Converting XC bike with high (road) gearing?

Northwind

I've just recently got myself a Carrera Kraken which is going to have to serve as dual purpose, road and offroad, for the time being. It does both pretty well for its price, the fork has a lockout and all it needs is faster tyres to make it capable on tarmac, but the one thing I have problems with is the long downhill roads on my commute- the gearing just completely spins out, I can't put any useful force on. it has fairly tall knobblies on just now but if I swap with my old marathons, that's going to reduce the circumference too.

The front set is 22T-32T-42T I believe, the rear set is SRAM 11-32

So... Is it realistic to fit a larger front sprocket? Perhaps offsetting that with a 34T rear? I know roadies tend to have a very large front sprocket and I'm not sure I want to go to huge lengths. I'm kind of assuming that a smaller rear sprocket isn't going to be an option, but, well, I know nothing really.

I can see the obvious drawbacks- most front chainsets with a high big wheel will have a fairly high little wheel too, and of course will be more vulnerable to grounding out. Is there anything I'm missing? i don't make an awful lot of use of the bottom gears as it is

Thanks for any help/advice.

stumpyjon

You could just go up to a 44 tooth large chain ring although that proably won't make a lot of difference.

I'm wondering about doing the same thing on an old mountain bike I'm using exclusively for commuting. I don't want to lose the really low ratios as I have some b*stard hills to get up but down the other side the pedals cease to be any use over 30 mph (have hit 48.5 mph on one of the long down hills).

You could put a road crank on like the Shimano 105 FC-5603 which is a triple ring with 30-39-50 sprocketts, you'll lose some of the very low ratios (standard XT FC-M770 is 22-32-44 so your smallest chain ring will now be similar to your current middle ring, maybe lose the bottom 2 ratios?). I think you'll need to change the front mech, standard MTB ones aren't designed to cope with much above 44 teeth, a road one will still work fine with MTB shifters.

Only other thing to consider is possibly the rear mech, if you've alredy got a long cage mech there's not much more you can do, if you've a short or medium cage mech it'll probably want swapping for a longer cage (will need to extend / replace the chain as well I would expect).

nicklouse

anything is possible.
But what end result are you wanting.

fitting a bigger ring on the front gives one result. Fitting a cassette with a bigger big cog has no relation to the fitting of a big front ring as you would never use that combo it the same way you would not use the two small cogs.

Cheeky Tosser

Just done the same myself with my "spare" bike which I am now using for commuting. I fitted a Shimano Hollowtech II triple set with big rings i.e. 48 - 36 - 26. I changed the rear cassette to a road ratio model 12 - 25 teeth. Although you now lose the very lowest ratios the closer ratio of the road cassette gives a much better feel when road riding with less jump between the gears. Needed to lift the front XT mech up the seat tube a centimeter to clear the bigger outer ring and shifting is fine - didn't need to put more chain links in or touch the rear mech (apart from slightly tweaking the shift limit screws.

Slick road tyres with reflective sidewalls (Conti City Contact Reflex), a Tesco 3w cree front light, Blackburn Mars 3 rear light and fleece-lined tights complete the winter kit. Bring on those cold, icy and dark mornings !!!

Northwind

Ah, now that sounds more the thing Cheeky- I don't really want to go to a massive ring but that could be a good balance. Thanks very much.

nicklouse wrote:

fitting a bigger ring on the front gives one result. Fitting a cassette with a bigger big cog has no relation to the fitting of a big front ring as you would never use that combo it the same way you would not use the two small cogs.

That was mentioned as a counter to losing low gears- any front set with a big top ring will probably also have a larger small ring, pushing up the low gearing, so using a cassette with a bigger rear ring will redress that balance slightly, though not completely. I wouldn't bother if it was a full road conversion though.

supersonic

Yes, if you replace all three.

Most front mechs can only handle a 'front large/medium difference' of 12 teeth - sometimes 14. So putting say a 50t chainring on what you have will give a 16 difference: too much.

Plus you have the maximum front chainring size, often 44 on standard MTB mechs.

11t cassette is the smallest ypu can go.

Northwind

Cheers Supersonic. If 12T difference is a good rule of thumb (it's a SRAM X5 front shifter) then that's what I'd end up with using the Shimano set Cheeky mentioned, do you think that'd be a goer? I know it'll end up at a point where the front cage is just the wrong shape for the job but I don't know where that strikes.

supersonic

Sounds a good option to up the gearing. Will nedd a new (longer) chain too.

merlie

go on treat yourself to a road bike ... soething cheap from gumtree or even try freecycle or ebay

Northwind wrote:

I've just recently got myself a Carrera Kraken which is going to have to serve as dual purpose, road and offroad, for the time being. It does both pretty well for its price, the fork has a lockout and all it needs is faster tyres to make it capable on tarmac, but the one thing I have problems with is the long downhill roads on my commute- the gearing just completely spins out, I can't put any useful force on. it has fairly tall knobblies on just now but if I swap with my old marathons, that's going to reduce the circumference too.

The front set is 22T-32T-42T I believe, the rear set is SRAM 11-32

So... Is it realistic to fit a larger front sprocket? Perhaps offsetting that with a 34T rear? I know roadies tend to have a very large front sprocket and I'm not sure I want to go to huge lengths. I'm kind of assuming that a smaller rear sprocket isn't going to be an option, but, well, I know nothing really.

I can see the obvious drawbacks- most front chainsets with a high big wheel will have a fairly high little wheel too, and of course will be more vulnerable to grounding out. Is there anything I'm missing? i don't make an awful lot of use of the bottom gears as it is

Thanks for any help/advice.

Northwind

I just did! But yeah, it's tempting. Or, my old bike could probably be resurrected. Just that with either of these plans, I'll get kicked in the nuts since I only got permission to buy this one because the old one was "too expensive to fix" and I'd save money this way... I think that would blow my cover

What's really annoying is that I can borrow my brother's revolution courier most of the time, lovely road bike... But it's geared too high for me, my right leg's not strong and so I can't get back UP the hill that the carrera's rubbbish at going down. Ah well.