GRX chainset.

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Barry and Janet wil
Posts: 7
Joined: 3 Aug 2017, 11:36am

GRX chainset.

Postby Barry and Janet wil » 14 Aug 2019, 3:36pm

My wife's road bike has a 34/50 11-32 Ultegra 8000 drive train. The smallest gear is still too big for her so I'm wondering about changing to a GRX 30/46 chainset. Also from 25mph and upwards she freewheels so a reduced biggest gear will not be an issue. How compatible is the GRX chainset with Ultegra?

I'm a trendy consumer. Just look at my SM-G950F using Tapatalk

slowster
Posts: 804
Joined: 7 Jul 2017, 10:37am

Re: GRX chainset.

Postby slowster » 14 Aug 2019, 4:41pm

If a 34 x 32 bottom gear is not low enough, will 30 x 32 be low enough? Moreover, if she freewheels above 25mph, that suggests she not only does not make much use of 50 x 11, but also that she does not make much use either of 50 x 12, and possibly 50 x 13 as well (unless she has a low cadence and tends to push big gears). Why are you considering 46/30, other than for the reason that Shimano is not offering anything else?

Fit a Spa Super Compact chainset, and she can choose from 46/30, 44/28, 42/26 and 40/24. For example, a 24 tooth inner ring would give three lower gears than 34 x 32, and a 40, 42 or 44 tooth outer might give her a more usable top gear and a better range of cruising gears than a 46 tooth outer.

Comparison of 50/34 with 46/30

Comparison of 50/34 with 40/24

Comparison of 46/30 with 40/24

Valbrona
Posts: 2192
Joined: 7 Feb 2011, 4:49pm

Re: GRX chainset.

Postby Valbrona » 14 Aug 2019, 6:55pm

Can the FD be lowered? Bracket type usually allows this.
I should coco.

Geoffroid
Posts: 15
Joined: 18 Oct 2011, 1:46pm

Re: GRX chainset.

Postby Geoffroid » 14 Aug 2019, 7:27pm

I think the GRX 2x chainset sits 2.5mm further out than a Shimano road chainset (to allow for wide rear tyres). It needs a GRX front mech, that will move out the extra 2.5mm.

NetworkMan
Posts: 536
Joined: 25 Aug 2014, 11:13am
Location: South Devon

Re: GRX chainset.

Postby NetworkMan » 15 Aug 2019, 8:48am

Strange how Mr. Shimano persists in offering chainsets that won't take small enough rings. This new GRX one looks like a re-cooked 4 arm one. Another lost opportunity - you'd thing that a 'gravel bike' would at last be a reason for having a wide range double or a triple. Perhaps it's intended for flat gravel!

pwa
Posts: 9838
Joined: 2 Oct 2011, 8:55pm

Re: GRX chainset.

Postby pwa » 15 Aug 2019, 9:04am

Any chance that Barry and Janet could simply put a 36 tooth max cassette on it? It is 11 speed and it looks like Shimano already provide 34t, and often you can go a couple more than they specify.... That, along with a longer chain, would be simplest if it were possible.

NetworkMan
Posts: 536
Joined: 25 Aug 2014, 11:13am
Location: South Devon

Re: GRX chainset.

Postby NetworkMan » 15 Aug 2019, 9:15am

pwa wrote:Any chance that Barry and Janet could simply put a 36 tooth max cassette on it? It is 11 speed and it looks like Shimano already provide 34t, and often you can go a couple more than they specify.... That, along with a longer chain, would be simplest if it were possible.

I'm sure that this "compact double doesn't offer low enough gears" issue has been addressed on this forum more than once before. Anyone an expert with the search tool? - I don't ever get on very well with it.

pwa
Posts: 9838
Joined: 2 Oct 2011, 8:55pm

Re: GRX chainset.

Postby pwa » 15 Aug 2019, 9:24am

Having had a brief glance at this GRX stuff, it does seem to dangle the prospect of a cassette going up to 40t with a new derailleur. If this were compatible with the existing Utlegra levers it would (not cheaply) make a difference.

slowster
Posts: 804
Joined: 7 Jul 2017, 10:37am

Re: GRX chainset.

Postby slowster » 15 Aug 2019, 9:41am

pwa wrote:Having had a brief glance at this GRX stuff, it does seem to dangle the prospect of a cassette going up to 40t with a new derailleur. If this were compatible with the existing Utlegra levers it would (not cheaply) make a difference.

I think the 40 tooth cassette can only be used with a single front chainring in a 1 x 12 set up. The rear derailleur designed for use with the extra large cassettes has a maximum capacity of 31 teeth, i.e. there is not enough capacity for a double chainset as well.

pwa
Posts: 9838
Joined: 2 Oct 2011, 8:55pm

Re: GRX chainset.

Postby pwa » 15 Aug 2019, 9:43am

slowster wrote:
pwa wrote:Having had a brief glance at this GRX stuff, it does seem to dangle the prospect of a cassette going up to 40t with a new derailleur. If this were compatible with the existing Utlegra levers it would (not cheaply) make a difference.

I think the 40 tooth cassette can only be used with a single front chainring in a 1 x 12 set up. The rear derailleur designed for use with the extra large cassettes has a maximum capacity of 31 teeth, i.e. there is not enough capacity for a double chainset as well.

Makes sense, unfortunately.

geocycle
Posts: 1694
Joined: 11 Jan 2007, 9:46am

Re: GRX chainset.

Postby geocycle » 15 Aug 2019, 10:47am

I’m having reasonable success with a spa sub compact 44/28 matched with 11-32. All 11 speed 105 which is not far from Ultegra. The two smallest sprockets are a bit noisy but I’m much more interested in the low gears. Importantly also the cruising range appropriate to your landscape without lots of front shifting. The spa chain set seems to be a modified triple with a chain guard replacing the big ring. I’ve set the height of the derailleur to 2mm above the chain guard (ie the faux big ring), I tried to use the 3mm above the actual big ring but it fouled the chain guard even when restricting the lateral movement as much as possible. Seems to work ok.

pwa
Posts: 9838
Joined: 2 Oct 2011, 8:55pm

Re: GRX chainset.

Postby pwa » 15 Aug 2019, 11:02am

geocycle wrote:I’m having reasonable success with a spa sub compact 44/28 matched with 11-32. All 11 speed 105 which is not far from Ultegra. The two smallest sprockets are a bit noisy but I’m much more interested in the low gears. Importantly also the cruising range appropriate to your landscape without lots of front shifting. The spa chain set seems to be a modified triple with a chain guard replacing the big ring. I’ve set the height of the derailleur to 2mm above the chain guard (ie the faux big ring), I tried to use the 3mm above the actual big ring but it fouled the chain guard even when restricting the lateral movement as much as possible. Seems to work ok.

I have put a variation on that (modified triple) Spa extra compact double chainset on two bikes for family members and it works really well with their 8 speed set-ups. 44t is a big enough ring for reasonably fast progress over rolling hills, and the 28t smaller ring is well worth having. If it also works with 11 speed Shimano stuff, that is great. Different bottom bracket as well, of course.

slowster
Posts: 804
Joined: 7 Jul 2017, 10:37am

Re: GRX chainset.

Postby slowster » 15 Aug 2019, 11:44am

I'm using a 40/24 Spa compact chainset. The bike came supplied with a 46/36 chainset and 11-28 10 speed cassette, which was just too high for my level of fitness and because I use it mostly for off road tracks. I had to replace the front derailleur because it fouled the chainstay when lowered to within 2mm of the 40 tooth ring. However, that derailleur was a Microshift Arsis, which looked to me like it has a longer cage than, say, Shimano Ultegra, so a standard Shimano front derailleur might not need to be replaced depending upon the choice of outer ring size, seat tube angle, and the shape and size of the chainstay. I fitted a CX70 front derailleur, and having anticpated that I could have some problems getting the shifting to work with such small non-standard size chainrings, I was very pleasantly surprised at how well it shifts.

Eventually I want to replace the 11-28 cassette with 11-32 or 11-34, not so much for the lower gears but rather to increase the gaps between cruising gears. The 11-28 has one tooth jumps from 11t to 15t which would be fine for fast riding on the road, but for off road riding the gaps are too small and I always seem to have to shift at least two sprockets at a time.