armybikerider

What is the largest "workable" difference between chain ring teeth on a double crankset? 16 teeth? or more???

And what is the limiting factor?

Wilfred Laurier

The limiting factor is the geometry of the front derailleur. I believe this number is usually given as the 'capacity' of the front derailleur (calculated completely differently than the capacity of rear derailleur). See specs from the derailleur manufacturer you are considering.

I think 16 teeth is a commonly published capacity of double-chainring FDs. You might be able to go a bit beyond that limit, but it might not work acceptably.

rhenning

Depends on the rear derailleur. Roger

tomtomtom123

If not considering the chain capacity of the rear derailleur, I use a 20T jump between double rings. It's works fine. With Shimano FD, it needs trim. SRAM YAW doesn't need trim but needs to be careful with the H limit screw so that it doesn't overshoot the chain and fall to the outside (because the front of the cage is very wide on the SRAM YAW in order to prevent chain rub when cross chaining on the outer chainring). For the inside, a chain catcher is mandatory for 20T jump. SRAM YAW FD comes with a chain catcher included. On 2x10, I can't use 8,9,10 cog when on the inner chainring because the chain will rub on the outer chainring, but that's fine because I don't need to use those 3 cogs since they're overlapping in range with the outer chainring gears.

On a workstand I was able to use 22T jump, but I never tried it on the road, and the SRAM chain catcher wasn't long enough for 22T unless you glued something to the end of it.





I have 12-34T with a 20T jump in the front for a chain wrap capacity of 42T. I use the 10-speed 4700 RD-GS, and it's supposed to have a capacity of 41T. 42T chain wrap is just at the limit of this RD, but since I don't use the outermost 3 cogs anyway (when on the inner chainring), I lengthened the chain by 2 links, so that the pulley drags on those 3 cogs, in case I want to increase my chainring size by 2T, without needing to change the chain. Or if the chain breaks, I can remove 2 links. I'm using TA chainrings, but Spa Cycles chainrings were shifting a little more easily because it's 0.5mm thicker so that the chain catches faster onto the pins (although my memory of the difference in thickness could be wrong).

DaveSSS 

Quite often, the crank mounting bolt pattern limits the difference to 16. Sub-compact cranks with 48/32 or 46/30 rings are becoming more common. I'm using a 48/32 with an 11-34, 12 speed cassette. I can still spin up to 35-38 mph and climb as slow as 5-6 on really steep hills or mountain climbs.

tomtomtom123

The BCD doesn't actually limit the teeth difference. It only limits the minimum number of teeth possible on the inner chainring. You can always select an outer chainring that is more than 16T greater than the inner chainring. The problem is that manufacturers keep selecting BCD sizes that are too large for the inner chainring. For example, Shimano chose to use 80mm BCD inner on the GRX crankset, which is physically limited to 28T inner chainring minimum (oh I see SRAM/TA has a 26T ring for 80bcd but GRX has asymmetric spider arms so it might be difficult to find a 26T to fit it). If you want to go smaller, then you have to buy a 110/74 bcd road crank, or else buy a mtb crank with 104/64 or 96/64 bcd with wide Q-factor.

Edit: here is a 25T 80bcd ring, but for 4 arm symmetric bolts.
https://www.apollocomponents.com/prod...-bcd-x-ring-2/
https://velosvet.sk/produkt/prevodni...g-25t-sram-xx/
https://www.rczbikeshop.es/es/b-o-r-...lts-black.html
BOR website doesn't work
https://www.facebook.com/pg/bor.germany/about/
If you know the angles of the bolt hole location of Shimano's proprietary design, you could laser cut a bolt hole adapter ring to get one of these symmetric 25T rings onto the GRX spider. Since the chainline is 47mm on the GRX and standard road is 43.5mm, you could make the laser cut adapter ring out of a stainless steel or aluminum sheet of 3mm or 3.5mm thickness.

16T jump allows for more tolerance with setting up the FD while still getting good shifting and clearance for cross chaining. But as I mentioned, I can do 20T successfully, just without being able to use the outer 3 cogs when on the inner chainring due to chain rub.

Retro Grouch 

Riding a bike isn't the same as building a bike that works.

I once built up a bike with (I think) a 46/36/26 triple crankset and a 14/18 5-speed freewheel. It provided all of the gear ratios that I ever used with no extras in a nice, logical sequential shift pattern. Once I came to a hill it was a PITA to ride. Starting up the hill was okay but when I came to the 18 tooth cog the next gear in sequence required a double shift all the way across the freewheel in back. By the time that happened, I had lost all of my momentum so I had to grind up every single hill. Those duplicate gears that guys so often scoff at - they're actually a good thing. They give you choices so that you can time your front shifts to match the terrain. I learned a lot about bike design from that bike. I'd never do it again, but I'm glad that I did it once.

tomtomtom123

I don't have problems shifting 5 gears all at once in a single trigger push with Shimano's rapid fire road triggers. So I see the 20T jump between my double rings as a big advantage in gear range. When shifting to the outer chainring, I have to shift 4 or 5 cogs inward, which can be done in a single trigger press and only takes maybe 1/2 a second longer than shifting 1 gear. When shifting to the inner chainring, I have to shift 4 or 5 cogs outward, which Shimano road triggers do individually (1 trigger press per gear), but 5 gears also only takes an extra 1 second compared to shifting 1 gear. You only have to push the trigger in 5 rapid successions (doable in 1 second) and the cable slack will simply allow the derailleur spring to continuously apply force to the chain until it shifts all they way across the cassette, which usually happens in 1 extra crank revolution for me (if I'm pedaling at 70-80RPM, I can complete the revolution at the same time I complete the 5 trigger presses). I also only shift the chainrings perhaps 2 times for every 10 minutes, so 1 + 1/2 second = 1.5 extra seconds of time isn't that big of an issue for me.

ThermionicScott 

Not just hardware, but finesse on the part of the rider plays a role when the jump is large.

CliffordK

54/34 or 50/30 would give you 20T.

Some kind of a chain catcher might help with shifting.

One issue is trying to keep the chain from rubbing on the larger ring when cross-chaining going small/small.

Sy Reene

There is something like the linked below; can't speak to how well it works, nor why Shimano/Campy etc haven't tried similar.

WICKWERKS ULTRA WIDE

armybikerider

Thanks for the replies!

I currently have a 52/36 and have just ordered a 34 t inner chainring.

shelbyfv

There's a good chance that if you need a 34, you may not miss the 52 vs a 50. There will always be weird situations and folks who imagine they understand more than the "herd" but the normal combos work for most.

tomtomtom123

You could probably get a 33t if you have 110bcd 5 arm.

CliffordK

True in many situations.

Personally, I was a late adopter of the 34, and mounted it specifically for one ride up in Portland that kicked my rear with the 39. Otherwise I would be fine for all my other riding with "Standard". But, chose not to swap back and forth between rides.

Of course, the overall gearing is a combination of what one has up front and behind, both high and low.

The move from 13/14T freewheels to 11T cassettes has also reduced the reliance on large front sprockets.

However, the other thing that has happened is a general move to both higher gears in the rear (11T), and lower gears in the rear (25T/28T/30T/34T/etc).

Say a freewheel, 14:21 gives one less than a 2:1 overall ratio.
On the other hand, a cassete, 11:33 gives a 3:1 overall ratio.

Say for chain rings 54/34 gives one about a 1.5:1 ratio. I.E. One can shift the 11/33 cassette, and have a few sprockets to spare on each end (to avoid cross-chaining), and still have gear overlap.

Personally I don't see any benefit of discarding the 50/52/54 sprockets to get something like a 46 or 48 and much more gear overlap.

Well, that is unless the 46 is the small ring.

armybikerider

I ride probably 75% of the time in flat to rolling terrain and live in the 52 and middle third of the cassette. I'd probably do fine with either the 50 or 52, but the 52 is new so I'll keep it. I want the 34 to help me out on some short but very steep local climbs.

evets11

Here's MY ISSUE ... Has ANYONE of you Ever ATTEMPTED this ... GRINDING/FILING down the 5 CR bolt holes from a 74bcd CR to fit 80bcd? For the LIFE of me I CANNOT find a 5 hole 80bcd Chainring. I NEED one in 28t. I scowered EVERYWHERE on the 'Net to No Avail! It's for a Modern FSA TEMPO' square tapered 80/110bcd Crank with 48/32t CRs. I need to gear down my Gravel Bike from it's Stock 48/32t Crank 12-34t 8 sp. Cassette to 48/28t Crank 12-34t Cassette so I can REALISTICALLY CLIMB Offroad with 29er MTB Knobbies installed. NO I'm NOT gonna swap Cranks for Austetic reasons, mine matches the 'FSA' handlebars Beautifully, and I'm NOT gonna rig an 11-40t Cassette with a Wolftooth Roadlink either (Tho I HAVE done that with a Bike with a Compact 34/50t Crank and (Custom made) 12-40t Cassette PUSHING THE ABSOLUTE LIMIT of a Modern 8sp. Claris RD, but it WORKED in Lg/Lg CR combos w/o Catastrophic Failure repeatedly Shifting into Lg./Lg. from 34 to 50t in 40t or 50t/34t to 50t/40t and the Chain NOT falling off when shifting into the Sm./Sm. combo and the Chain NOT sagging Too much in the Sm./Sm. Combo). I THINK I can get a 74bcd Chainring onto 80bcd if I CAREFULLY 'Expand' the CR bolt holes. Has ANYONE of you tried this before?

Germany_chris

https://www.benscycle.com/ta-special...tes-pro5-444/p