Race Frame/MTB Mongrel

[djnotts]

I'm planning on kitting this



with a 3 x 9 MTB set up. Flat bars obviously. XT shifters. Can't see any difficulties with rear end - XT mech and cassette up to 32 or 34T.
Deliberately chose frame with standard threaded b/b and band on front mech and intend using a 44/32/22 external bearing XT chainset. The only problematic bit seems to me to be a front mech that will reach the rings sufficiently well given the seat/stays angle. Have I missed anything else? Front mech suggestions?

I need light and I need low gears and this the only obvious route.

Thanks.

[Bazooka Repairs]

You're set - If you want compatibility you should look at other Boardman ranges and see what Front Mech they use.

I would heavily recommend Sram Apex front mech. I just upgraded to it on my Boardman Frame - I got mine from £14.99 (£30.00+ RRP) and the shifting is smooth and precise.

I believe your frame will be a 35MM clamp band for the front mech - Unless i'm mistaken.

[cycleruk]

If using XT shifters and an XT chainset then you will require an MTB front mech'. This should be fine with the chainline for MTB's.
(Also MTB FD's have a different cable pull than road mech's.)
If going for 9 speed then make sure you have a 9 speed rear mech'. 10 speed XT mech's changed the cable pull so won't work with 9 speed shifters.

P.S. Need to double check that the chainset will clear the chainstays.
(MTB frame chainstays sometimes have dimples to allow the granny ring to miss the frame.)

[531colin]

If it takes a compact double (road bike) then an MTB triple is usually fine......
MTB middle ring (32T?) is further out and smaller than Compact inner ring (34T)
.....MTB "granny" is so small diameter that its not a problem.....although my experience is limited to metal chainstays

[djnotts]

Thanks all! Not been able to identify an mtb front mech that will fit - new 9 speed ones for example are mostly dual pull and simply will not squeeze into space twixt down tube and tyre! Low mount ones will of course not fit low enough because of bulging carbon as blends into bottom shell. Road mechs don't like either the shifters or 44T outer ring.

For now a NOS Exage CX will pull with the XT shifter - but the clearance is very tight even with a x23 tyre when clamp arm reaches out to the outer ring.

Looks pretty though!



Mech as fitted. ANY suggestions for a "better" fitting triple that will sit with a 44T outer and work with a mtb shifter gratefully accepted! Cost is not the issue at this stage. I've met all 3 of my targets. Weight sans pedals 8.5kg, come in at about 475 quid (NO new parts except the yellow cables!) and gearing is 17" to 105".



If I could buy such a machine ready made then I probably would.

[djnotts]

Wrong second snap!

[Brucey]

you may have noticed that many MTB mechs (that work fine on an MTB) are in danger of clouting the chainstay on a road frame; this is because the chainstay angle is different on a road frame vs an MTB one.

FWIW I think there is an XT bottom pull, bottom swing mech (a M750 version? -nb there are several with very similar model numbers) which will work on your frame, as part of a 'trekking' groupset; this is meant to work with your chainset and is designed with your chainstay angle in mind too.

cheers

[djnotts]

Yes I have noticed that!

"....XT bottom pull, bottom swing mech (a M750 version? -nb there are several with very similar model numbers) which will work on your frame, as part of a 'trekking' groupset; this is meant to work with your chainset and is designed with your chainstay angle in mind too."

But none I think I designed to fit within the seat tube to tyre gap of 10-12mm? I shall however carry on researching!

Edit: "down tube" replaced with "seat tube"!

[Brucey]

well you'll have to try it to be sure but IIRC the top part of the mech looks like a lot of road mechs; I'd fancy it as squeezing in to a small gap better than most.



http://bike.shimano.com/media/techdocs/content/cycle/ev/bikecomponents/fd/ev-fd-m751-1803_v1_m56577569830611893.pdf

There are also plenty of 8s MTB mechs that are built the same way.

cheers

[djnotts]

I agree that would be worth a try and did indeed search for a suitable "retro" model, 9 or 8. Price put me off but I shall probably try one (altho' as far as I can tell from the many photos I've skimmed they give no better clearance than the one I've fitted).

Thanks for the thoughts.

[peetee]

That looks so similar to the old LX/DX/XT mechs from the 90's
I am a big fan of them. they work on anything with a precision and silence that put most modern stuff to shame. I fit and replace a lot of front mechs these days and finding one that is 'right' can be a nightmare. One guy wanted to change his gearing so I suggested he substitiute the 34 tooth middle ring for a 36. I then had egg on my face because the front mech simply wouldn't shift on to the big ring. They are soooo fussy.

[Brucey]

I found the SI techdoc with the specs for FD-M751

http://www.sillygrin.co.uk/techstuff/service/shimano/f650d.pdf

[needless to say I didn't find it on shimano's site, which (it seems to me) is incomplete and somewhat tortuous to navigate these days...]

It appears to be as I recalled; FD-M751 allows for a smaller chainstay angle than other mechs, i.e. it is designed to fit a frame that takes 700C wheels.

If you have a front mech that works already, there may be little to gain by sourcing another. However front mechs are tricksy beasts; IME if you get 'the right front mech' it all just works strangely perfectly by comparison with one that isn't quite exactly right.

Some folk would go for the XT mech every time, just so that it all looks right. That kind of thing doesn't usually worry me, but hey, its a free world...

BTW if the clash is the cable itself fouling the tyre, it may be possible (with a little engineering) to mount the cable on the other side of the pinch bolt arm and get a bit more clearance that way.

cheers

[djnotts]

As far as I can judge, there is very little if any difference in clearance between the late '90s mtb mechs (and 34.9 clamps are not easily found) and the Exage CX which is from much the same era.

With some demands on my patience the indexing front and rear is good. I can engage all f & r combinations without any cage rubbing etc (altho' obviously I wouldn't do so in use). Nonetheless the pinch bolt to tyre (not the cable, Brucey) is very tight - certainly wouldn't take even a 25 mm tyre.





If I find that this is a "problem" in practice (or if simply preys on my mind!), then I shall switch to a two ring set up, 38/24 on same cranks. This will mean the cage has less far to reach and thus increase the "gap" by maybe 5mm. Gear range will decrease to 19" to 91" - entirely sufficient for my needs.

Short spin along the street reveals a nicely riding bicycle, light and quick! Do for me for half a grand.

Thanks all.

[Brucey]

FD-M500 is a pretty decent mech for sure. But it isn't intended to work with your chainset. Specifically

- it is meant to work with a 47 to 47.5mm chainline
- it is meant to work with a 48T (biopace) chainring
- it is meant to work with a 10T middle to big ring interval
- it is a 7/8s mech not a 9s mech

The second of these means that the curvature of the cage is quite a lot less than it would be in a mech meant for a smaller ring; in extremis this can lead the chain getting jammed in a wedge-shaped gap as you go for the big ring. It also means that the tail of the mech may be in danger of clouting the chainstay unless the mech is raised.

The third thing means that with a 12T interval in the chainset the small to middle shift may not work that well and if the mech is raised, this will only compound this issue. Once the mech is raised high, you stand a better chance of overshifting off the big ring, too. The slightly wider 7/8s cage also lends itself to that.

However if you run this mech on a 50mm chainline, the arm and the pinch bolt will swing further to the left than normal (on the big ring), which could perhaps lie at the root of the issue that you have with the pinch bolt clouting the tyre. It also leads to the possibility that the middle ring position won't be quite lined up with the chainring and/or that the cable will be under undue tension in the big ring position (the shift ratio varies through the stroke, and therefore with variations in chainline, too).

http://www.retrobike.co.uk/gallery2/v/Manufacturer+Archive/Shimano+Archive/Service+Manuals/fc-mt60+m730+m550+m500+m400+m300+m200.JPG.html

Having said all that, it isn't the worst match ever, and if it works well enough for you, fine. If you do experience any of the various possible annoyances I have outlined above, I would suggest that FD-M751 might alleviate them. However without trying it out, I couldn't say for sure that the clearance between the pinch bolt and the rear tyre on your frame would be greatly improved or not; it ought to be (because unlike FD-M500, FD-M751 is designed for a 50mm chainline) but that isn't certain.

I might have FD-M751 in a box somewhere; if I find it I can maybe see if the arm is likely going to stay clear or not.

cheers

[djnotts]

I pretty much understood from the start all these issues - but the "design" of such a mongrel was always going to involve compromises. And with these I am prepared to live! If someone points me to a different way to achieve my 3 essential objectives then I will happily start another build - I have not come across any off-the-shelf options at any money. Plenty that match 2, but not all 3!

But when I spot an FD M751 to fit then I' will give that a try!

Thanks.