jppe

I’m just getting started looking at the Shimano GX stuff. I think I have the Shimano R8000 Disc groupset on my gravel bike. I have the 46/34 chainrings. Do you happen to know what the minimum changes are that I’d need to make to go with the smaller chainring? Is it possible to just replace the 34 I now have with the new smaller one (31 or 30) and adjust the FD?

I’ll try and read up on the new stuff when I have time.

tomtomtom123

For Shimano products:
Your FC-R8000 crankset is 110 BCD 4 arm. The minimum size physically possible for 110 BCD is 33T.
https://si.shimano.com/#/en/EV/FC-R8000
https://specialites-ta.com/double/10...denture,33-dts

If you want the FC-RX600-11 or -10, it is 110/80 BCD 4 arm. The minimum possible size for 80mm is 26T. I was wrong when I said 80mm was minimum 28T. TA makes rings for 80 BCD 4 arm down to 26T, label says 10 speed but the webpage says 11 or 10 speed. But they don't seem to make 80mm 5 arm. Both FC-RX600 and the FC-R8000 use the standard Shimano road BB with 24mm spindle, so you can simply reuse your old BB. However, the bolts thread directly into the flange, so there is no way to add a bashguard.
https://specialites-ta.com/accueil/3...denture,26-dts

If you want smaller chainrings down to a minimum of 24T, you need 74 BCD. FC-5703 and FC-4703 are 130/74 BCD 5 arm. You can use the inner 2 and put a bashguard on the outer 3rd. The only problem is that your chainline will be around 2-2.5mm further inward. You could push it out by 1mm if you use the mountain BB cups, with the road water sleeve (but you have to shave off the plastic stopper edge). The Mountain BB cups are each 1mm narrower than the Road BB cups (on each side). So your spindle will get an extra 2mm of length. Then you add 2mm of spindle shims to the right side crank (or 2mm shims to the right side BB cup, which shifts your chainline by 1mm to the right. You could possibly get away with adding an extra 0.5mm of shims.
https://productinfo.shimano.com/#/sp...t%20Chainwheel

Otherwise, I use the Sugino OX cranks, which are 110/74 BCD 5 arm double, which has a standard road chainline. It also uses a 24mm spindle, and the Sugino BB is somewhere in between the widths of the Shimano mountain and road BB, about 1.6-1.8mm narrower overall than Shimano road BB. So I used the Shimano mountain BB with shims. I was also able to add a bashguard to the outside of the Sugino crank but I had to make custom spacers for it. The large ring is in the "middle position" since it is mounted on the inside face of the flange. I've only found maximum 46T middle position from Spa Cycles, TA, and Sugino.
https://www.suginoltd.co.jp/us/products/road/ox.html

Otherwise, you could go with square taper JIS BB and use any cheap 110/74 triple crank.

If you want to go even smaller, the Shimano mountain cranks are usually 96/64 BCD 4 arm. 64 BCD goes down to 22T. However, I've only seen up to 38T for the 96 BCD 4 arm middle position, and TA doesn't make any for 96, only 64, 104, 110 BCD. You could laser cut an outer bashguard that serves as an adapter between 96 and 104 BCD (or even 110 4-arm) by rotating the holes by 30 to 90 degrees. Then you can use TA 104 BCD middle position up to 42T, or TA 110 4-arm outer-"middle" from 42T through 56T. (You could even make an adapter for 5-arm) The Q-factor of the Shimano FC-M9000 or FC-M9020 are the narrowest of their mountain lineup. Chainline is at 48mm, but you can space it inward, but you'll have to use your own longer bolts, like from Truvativ. Otherwise, there are older Shimano mountain cranks with 104/64 BCD, but the Q-factor is wider.
They use the Shimano Mountain BB, which is 1mm narrower on each side (total 2mm) than the Shimano Road BB, so you will have to change your BB.
https://specialites-ta.com/triple/38...denture,32-dts
https://specialites-ta.com/double/11...denture,52-dts

jppe

So it sounds like if I'd like the 46/30 then I just need to change the R8000 crankset to the FC-RX600. It looks like they are $150. Sounds correct?

shelbyfv

The GRX crank will be compatible with the rest of your set up, with the possible exception of the front derailleur. Read about that earlier in this thread.

tomtomtom123

Well, he says he's got 46/34T on his FC-R8000, so assuming the FC-RX600 46/30T has the exact same chainline, then he shouldn't have to change anything else other than swapping the crank. The chainline will always have tolerances though, so you'll still have to adjust the sideways limits and cable tension for the new crank.

I looked at the photos of the cranks again, and saw that a lot of the Shimano 4-arm chainrings have holes that are asymmetric. So some of the TA chainrings might not be compatible. So your chainring options will be limited.

tomtomtom123

I'm looking at SRAM cranks, and the ones that have small chainrings are usually mountain cranks with wider q-factors. If you don't mind the width, there is the 1000 GXP with 104/64 BCD 4-arm, symmetric, which means there are more options for compatible 3rd party chainrings. I believe it runs on a 24mm spindle.
https://www.modernbike.com/sram-gx-1...crankset-no-bb

Other SRAM cranks have removable spiders with various BCD sizes. But it's really difficult to find specifications since SRAM doesn't publish them. One of these cranks would be the SRAM/Truvativ X9 which comes in both GXP and BB30. The standard double is an odd 120/80 BCD. There is a conversion spider to change to 104/64. A Canadian company North Shore Billet, also makes an alternative double spider with 104/64 BCD, with the possibility of adding a bashguard to the outside surface. With 64 bcd, you can go down to 22T. But I'm not sure if you'll be able to find for the 104 middle position anything higher than 42T.
https://www.modernbike.com/product-2126277337
https://www.bike-components.de/en/SR...racket-p39525/
https://www.modernbike.com/north-sho...p-cranks-black
https://northshorebillet.com/collect...ant=1112875409
https://all4bikes.be/shop/en/sram-ch...p-1562531.html
https://www.bike24.com/p245121.html

The mountain cranks will have a mountain chainline, around 48mm. So you'd have to space the chainrings 4-4.5mm further inward if you want a road chainline, for example with a 130mm dropout.
I'd wish that SRAM would actually publish specs, like Shimano does. It's one of the reasons why I don't buy SRAM products, because I can't find specs.

For Shimano, there is the older FC-M785, also with symmetric 104/64. Q-factor 175.
https://www.bike24.com/p2263282.html

shelbyfv

It doesn't and that's the issue with GRX discussed earlier in this thread and elsewhere. I expect we'll see some reports soon, since the cranks are finally available. I think OP posted that he had one on order so maybe we'll hear about it shortly.

tomtomtom123

Ah, yes. The specs sheet says FC-RX600 chainline is 46.9mm.
https://productinfo.shimano.com/#/sp...t%20Chainwheel

Standard road chainline is 43.5 to 44mm. So it's 2.9 to 3.4mm further out. To fix this, there are 2 things you can do.
Switch your road BB to mountain BB cups, but use a road water sleeve with one end's stopper tab shaved off. Mountain BB cups are 1mm narrower than road BB cups, so a total of 2mm narrower width. This will shift your chainline 1mm inward, and makes your spindle protrude 2mm longer, so you'll have to check if there is enough tolerance for the left arm. If not, then you'll have to put 2mm of spacers on the left side, either on the arm or the BB cup.

Otherwise, simply use 3-3.5mm chainring bolt spacers to bring the chainrings closer inward. 44mm chainline is fine for 10 speed 130mm dropout. Then, use longer chainring bolts. The shimano documentation says the original bolts are 8mm x 8.5mm long. So you'll need 11-12mm long bolts.

jppe

I’m now seeing where there’s a 2.5 mm difference in chain line so probably best to get the new FD, too. Looks like $200 for the crank and the FD just to get the 30 vs the existing 34. I sure wish Shimano had the stuff two years ago.......I’ve climbed everything put in front of me in my 34-34 combination but I know it would be nice to have the easier gear.

I guess it’s safe to assume the shifting of the GRX FD will be compatible with the R8000 left shifter???

tomtomtom123

You don't need a new FD for a small change in chainline. You simply use the adjustment screws and cable barrel nut to move the FD further out. Unless the FD range of motion cannot reach out to the extra 3mm. But if you want to maintain the same chain angles, it's better to move the chainline inward, which is very easy to do. If the chainline is too far out from the cassette centerline, the chain might drop when you're near the big-big combination.

You could simply add chainring spacers to push the chainrings further inward. Spacers cost less than $5. If you don't want to search for new bolts, just 1mm of spacers shouldn't reduce the amount of engaged threads on the bolts by too much, although you should measure them. But longer chainring bolts are very easy to find, and cost $10.

Surly Mike

I’m wanting a gearing setup with: 30/46 crank & 11-36 cassette for a road bike build.

Reports from others say that Shimano 105 & Ultegra setups can do it.

Can the new Shimano GRX do that (or 31/48 & 11-40) ?

Can I make that work with a SRAM Rival groupset?

Thanks for all advice,
Mike

tomtomtom123

30/46 and 11-36 is a range of 46/30*36/11 = 502%, with a chain capacity of 41T.
I don't think you'll find a 31T chainring, so change that to 30T.
30/48 and 11-40 is a range of 48/30*40/11 = 581%, with a chain capacity of 47T.
Lets assume that you have a real diameter of 26 inches.
Your top gear is either 108.7 GI, or 113.5 GI.
Your low gear is either 21.7 GI, or 19.5 GI

Instead of going up to a 40T cog, why not drop the inner chainring size to get a similar range, and with the benefit of a smaller chain capacity? You'll also get closer gear spacing this way.
26/46 and 11-34 gives you a range of 547%, with a chain capacity of 43T, and 108.7 GI to 19.9 GI.

However with a 20T jump in the chainrings, you won't be able to use the outer 3 cogs when on the inner chainring due to the chain rubbing on the outer chainring, but this is fine since those gears are overlapped anyway, and are available on the big-big combination. You can also add 0.1-0.3mm shims to the inner chainring to increase the gap to the outer chainring to make the chain clear an additional cog. 20T jump also requires a chain catcher and some front derailleur adjustments.

I am a slow rider that's not good with climbs, so I have 14 to 72.6 GI, with a 519% range.
On flatter surfaces, I would prefer 17 to 82 GI. I am rarely ever going faster than 82 GI. However, your requirements will differ than mine.

You should plug in the data into a table to see which gear combinations work for you, and whether the gear spacing from inner chainring 7th to outer chainring 2nd or 3rd is sufficient in a 20T jump.

You should look also the Shimano specs website to see the total chain capacity and max cog size to determine if the rear derailleur will be able to take the teeth combinations that you want.
https://productinfo.shimano.com/#/sp...r%20Derailleur
https://productinfo.shimano.com/#/sp...r%20Derailleur
https://productinfo.shimano.com/#/sp...r%20Derailleur

The gravel RD-RX400 rear derailleur can only go up to 36T cog with a 41T chain capacity. You'll have trouble if you want to change to a setup with a larger capacity.

Instead, the trekking RD-T6000-SGS can go up to 36T cog with a 47T chain capacity. The extra 5T in chain capacity will give you more options.

Some of the mountain RD for 10 speed go up to 36T cog with 43T chain capacity.

If you want to go up to 40T cog, you might be able to use the derailleurs that are rated to 36T, but you'll have to research the internet to see if other people have been successful. If not, then you could try the Wolf roadlink to increase the gap between the pulley and the cogs. Otherwise you could try the mountain RD-M8000-SGS up to 46T cog with 47T chain capacity, for a very wide range of options. However, it's made for 11-speed. But some people report that they could use some specific 10 speed mountain shifters on the 11-speed mountain derailleurs. Although, other sources claim that the pull ratios are different. So you'll have to do some research and experimentation.

https://forums.mtbr.com/drivetrain-s...in-947611.html
https://forums.mtbr.com/drivetrain-s...y-1017063.html

bargo68 

Kind of a wrenching newbie here, but I just switched from a 48/36 to a 46/30 on my Cross Check with the stock Andel crankset. I bought 2 chain rings on 'that auction site' from a company called bikinGreen and both chain rings are 110 BCD. Granted, it's a square taper bottom bracket, so no headache there, but with my Shimano HG 11-34 cassette it works perfectly! Chainline is nice and didn't have to swap out the bottom bracket. The only weirdness is to maintain cadence while downshifting in the front, I have to upshift one cog in the back. A little wonky, but it's on a commuter bike and I appreciate the available lower gears.

tomtomtom123

You are supposed to shift the rear derailleur at the same time that you shift the front derailleur, if you want to maintain the same cadence on the same surface.
Your 48/36 is a 33.3% step. If you have an average gear step of 15%, you would need to shift 2 rear cogs to maintain the same cadence. 1.15x1.15 = 1.32
Your 46/30 is a 53.5% step. You need to shift 3 cogs to maintain the same cadence.
I have 44/24, an 83.3% step. I have an average gear step of 12-13%. I need to shift 5 cogs to maintain the same cadence.

The shimano RD shifter that I use can downshift by 5 larger cogs in 1 press, but only upshifts 1 at a time. But it's not an inconvenience. I simply press it at the same time I use the FD shifter. It takes 1-2 seconds to complete the shift. Perhaps the cheaper shifters can only shift 1 and a time in both directions. The high end models can upshipft 2 at a time.

I have only seen a standard 110 bcd as low as 33T. It's not physically possible to get 30T on a standard 110bcd without hitting the chainring bolt or bolt flange. But the bikinGreen 46/30 110 bcd double set seems to connect the inner ring to the outer ring by a small tab without a chainring bolt, and the outer ring seems to have threaded holes to be mounted on the middle position of a triple crank, or on the inner side of the crank flange. But the photos don't show a closeup, so I'm not sure exactly how it works. You would also have to bring out the crank to compensate for the chainline.

How are you mounting it on your crank?
https://www.ebay.com/itm/bikinGreen-...-/123706511747




You can see a list of minimum teeth for common sizes of BCD here:
https://www.harriscyclery.net/articl...uide-pg157.htm

If you only have 110 or 130 BCD holes, you could also use the TA Zephyr K adapter (triplizer) for 110 BCD or Alize K adapter for 130 BCD to allow you to attach a 74 BCD inner ring, down to 24T. However, you'll have to mount this on the inner side of the flange. But I've only seen the 110 Zephyr in 36, 38, and 40, and the 130 Alize in 38, 39, 40, and 42.
https://specialites-ta.com/triple/19...denture,38-dts
https://specialites-ta.com/triple/20...denture,38-dts




Stronglight also makes something similar for 130 bcd, for 38,39, 40, 42, 44T, but strangely, the photo of the silver version doesn't show any pins while the black version does show pins.
Stronglight - Road Type 7075 "S" 130/74 mm
Stronglight - Road Type 7075 "S" 130/74 mm
https://www.spacycles.co.uk/m2b0s149...D-Adaptor-ring



I don't know whether or not the bolts on the TA are permanently attached. But the Stronglight seems not to be.

shelbyfv

Yes

Surly Mike

Wow, lots of info! Thanks Tom for taking the time for this. It has taken me a while to digest and research all of it.

I do use the gear-calculator (.com) to check gear options, etc.

Your suggestion of using a front crank with 26/46 chainrings & rear cog of 11-34 sounds excellent! I don’t know if any brand makes them however. I searched the web and didn’t find any. Most brands make front derailleurs that can handle a 16-tooth difference, not 20. The new Shimano GRX can do a 17t difference with their new 31/48 crankset.

I certainly want to be able to use the full range of a 2x11 system. That’s the primary reason that I prefer not to go to a triple front crank; I know I will absent-mindedly cross-chain the gears, etc.

I’m in good shape for an over-50 rider...can keep up with strong riders my age, but not the young guns or pros of course. I like long rides, especially into the mountains, so I really need the full range of gears. Today, the grades got up to 17%, and some rides 20%.

Tom, BIG THANKS on the Shimano product pages!

It is quite a jungle of options and compatibility issues to sort out a good solution.

The quest continues. Thanks,

Mike

bargo68 

Ah, but I use downtube friction shifting and really, it's only an issue on sustained climbs of over 5%. I'm in the big ring most of the time. As to the connection points on the 30T, the holes are almost in line with the tops of the teeth. They fasten with with 5M hex bolts, IIRC, and the 30T is tapped to receive the bolts. I was a little suspect to the strength of this type of connection, but so far no problems. I've put a couple hundred miles on them and not a peep. They are meant to work with any 5 arm crank with 110 BCD.

Thanks for all the technical info!

Jeez, if I'da know there was going to be math involved.... )

tomtomtom123

You won't find anyone selling double cranksets with a 20 tooth jump. Because there is a very small tolerance on how much you can adjust the FD to get the chain to shift. If you don't get it adjusted correctly, the chain won't shift at all. On my FD, I have the high stop set a little further out, so I can press and hold the shift lever down all the way, wait for a successful shift, then release the lever and it will return to normal index position. The slightly extra outward movement when I have the lever depressed will help to push the chain further against the outer chainring to make it easier for the shifting pins to catch the chain. Also with a 20T jump, you won't be able to use the outer 3 cogs while on the inner chainring, but it doesn't matter in practicality. since you don't need those gears.

If you want a 20T jump, you have to switch the chainrings yourself, and use a crank that has a bcd that can take the small size chainring that you want. Then you have to find the actual chainring. 74mm 5-arm is readily available. Some of the Shimano 4-arm mountain cranks have small chainrings. But the 4-arm asymmetric mountain and road are more difficult to find. SRAM 4 bolt symmetric chainrings are easier to source. I mentioned some of the options in the previous posts.

Similarly, double FD specs will always say 16T capacity, since they will never tell you that you can use a 20T jump. You just have to try it yourself. But you should look for a FD that is made for a smaller chainrings, because the radius of the side plate is smaller and will shift better on small chainrings. Smaller chainrings will also need the FD to move lower. If you have a clamped FD, then it's less of a problem. If you have a braze on mount, you may need an extender. One issue is the possibility of the FD touching the chainstay if it's moved too low. The FD specs will state a chainstay angle. The various Shimano models have 61-66, 63-66, 66-69 degrees. Usually the smaller angle will have better clearance to the chainstay, but I could be wrong. You should do some research. The models that have a wider range of angles usually have a longer cage length, so it is long enough to clear the chain when you have a 20T jump. If the angle of the braze-on tilts the FD too far forward, there is a chance that the chain will rub the bottom of the cage. Then you can use some cheap $5 SRAM FD braze-on shim/angle adjusters to tilt the FD further rearward.

If you use downtube friction shifters and don't like it, then you could try switching to index shifters. A quick google search comes up with this, but I'm sure there are other options. You just have to look for it.
https://thebicyclerebuilder.wordpres...tube-shifters/

Road Fan

I would not accept that in my own work on my bikes, but if I had to have it that way until I could finish the job I would ride it. But I would absolutely not accept it from someone supposedly with skills and whom I am paying to give me a working, tuned gearing set.

jppe

I’ve received my 46/30 GRX crank that I plan to put on my gravel bike to replace the 46/34. I thought that I might just buy a 50 chainring and convert the 46/34 to a 50/34 for one of my road bikes. However after looking at the prices for just a 50 chainring I think I’ll just sell the 46/43. I didn’t realize how pricey they were. Heck, for not much more I can get a whole crankset.

shelbyfv

Yep, they are proud of their large rings. I'm using a 46-34 on one road bike and I rarely miss the 50. If it's a nicer crank than what you have on another bike, may as well give it a try.

jppe

What is on the road bike is an Ultegra 52/36 11sp so the 46/34 would be an equal swap quality wise. I was hoping to get slightly easier gearing going to the 34. I have 50/34s on all my other bikes except for the 53/39 on my TT bike. I’ve climbed some really steep stuff using the 52/36 and it’s perfect gearing for riding around the house. But having a little easier gearing is always nice especially for climbs at the end of long rides and also deep into multi day rides. I haven’t really thought about using a 46/34 but might consider it.

horatio 

Planning to order a 46-30 crank very soon.

meyers66

I’ve ridden a Bianchi Volpe 07 with a 48/38/28 front that gives me a low gear inch of 28”. If I were to buy another bike I’d buy something that gives me even a lower gear inch like 24”. Screw the market pushing everything higher. I’m the one on steep hills. By the way I think the stock Sugino (500?) crank works for me. Aftermarket hardware seems so expensive there’s an argument to buy a new bike in the $2-1500 range. Originally I thought I’d have to

shelbyfv

wut?