GRX Crankset, Chainring & Cassette Options - Could a RX800/600 hybrid drivetrain deliver awesome gear range?

11 replies · 4,877 views

saker
5 posts
#1Sep 22, 2020, 10:22 AM
Hi Everyone, I have a couple of questions about the RX600 and RX810 crank and chainring compatibility:

Is it possible to use the 46/30 chainrings from a FC-RX600-11 crankset and install them on a FC-RX810-2 crankset?

If this can be done, you would get the lower gearing from the GRX 600 chainrings and the lighter and higher spec. of the GRX 800 cranks. I read somewhere that they should fit, as the Bolt Circle Diameter (BCD) for both big and little chainrings is the same. However, I also read that there may be an issue as the the bolts for the 600 and 800 series are a different size (can this be confirmed as an issue by anyone).
Alternatively, would it be possible to just replace the 31T chainring from the FC-RX810-2 crankset with a 30T chainring from the FC-RX600-11, and still have a fully functioning FD-RX810-F front derailleur?

If this works, you would have a Hybrid RX-810/600 crankset with 48/30 rings. I understand that the front derailleur range will increased by one tooth from 17T to 18T, and technically this will not work, but Shimano tend to be very conservative with their performance specifications, so it might.
As a final modification, would it be possible to use this 48/30 GRX 600/800 hybrid crankset with an 11-36 SRAM or Sunrace cassette?

I understand these cassettes are widely accepted and compatible alternatives/improvements to a Shimano 11-34 cassette. I believe the rear derailleur has a capacity of 40T (some Shimano sites and other websites state 42T) and this combo of 48/30 & 11-34 results in a total capacity of 43 which will not technically work, but as stated previously, Shimano is conservative, etc. so will it though?
I look forward to hearing thoughts on this potential option to significantly increase the gear ratio offered from a Shimano GRX (mainly) drivetrain. If it can be done as described above (48/30 & 11-36), the gearing would potentially have a range of 524%, with a highest ratio of 4.36 and a lowest ratio of 0.83, which would be awesome!

Fingers crossed....
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dagowey
8 posts
#2Sep 22, 2020, 3:01 PM
I would just email Shimano about the bolt size issue to be sure. I don't see a reason they wouldn't give you an answer. You could maybe even ask Wolftooth since they make GRX compatible chainrings. I run the SRAM 11-36 with a GRX 600-11-2 crankset and it shifts fine. It generates more noise than the 11-34 shimano cassette I had on there, which was nearly silent after installing a freshly waxed chain. I'd advise just going with the FC-600-11 and not worrying about it. Seems like a waste to do all this swapping for 100 grams which in the scheme of things matters so much less than proper hydration/nutrition and sleeping well for your riding performance but I understand the need to make things how you want it on a new bike!
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saker
5 posts
#3Sep 22, 2020, 6:02 PM
Hi dagowey, thanks for the response. Good to hear that you are using an 11-36 SRAM cassette - it is my preferred choice. I agree with you re the weight - 100 grams is insignificant (I'm 10kg heavier than I should be!). It was more the additional gearing range you could get from 48/30 rather than the stock 46/30 you get with the RX600 - you would not spin out so quickly on descents with a bit more at the high end. Anyway, I'll email shimano to see what they say about the bolts etc. Thanks again.
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ccand51997
265 posts
#4Sep 22, 2020, 7:06 PMedited
Keep in mind too that when you start to have large separations in your chainrings, you become very limited in where the small chainring can go down the cassette before it starts rubbing the large ring. As an experiment once, I ran a 50/30 without a front derailleur because I was doing a ride that was mostly climbing or descending. The 30 did not clear the 50 well enough to go down the cassette very far. Then the 50 would end up in the really big cogs most of the time when not headed straight downhill.
There is a company called Wickwerks that makes a 53/34 chainring combo that covers 19 teeth for a road usage, but they did a lot of engineering to make those two work together smoothly and even then that is a huge jump that will require more than the usual amount of double shifting. If they made a 46/27 or 28 I might be into that since some of my 'gravel' requires some very steep, long climbs.
I ran a shimano 4-bolt road crankset with 46x30 Absolute Black rings paired with a 11-42 rear Sunrace cassette, an SLX rear derailleur, 105 11-speed shifters and a Wolftooth tanpan. It worked really well until I decided to go lower with an XT crankset (40x26). I rarely miss the 46 up front but I really like 26x42 on the back in a pinch. With that combo it used the entire chain out of the package but I could run it 46x42 without it binding (not that I did it on purpose, but sometime you forget what ring you are in when it is rough) and it would also wrap up 30x11.
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ccand51997
265 posts
#5Sep 22, 2020, 7:22 PM
Oh, and there can be one other issue when going with really wide rings. You have to set up the FD for the big chainring and when you drop down to the little, it can rub on the bottom part of the cage (the part that normally does not touch the chain, it is just there to connect the two sides of the FD). If you use oval rings like the ABs I mentioned above, it gets worse. I initially tried an XT front (needed a top pull) with a stated max of 14 teeth. It would work pretty well with the oval 46x30 but I had to adjust it so high up the seat tube that it would rub the derailleur in anything but 30x42, 36, 32 and maybe the 28. I ended up find a top-pull CX FD that worked pretty well across that range and also with the oval rings.
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brian_m
14 posts
#6Sep 23, 2020, 12:16 AM
I very recently built up a new gravel bike with GRX800 and did the same thing; went for the 600-series crankset to get the smaller 46/30 chainrings. (I didn't explore swapping the rings from 600 crank to 800 crank arms.)
My impressions so far on riding --paired with the 11-34t cassette-- have surprised me. I'm using a lot of the cassette on the 46 ring, more than I would have thought. GRX800's 48t big ring doesn't actually sound like such a bad thing to me now. And 1-tooth difference in small ring likely isn't very discernible in most real-world situations.
Am not going to change anything, but in hindsight I do feel like the complete 800-series package with its 48/31 rings would have been fine, and a little bit lighter.
YMMV of course.
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saker
5 posts
#7Sep 23, 2020, 11:50 AM
Hi ccand51997, thank you for sharing your experience and information on cranks, cassettes, gearing etc. It is always helpful to hear from someone who has actually done - good to learn that you were able to go 46/30 with 11-42 successfully.

Thanks also for the information on the the front derailleur - I suspected there would be some issues with going from a 48Tto 30T chainring and I feel this will be the biggest challenge to my thinking. Firstly finding a crank/chainring combination that can actually do it and then making the front derailleur work with the large gap in numbers of teeth....
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saker
5 posts
#8Sep 23, 2020, 11:57 AM
Hi brian_m, thanks for your interesting response. It kind of supports / justifies my thinking and idea posted above. I think 46/30 is great for the climbing / hills and bikepacking I will do, but I just feel I would appreciate having some more at the high end, as you have described with your experience. If I cannot do what I am thinking I may well compromise on the low end and do as you suggest and go RX800 all the way (48/31) Thanks again!
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Mark_S
144 posts
#9Sep 29, 2020, 6:02 AMedited
I don't have direct answers to your questions, but I can confirm that +4T works well with my similar, R7000 (105) derailleur. I am using 46/32x11-40 (Sunrace). I've only ridden this about 25 hours so far but I'll probably stick with it. I like that it allows for better cadence on steep climbs and also that it requires less front shifting than the stock 46/30x11-34 did. I'm not using an extender and there's no issue getting onto the big ring, but I do wonder if using one might smooth out the shifts in the higher gears.
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Zurichman
1,526 posts
#10Sep 29, 2020, 10:07 PM
I have the 48/31 with a 11-40 GRX Ultegra installed on my Roker this spring before Covid hit. The only - it doesn't like the 2 low gears in the high ring as they grind a little bit.
Zman
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Mark_S
144 posts
#11Sep 30, 2020, 12:57 AM
Precisely my experience - last few cogs (the smallest - what I call highest) grind when the chain is on the small ring. I'd add that when on the large ring the shifts in those last gears are a bit rougher now as well. I attribute that to the slacker chain, having set the length to 46-40.
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saker
5 posts
#12Oct 3, 2020, 2:46 PM
Hi Zurichman / Mark_S, Thanks for your input - it is good to know that your expanded gear range works, albeit with the last few sprockets being a bit rough. I am probably going to use the GRX 600 crankset (46/30) with a SRAM 11-36 cassette to give me some additional gear range. It will be a range of 41T, which is only one tooth over the official capacity of 40T - and going by all the feedback so far, this should be no problem.
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