Double with bash crankset compared to triple

chrisga

Would a double with bash Shimano crankset have a longer spindle (can't think what else the part that goes through the b/b) than the corresponding triple to allow the chainring side to be further away from the centreline of the bike to get a better chainline?

In my mind the gap between the double rings should line through with the centre of the cassette. On a triple it would be the middle ring. If I was converting my bike and wanted to move the driveside crank out a bit to get the ultimate in chainlines, the non driveside crank might not sit on the end of the spindle by the corresponding amount that the right hand side crank moved out by if that makes sense. That seems like it could be a bit iffy to me, (I'm not the lightest rider).

The Northern Monkey

nope.

There are 2 types.

68/73mm (can be either 68mm or 73mm - depends on length needed and is adjusted using spacers).

83mm. Usually only found on big bikes... ie DH bikes.

chrisga

Isn't 68/73 the length of the bottom bracket case on the frame not the spindle attached to the right hand side crank though? Or do you mean there are different length spindles available for the same cranks?

When i look down crc for example the only options for slx are 170 or 175 which is crank arm length. Nothing about different spindle lengths.

The Northern Monkey

2 different spindle lengths....

You cant use a 68/73mm spindle in a 83mm BB as it'll be too short!

chrisga

Ok, but the 68/73 is only one spindle length? Not two cranks available.
So if i fitted an slx double with bash in theory the inner ring would sit on the same chainline as an slx triple?

The Northern Monkey

no, like I said you use a spacer.... all included with the crankset and instructions that come with them.

And yes...exactly the same.

You can take the outer ring off a tripple and fit a bash and have the same as an out of the box double crankset.

The 68/73mm or 83mm is determined by your BB shell width. nothing else.

chrisga

So you space out a double with the spacers more than a triple. i might be being a div but don't understand the seemingly simple shimano instructions!

I dont think I'm explaining what i mean very well though. I know that you get 68/73mm shell widths and these are taken account with the spacers that come with a bottom bracket when installing. But, I had a triple, setup from the factory with relevant spacers in all the right places. I ditched the outer ring and put a bash ring on. Now in my engineering head the sensible thing to do is put another spacer in to move the right hand crank out from the bike so that the gap between the 2 rings is where the middle ring used to be so you get a better chainline at the extremities of the cassette. If I were to do this though, the corresponding amount of spindle appearing through the b/b bearing on the non drive side would reduce by the same amount. This is what I'm worried about.

If iwent out and bought a double with bashring off the shelf rather than convert a triple, do they have a different offset on the chainrings or are they just the same as a triple but with a bashring bolted on and you have to make do with a slightly rubbisher chainline if you want to run 2 rings instead of 3?

Matt 24k

I think you are trying to get your double chain rings more centrally loacated to the the chain line? This makes a lot of sense because as you say your front chain rings will be better positioned to use both ends of the rear cassette.
I also notice that there is a Shimano SLX double front mech available with a different quoted centre line measurement. With a 73mm frame and a Hollow Tech 2 type BB you would have one spacer on the drive side. Adding another spacer to the drive side would move your chain rings over but the concern is that there will not be enough crank shaft on the non drive side for the crank arm to safely attach to.
I hope some one on here knows the answer as I want to do the same thing to make a 2 x 10 set up using an LX chainset with 24t and 36t chinrings mated to an SLX 11-36t 10 speed rear cassette.

chrisga

Yep you got it. Thats exactly what i mean. I understand the whole spacers and spacing the bottom bracket to achieve the best chainline for a double ring setup. But its the reduction in spindle showing on the non-drive side for the crank to clamp to that is niggling me.

delta5

I don't think there's any point trying to space the middle ring further away from the BB shell.
Think about it - if your old middle ring was lined up to optimise the angles to the extremes of the cassette, your new 36T chain ring will also be best in that same position.
Moving it further out will approach the position of the old triple outer chain ring, where you need to avoid the big-front-to-biggest-rear gear combo. That's okay if you don't want to use that combo, but remember that moving it will also affect the crank to chainstay clearance on the non-drive side.

I recently fitted an SLX double & bash, and if anything I'd prefer the double slightly closer in than it is with the spacers provided. Looking at the chain-line, there is a fair bit of chain bend when running from the biggest rear cog to the 36T, but luckily with a 34T cassette I seldom need that combo. On mine the outer-to-outer is already a much straighter chain-line.