Single to Double front chain wheel Conversion /Bottom braket help ?

[BADGER.BRAD]

Hello all,
I have come to the point where my Chain set is on it's last legs needing a new front ring, rear cassette,jockey wheels and chain ( again!) now I'm looking at converting to a double front chain ring set up to give me higher gear options when I get on the flat or downhills ( at present on my commute I have various bits where I end up spinning out in top 44/11) As I have a 4 hole crank at the moment I'm limited to 48 tooth as a max top gear but really need 52 as the highest this means I need to go to a 5 hole crank but haven't clue as to how to work out which bottom bracket I need in order for the smaller chain ring to clear the frame can anyone offer any help. The bike is a MTB based hybrid which at present has 44/11 as a top gear and 44/34 as a low gear the low gear being the lowest I would wish to go. As I intend to buy the chainset from eBay to keep costs down I will need info on how to work from this to a suitable BB .

Any help would be much appreciated

Brad

 

[RecordAceFromNew]

Which bike is it?

If you want to run a double from where you are you obviously will also need to get a front shifter and a front mech.

Since you have a hybrid I assume you have flat bars. With only a few exceptions flat bar front shifters generally pull too much cable for road front mechs. However if you are never going to run a triple then such shifters will drive road front mechs fine.

Regarding chainset, do you know what is the existing's BCD? If not what make and model? If and when you are sourcing a new double chainset it will/should have associated BB specification that will deliver the requisite chainline for the front mech. Choice of specific front mech is driven by the chainrings' tooth count., the seat tube diameter and front gear cable run possibilities.

A double chainset might exceed the chain wrap specification of the current rear mech. Knowing the cage length of the existing rear mech will help inform what chainring range it should be good for.

By this point I don't know if your intention to keep cost down is still intact. If not, then

a) I suspect someone will come along and say since 44x11 on e.g. 700c should deliver ~30mph already at 90rpm at the pedals, you could/should simply try spinning faster.

b) If 44x34 is a little lower than you need you can always stretch the high by running a bigger single front chainset, e.g. 50T, and with a 11-36 cassette. That will increase the low by 7% and high by 14% approximately.

 

[BADGER.BRAD]

Sorry for the delay I posted to this but some thing must have gone wrong ! I have since realized that there are no Cable stops on the frame so I will need to find a front mech which has the stop included ? The bike is a Revolution Courier MTB based Hybrid with 26 inch wheels. It can be spun up to faster speeds than the 22 mph comfortable cruising speed but not for any length of time. I did consider going completely the opposite direction with this and going single speed but this would require a chain tensioner so I gave up on that Idea ( They seem to take away from the simplicity of single speed) plus I already have a battered old single speed MTB which I share with my son. It does look as if I'm going to have to return the same gearing/set up as 4 hole sprockets ( in the UK ) seem to go to is 48 teeth and the rear 8 speed cassette 11 to 34 rather than 36. The bike works well as it is really but as I was needing to change everything I thought I would go for something a little different !

Thanks all

Brad

 

[mickle]

Dont do it.

 

[MacB]

to be honest you can pick up a new chainset for not much different to the cost of a chainring upgrade/replacement.

As it's a 1x8 at the moment then the existing chainring will be placed so that you can hit all 8 gears at the back. If that's aligned with the centre of the cassette then it's possible that a larger chainring in that position would connect with the chainstay. Obviously you can move it out but then it may not play nicely with the low end of the cassette.

You could grab a cheap square taper trekking chainset for about £25, with say 26/36/48, either ignore or remove the inner ring and set it up with a new BB to give the chainline you want.

If you currently have 26" wheels with a 44t ring and 11-34 cassette that will be giving you approx 32-100 gear inches.
The same rear with a 48/36 double would give you 26-110 gear inches.
If you don't need/use your existing low then you could go for a closer range cassette with the double.

The problem you'll have is the lack of facility for a front derailleur setup, you'd need some sort of clamp on cable stop for the DT. Or you could go old school and just use an old spoke to manually lift the chain between the chainrings.