is the proper size allen wrench to loosen the left (non-drive) side and a tool to loosen and remove the end plug. If I remember correctly, they are available in cheap plastic but I have a steel one that also can be used to tighten and loosen the BB bearing holders. Typically this tool works best on American threaded BB's (1.37" x 24 threads per inch) On Italian threaded frames, you have to tighten each side clockwise and you have to use thread locking compound on the drive side because bearing forces try to unscrew it. American threads are left hand thread on the drive side and right hand threads on the non-drive side. This uses the bearing forces to always tighten each side. But you have to make sure that the left and right hand threads are tightened all of the way or the tightening forces can over tighten it to the point that you can't get it off again.


If you want to use a SRAM, you will need a new bottom bracket because although the shaft is 24 mm, the non-drive side bearing is (if memory serves) a 22 mm. This avoids preload problems. so that you can simply tighten the crank on until it hits the stops and then tighten the non-drive side crank lever with a standard allen wrench.


But if you're changing Shimano for Shimano it is normally a pull the old one out and insert the new one though it is usually best to replace the old BB, especially the older cheaper styles. The bearing lands on the cranks are pretty tight so installing the new crank may include taping it with a heavy rubber mallet. Tap on the shaft end to get the old one out and the outside of the drive side to get the new one in.

badger1

Yes. Should be a straight swap. Had a GRX 810 (40t) put on my '19 Sirrus X carbon last year to replace the heavy no-name stock one. The latter was probably the same as yours. No issues at all.
I'd get a new Shimano b/b while you are at it (in fact, I believe you might have to). I used the Ultegra one (BBR60), threaded external cups for 68mm (or 70mm) shell. You could also use the 'Tiagra' one (BBRS501).

Iride01 

You might check what the chain line is on that bike. You can simply measure in millimeters from the centerline of the ring to the centerline of the seat tube.

The GRX FC-RX810 is a chain line of 49.5. If you have much difference, say 4 or 5 mm, then consider that shifting to the side of the rear cassette that is furthest away from that chain line might suffer.

If on that 68mm BB shell that the specs show your bike has, the bb for the GRX crank has spacers, then you might be able to move the chain line over.

https://www.specialized.com/us/en/si...=301081-187461

Mrbilll

Thanks for the replies. Besides weight, is there any real differance between the Ultegra and Tiagra bb"s

Iride01 

Weight is the big difference if that's enough to bother you. Some people will think Ultegra gives them more prestige or snob factor. Either will last you long enough.

You also should consider the larger diameter of that 42t ring. Do you have enough chain stay clearance that far out. Might be almost 4mm more radius.

KerryIrons

[QUOTE=bboy314;23212829]Also, looks like the bike is stocked with a 40 tooth chainring, so you may need a longer chain with the 42./QUOTE]

Unless the chain was right at the limit, adding 2 teeth to the chain ring would NOT require a longer chain. Remember, chains add length in one inch increments, while 2 teeth on the chain ring requires only a half inch more chain.

bboy314

Hence the word “may” in my advice.

2 teeth would require a full link, or one whole inch, no?

grumpus

No, because the chain only wraps around about half of the chainring, so one more tooth, and the points where the chain engages with the chainring move apart only slightly. So more than a half link, but not by much, if my brain is working.

bboy314

Ah, makes sense!