Road Chainset on MTB

cooper.michael1 Posts: 1,787
edited March 2009 in Workshop
My commuter MTB is looking a little worse for wear after the winter. I have a spare Shimano 105 octalink chainset and Ultegra bottom bracket, would it be possible to fit this to a MTB frame..or would there be any issues?

Im thinking that there could be an issue with chainstay clearance? I mean the crankarms are narrower and the chainstays are wider???


  • Wappygixer
    Wappygixer Posts: 1,396
    Its your rings that may cause an issue as they are bigger.Bottom bracket length may also be an issue.
    You will also need to change the front mech to cope with the larger rings.The issue here is nearly all road mechs are bottom pull and mtb's normally use top pull.
    It may well work depending on the frame.I would give it a go as its not a big job to swap it back if needs be.
  • maddog 2
    maddog 2 Posts: 8,114
    suck it and see

    cranks/rings may clear the chainstays, may not.

    The mtb shifters will pull too much cable for a road front mech, but you can adjust this out easily as long as you just have two rings.
    Facts are meaningless, you can use facts to prove anything that's remotely true! - Homer
  • barrybaines
    barrybaines Posts: 273
    If it is a double 53/39 I think you will have clearance issues, a compact MAY fit? I put a triple on my Giant Escape M1 after trying to get a 53/39 on with no luck
  • dilemna
    dilemna Posts: 2,187
    First up what bike have you got and what is the current set up ie bottom bracket dimensions and chainset and ring sizes? You then need to look at the chain stay to see if it will accommodate fitment of a double or even triple as the angle is different to a road bike as the spacing between MTB dropouts the OLD is 135mm whereas road bikes is 130mm typically. Also MTBs have 26" wheels and road bikes 700c which are bit larger. Once you decide whether your chianset fits you need to decide whether you can retain your current triple MTB front mech as this will be designed for an MTB chainset which has much smaller rings. At best you will have to move it up the seat post at worst you will need a road front mech but owing to MTB frames being of larger dimension may not be feasible........The curve of the MTB front mech is suited to smaller ring chainsets and not bigger 52 or 53 ring road chainsets.

    Next is choice of bottom bracket. MTB bottom bracket shells are typically 73mm. Your Ultegra bottom bracket will be a 68mm (English) thread. Check to see if you agree. If it is 109mm wide it is for a double ROAD chainset and if 118mm for a triple ROAD chainset. Unfortunately 68mm shell bottom brackets will not fit in a 73mm MTB frame. Shimano did not make their road Ultegra bottom brackets in anything other than 68mm (English thread) or 70mm (Italian thread). Unfortunately I don't think Shimano do a complimentary Octalink bottom bracket in 73mm shell to take an Octalink road chainset. I stand corrected if they do. The ES51, ES71 Octalink bottom brackets for MTB chainsets will not fit Shimano road chainsets. Maybe some one else can advise on other makes of bottom bracket to suit or chainset combos? I tried finding a suitable Octalink bottom bracket for an MTB that would take an Octalink road chainset at first as this seemed the easiest................unfortunately it wasn't that simple and so have now ended up with something totally different but just as good if not better, but not before nearly giving up as I appeared to have hit an insurmountable brick wall.

    You may also very likely need to fit a smaller cassette as running on your MTB large 27T+ rear cog with a front ring any larger than 42T will rip your rear mech to shreds as the chain will simply not be long enough. Conversely you can't have a chain too long because then you end up with too many links if you run on the smallest 11/12 ring of the cassette on the rear with the 30T or 39T ring of the front chainset. I've added a few links to what I originally thought when I first set it up. I think it is now about 114 links. Will count next time it is off.

    Also you may need to change the shifters and or rear mech if they are shagged or readjust to achieve a half decent shift.

    There's a lot to think about as it is not that simple made deliberately so by Shimano for obvious reasons so you can't chop and change.

    For my bike a Trek 7500 hybrid which originally was bought as a cheap commuting bike which had half decent Deore LX components but a hideously crude MTB triple chainset, which I found too lowly geared for any decent road use. I was frequently spinning out in top. I used it for 2 years and when everything had worn out after about 15,000 miles I bought another bike to use in the meantime and set my mind to my Trek as a project replacing the worn out chainset and other components with road stuff as the frame was still excellent making a potentially excellent touring/expedition bike, so I didn't want to lose it.

    It now has a Campag Record Triple front chainset 53-42-30 on a Shimano UN 54 73mm x 113mm square taper bottom bracket. Cassette is Shimano Ultergra 12-25 and rear hub is Shimano XT 36 holes currently on Mavic Open Pro CD rim. Front is Mavic Open Pro CD on 32H Campag Chorus hub. Chain is Shimano HG93. Front and rear mechs remain as originally supplied Shimano LX, although rear is shortly to be replaced with XT long cage. I have to make a few adjustments but no more than you would have to do so in setting up a new mech or replacing an inner cable. I found I only had to slide the front mech up the seat post tube a small distance but can't say how much to accommodate the Record triple chainset which has a big 53 outer ring. Originally I tried the Shimano Octalink route as you intend but couldn't make it work. I got a Shimano Ultegra triple chainset which was a waste but I now use on another bike.

    Quite by chance I found my old Campag triple chainset set sat nicely on the original square taper bottom bracket. As this was shagged and of an unkown provenance I replaced it with the UN54. I was looking for UN73 but couldn't find any. Once on properly the small inner 30 ring of the chainset cleared the chain stay by 1.5-2.00mm which was a bit of luck. If gap is any larger than this then front mech will likely not be able to stretch across to move chain onto larger outer ring if a triple. My innermost 30 ring doesn't foul chainstay at all neither from flexing when on the move as the chainset is top quality Campag Record which is extremely rigid, strong and light. Changes onto the big ring are fine if at the limit of adjustment of the original LX front mech. Sorry don't know about doubles as I wanted to fit a triple. I would have thought principle is the same for doubles. I thought I might have probs with the Campag chainset going onto the Shimano square taper bottom bracket as the tapers between the two manufacturers aren't quite the same but I've not had a single prob in the last 2,500-3,000 miles. I check it from time to time but it is always rock solid. I took the chainset off last weekend to give it a good clean and it was still well and truly on. Came off easily with the puller though. The front mech makes changes smoothly onto small inner ring and also up onto the big 53 ring from the middle 42 ring. I could fit a 52 outer ring I suppose but haven't got round to it. The rear mech changes fine although I recently buggered about with the shifters dislodging a small spring which I think has had a slightly detrimental effect in their operation. I need to take another look at it to fix it. Anyhow I have an XT shifter to go on when I get time.

    All in all it took some time to arrive at where I am now, a year or so maybe more. It has been very much worth the hassle. It wasn't easy but all is working ok now. So I now have a new bike in effect with pretty good components on it. The frame is still excellent that I would otherwise have scrapped or let gather dust. You will probably only sort out what you want by trial and error. If it starts getting mega expensive which it might well do given that all components have just rocketed in price then ditch the idea and get another bike suited to what you want to achieve.

    I tried no end of bike shops several local in Cambs and a fair few in London, some were more helpful than others, alot just talked out of their asre or didn't know what they were talking about although most just told me what I was trying to do wasn't worth it, to bin the bike, as it was shagged anyway, and buy a new bike preferrably from them. Aside from this I like the idea of recycling whereas alot of bike shops don't seem to. The most helpful was Greg at Condor although I didn't buy the components from them as they had to order them he spent some time going through all the Shimano catalogues and his knowledge of bottom brackets and chainsets is encyclopedic. I got my hubs and rims from them and wheels built by Monty though so they made a bit out of me. I've also bought a fair amount of stuff from them in the past such as my lovely Barrachi 853 for one and loads of other stuff.

    Hope my experience is of help. Good luck and keep us informed how you do.
    Life is like a roll of toilet paper; long and useful, but always ends at the wrong moment. Anon.
    Think how stupid the average person is.......
    half of them are even more stupid than you first thought.
  • Thanks all, especially dilemma...I think my resolution is simply to get £30 for the 105 chainset on Ebay and put that towards a new MTB one...otherwise it sounds like i'll spend more money on bodging things...