Making a Super Compact Chainset?

[MacB]

I've been musing about what I'd like as a chainset and keep getting drawn to the idea of a super compact, partially inspired by Sheldons note about the 28/51 he ran. My thoughts so far:-

standard road triple chainset(130/74 bcd), bashguard in outer, 44t middle and 24t inner - running with a 9/10 speed 12-27 cassette
does it need a slightly longer BB to position the larger ring for better use of the bigger gears?
can you get a band on double FD that will cope with a 20 tooth gap? as far as I'm aware a double is a 16t capacity and triple a 22t, the FD itself should be able to be positioned a bit lower on the ST. Or could the shape of a triple FD be messed with a bit to do the job?
I'm guessing that this wouldn't play well with an STI shifter so was thinking of a bar end for front shifting only.

On a similar subject I'm hashing out details on the custom frame, one of the requirements was to match clearances to the forks I've got. This means up to 700x44 tyres with mudguards. My frame builder has come back to me indicating that there may be chainring clearance issues with this unless I run MTB chainsets or use square taper, with longer axles, for road chainsets. In theory I'm ok with this but three thoughts spring to mind:-

if I widen like this then a road FD won't reach so that means MTB FD and therefore definitely no STI for front shifting
q-factor(width distance between pedals) I have no idea what it is on my current bikes but keep reading that the narrower the better(would I even notice?). Exactly where is q-factor measured from and to? As in, is it inside of crank arm, where pedal attachs, to the same the other side, or is it from the outside?
rear shifting - by moving a chainset in/out are you not interfering with chainring to cassette chainlines? is there a chainline for a cassette to compare against? the bike will have 135 rear spacing if that matters.

Really I'm looking at a 1x9 or 1x10 combo here that has the bonus of a teensy granny ring for steep stuff.

 

[alecstilleyedye]

you will need a longer bb axle and i would use a triple front mech with the limit screws preventing the chain trying to move onto the bash guard.

i wouldn't worry about q-factor. i ride triple, double and fixed, and don't notice the difference in q-factor at all.

 

[MacB]

Thanks Alex, I've been trying to measure the q-factor on my bikes and it would appear that I have no preference, they're all different, so it would seem you're right.

 

[TheDoctor]

If I was doing a 44/24 front, I'd deffo use an MTB mech. The cage is shaped for a ring around that size.
Next width of BB up or so, and game on

 

[frank9755]

You might get away with a triple road front mech and hence STIs. As you will know, it will depend on your frame geometry.

My audax bike has a 105 triple front mech and is able to manage a 26-39-50. On the 26 it will get all but the smallest sprocket (11T) before the chain rubs the bottom of the mech. Given you would never want to cross-chain to the extreme, a 24T might be acceptable.

 

[MacB]

Hmmm, so, further thoughts:-

does the chainset matter whether it's MTB or Road if I go with square taper, can't I just choose a BB for chainline?

Chainline - I was more concerned that if I have the chainrings are too far out I'll be compromising rear shifting/chainline

STIs - I'll get them anyway and give it a go, I believe that you can use 9 or 10 speed with bigger cassetes as long as you use a MTB 9 speed RD? It is just a dumb reaction whereas the newer 10 speed RDs aren't? It's only really the rear shifting that matters, if the front will shift with the STI then great otherwise I'll just use a bar end shifter. If I'm going with a 12-27 cassette then pretty much any 9/10 speed RD will do? The inner chainring is really only for backup(Surrey Hills/Ditchling Beacon), I wouldn't expect to use it very often.

 

[MacB]

You might get away with a triple road front mech and hence STIs. As you will know, it will depend on your frame geometry.

My audax bike has a 105 triple front mech and is able to manage a 26-39-50. On the 26 it will get all but the smallest sprocket (11T) before the chain rubs the bottom of the mech. Given you would never want to cross-chain to the extreme, a 24T might be acceptable.

Thanks Frank, that's really useful, now it's really just a question of whether the FD will handle a climb from 24 to 44t in one go. As Sheldon managed it then it has to be possible!

 

[RecordAceFromNew]

If you are planning to run 44/24 up front, assuming the front chainline is standard (i.e. by not using a bb significantly longer than the chainset manufacturer's recommendation - see below why you might not want to), you will find that when you are on the small front the chain will clash with the larger front ring for some of the small sprockets at the back. Worse for road chainsets (front chainline ~45mm) than mtb (47.5mm or 50mm).

If you want to calculate how many will become inaccessible, these figures should help: chainring pitch is ~6.5mm, and, specifically in your case chainring size difference 40.4mm, chain width ~7mm, chain ring thickness 2mm to 3.5mm dependent on make/model, chainstay length whatever it is probably ~400mm, rear chainline ~45mm, and finally cassette width 36.5mm. It is likely at least two sprockets are out, especially given the chain has lateral play.

I think in terms of lost ratios it is unlikely to be an issue in your case. An upshot is that since you can't use small/smalls anyway the official range of the front mech becomes somewhat moot. I also think you might be able to establish that you can use a road triple mech with a mtb chainset, and, if so, front sti. The potential issue of a road front mech having inadequate reach for a mtb chainset won't be an issue here assuming as aforesaid you are not going beyond manufacturer recommended bb length, since in any case you won't have an outer ring.

I have a table of standard q factors of Shimano chainsets somewhere if you need it. IIRC all road ones are close to 150mm, while mtbs' can be anywhere between 170mm and 190mm. Deducting the crankarms' thickness will give you some idea about potential clash with wide stays.

 

[MacB]

Great info there RAFN, you guess correctly small/small chainlines are not in my consideration. 44/24 and 12-27 gives me 24 to 54 and 44 to 99 gear inches in small/big respectively. I'd only envisage using the largest 4/5 cogs with the small chainring and would expect to be able to hit the whole cassette from the large chainring. But I would probably bias the large to the small cogs and avoid the big/big combos as well. I'm sort of thinking a 2x10 that has 8 gears in general use from the outer ring and a 4-5 gear overdrive if I need it. Sort of like a range of 56 to 99 and a sub range of 24 to 40. I can get up anything around my way without dropping below about 60 gear inches, and this gives me a nice 5 gear cluster from 62.5 to 85 inches(I'll be using 700x32 in case you're wondering at my gear inch results).

Just to clarify on the q-factor, I have two sets of cranks off the bikes and have done them up so that the crank arms are parallell. It seems to me that you measure from outside edge to outside edge. One is an Ofmega Alfine triple and I get 170mm for that, the other an Ultegra road double and I get 150mm there. Does that seem right to you?

 

[RecordAceFromNew]

MacB given your large ring is actually the middle ring in terms of geometry, if I were you I wouldn't avoid running large large-ish. That will take full advantage of your design, and there wouldn't be a gap between 56 and 40. I have never seen any scientific evidence showing cross chaining is either inefficient or damaging to the chain for a derailleur set up.

q-factor is indeed measured outside to outside. Ultegra should be within 150mm plus or minus 5mm so no surprise there. I have not come across an Ofmega Alfine triple. I presume it is vintage? Is it installed with the recommended bb? Perhaps more importantly is the chainline reasonable?

 

[MacB]

Ah, the Ofmega is going in the spares bin, it was the chainset that I used with the 9 speed hub gear and is being replaced by the Hollowtech ones as I'm using this new fangled standard shell EBB, that only works with OBB cranks. The Ofmega will have a future use as the crankset on my 3 speed isn't a replaceable chainring so I'll need it sometime.

I think I'm sort of there mentally now and have been playing with various calculations, very attractive right now is buying a MTB 22/32/42 triple 104/64 BCD. Ditch the 32, move the 42 to the middle and put a bashguard in the outer(saves sodding about with different chainring bolts). I know the q-factor would be bigger but it obviously hasn't bothered me so far, the Ofmegas have done over 3k miles. But I use big platform pedals, can move my feet about, so maybe that has an effect as well. Run the 22/42 with a 12-27 or 11-25 and it works out quite nicely.

That just leaves the FD to sort, if I want to try for STI style front shifting then I could use a road double FD. I imagine I'd need to use a slightly shorter spindle to move the large ring into about 46mm. Then it would be a case of clearances, can I get the FD mounted low enough on the seat tube. Though Saint Sheldon did it originally with a 28/51 2x5, then a 28/40 2x9, that's quite a height difference for the front mech. Still it'll be trial and error, at least I have a plan B this time, ie the bar end and any FD that'll work.

I do have one further question though, slightly different, I'm due to install the new cranks and EBB tomorrow, once the BB shell is faced. This will be my first attempt with OBB cranks and, if installed as normal I'll get a 46mm chainline for the single ring. The rear chainline is 48.8mm, I can run as is, the hub has a published +/-5mm tolerance for chainline. But I'd quite like to space out the driveside by a couple of mm if possible. How does this work, if it does at all, on Hollowtech chainsets? For the EBB I'm not allowed to put any spacers between cup and BB shell but I did order the MTB spacer kit with it. This is designed to go on driveside, between chainset and BB cup, and create enough additional space that the chainset doesn't foul the EBB cups(they're larger than normal ones). Is there a minimum amount of axle protrusion on the non driveside required for crank attachment? I can't imagine 2mm less would make a big difference but I'd rather not bugger it up with blind experimentation.

 

[RecordAceFromNew]

I do have one further question though, slightly different, I'm due to install the new cranks and EBB tomorrow, once the BB shell is faced. This will be my first attempt with OBB cranks and, if installed as normal I'll get a 46mm chainline for the single ring. The rear chainline is 48.8mm, I can run as is, the hub has a published +/-5mm tolerance for chainline. But I'd quite like to space out the driveside by a couple of mm if possible. How does this work, if it does at all, on Hollowtech chainsets? For the EBB I'm not allowed to put any spacers between cup and BB shell but I did order the MTB spacer kit with it. This is designed to go on driveside, between chainset and BB cup, and create enough additional space that the chainset doesn't foul the EBB cups(they're larger than normal ones). Is there a minimum amount of axle protrusion on the non driveside required for crank attachment? I can't imagine 2mm less would make a big difference but I'd rather not bugger it up with blind experimentation.

Please hold for a second! I imagine you are contemplating extra driveside spacer to match your 48.8mm rear chainline? If so I would suggest you recalculate/check your rear chainline, because it can't be 48.8mm for a 135mm OLD and a 9 speed cassette that you said in your OP that you are having.

A 9 speed Shimano cassette is 36.5mm wide end to end, if there is no gap between the cassette and the right drop out, the chainline would be (135mm/2 - 36.5mm/2) or 49.25mm. Therefore your chainline can only be 48.8mm IF the gap is (49.25mm - 48.8mm) or 0.45mm, which I suspect you would agree is impossible.

As I mentioned earlier your rear chainline should be 45mm. I suspect you wouldn't want to do anything to the front chainline.

 

[MacB]

Ah, sorry, I'd jumped subjects there, the super compact chainset is my advance planning for the new custom Ti frame which will be run with deraillers. So far I've only actually bought the forks and, hopefully, I've finalised the frame diagram today. For that build I can use any crankset/BB combo I want. I'm still agonising over wheelsets and colour schemes though. Part of me wants to go with bare Ti frame and then red and black everywhere else. But all of the 'boutique' red rear hubs seem to come with bloody noisy freewheels....no doubt I'll agonise over every single piece.

The Hollowtech crank question was in relation to the new Salsa Vaya frameset that arrived this week. For that I'm running the I-9 internal hub gear which has the 48.8mm chainline. The chain tension is being done by a Forward Components standard shell EBB, which allows for 7mm of lateral chainstay length adjustment. That, and maybe a halflink, should be all I need for chain tensioning and means that I can run the hub gear in vertical dropouts without a tensioner. Basically the EBB is a pair of slightly oversize OBB cups and then there is an insert that takes the crank spindle. The insert slots into the cups, has the bearings and is held in place by 6 set screws on each side. This is a good review/explanation:-

http://forums.mtbr.com/showthread.php?t=490528

That said I had the BB shell faced, the FC EBB installed and the cranks installed today, and I left it without spacers to start with. I think the EBB cups are a bit bigger as the 46mm chainline seems closer to 48mm now. Regardless the hub specs indicate a tolerance of +/-5mm for the 48.8mm chainline so the 46mm was always within acceptable range, it certainly looks straight to me.

Sorry about that, my fault I threw in a different subject mid discussion, but that was great info on the cassette chainline. So a 9 speed is 49.25mm and that would work very well if I used a MTB chainset and only the middle and inner ring positions. As you already mentioned I should be able to hit the whole cassette from the large ring and at least half from the small.

 

[Tollers]

MacB. You deserve a tinkering award. I'm pretty sure you're going to accidentally hit on a goldmine idea someday.

 

[MacB]

MacB. You deserve a tinkering award. I'm pretty sure you're going to accidentally hit on a goldmine idea someday.

Hopefully the Vaya will be all built up today, just back from getting headset fitted and chain is all done. There'll be a natural gap, maybe 3 months, until Ti frame is ready and fully built up. In the meantime I thought I might actually stop arsing about and get out and ride