A friend’s asked me to spec him up a lightweight commuter with flat bars. There’s some cheap and light road frames around and I can pick up entry level MTB transmission groups pretty cheaply as well.

Is there any reason that a MTB group wouldn’t work on a road frame? I know the rear drop out spacing is different but I’d be using road wheels anyway. Would there be any issue with BB spacing or chain line?

I can’t find any info online but I’m sure the Singletrack collective will set me straight!

Would there be any issue with BB spacing or chain line? no

Check front mech mountings some road bikes have a braze-on/fixed mount but you could use a road mech
Cable stops are positioned differently put no issues.
have a look at a Cotic Roadrat, if they are too much for the budget they might still give you some inspiration

Chain line for mtbs is wider than road but it can be made to work.

That said, you’d be better doing what you’re planning but getting a cheap road chainset instead of the mtb one.

Should be fine in most respects, as long as the front mech fits (road frames often need a braze on mech) and the BB isn’t a weird one.

Biggest problem will be the gears – even with a triple 22/32/42 the gearing will be very low. They’ll spend most of the time in the big ring even in hillier areas.

A road triple groupset with flat bar shifters might be better – you could get a road groupset and sell the STI shifters and get flat bar ones – road STI’s are always in demand and fetch high prices even second hand.

You would be better off putting a compact double on the front i think.
My Boardman hybrid runs SRAM X5 11-32 out back and a 50/36 compact up front. Works wonderfully in that you have enough top end to motor along at a good lick yet has enough low end that you could comfortably ride hills with full panniers etc.

I picked up a 2nd hand (almost new) Land Rover branded aluminium flat bar road bike for £200 the other week, would probably be ideal for what you want, although not that light, way better than an mtb on the road.

May be worth looking online or lbs…….

don’t road frames have a shorter TT to allow for drop bars ?

may be better with a cheap mtb frame for flat bars

You can get “touring” chainsets with 26-36-48 chainrings.

A 48-11 max gear is 115 gear inches. (Touring style MTB gears)
A 53-12 max gear is 116 gear inches. (road standard)
A 50-12 max gear is 110 gear inches. (road compact)

so it should be fine.

Top tubes are shorter on road frames, you could get a size up though?

Would there be any issue with BB spacing

Yes – Road BB cups are shorter, so if you bought a MTB crank, you’d have to replace the BB with a road one. I think the cranks would still fit, but not 100% sure (I’m talking HT2).

Better off buying some cheap Shimano road cranks. Shop around, shops like Merlin sometimes have good offers (or eBay).

Thanks all – plenty to think about

The gearing should be ok as the group I’m looking at has a 48:11 as top gear. He’ll never use the granny ring but it’s a cheap solution! Swapping the chainset out for another would push the build out of budget.

Having just researched further, both road and MTB frames usually have a 68mm BB shell, so assuming the front mech fits, I reckon it should be ok. The group set has an old skool internal sealed bearing type BB.

I thought about using a MTB frame but it needs calliper brake mounts and clearance for 700c wheels. It would also be heavier and burlier than required.

The geometry’s the tricky one alright. I’ve converted drop bar bikes before and they’ve been ok, but each frame is different. They’re not always designed differently, as the Specialized Roubaix drop bar bike has identical geometry to their flat bar Sirrus, but the frame I’ve had my eye on might not work quite as well…

I think I’d better use the geometry of the bike he had nicked as a guide and then tweak the position with stem length.

Thanks for the help folks!

both road and MTB frames usually have a 68mm BB shell,

But it’s the thread on the cups that is different.
I’ve got a photo somewhere, but it’s on my other ‘pooter.

i run an mtb touring chainset on my CX which I also use for errands around town 48-11 spins out for me at about 28mph+ generally works well
as DezB says HT2 comes in two flavours and the cups for MTB say it on them I’ve always assumed its because MTB cranks are a bit thicker than road so need a “thinner” cup to maintain chain line reading above if all part of a groupset should work ok

I’ve a Deore MTB crankset on my Spec. Tricross winter bike, with a Race Face Ext bottom bracket. I had to space out the nearside bearing with two spacers and the offside with one due to crank axle length. I ditched the 22T inner ring.
Oh and set the lower stop on the front derailleur to avoid shifting into thin air.
It’s ugly but it works.
There was no difference between the threads for road or MTB cups. There is a difference in the spacer length, but that is to suit the crank axle length, as above.

As suggested above if using an HT2 chainset you can move o ne of the 2.5mm spacers on the driveside to the other side and get the MTB chainline 2.5mm closer to a road chainline.

But if you are using an MTB front mech you may want to leave the cranks where they are to avoid shifter issues. If the expected use is more on the smaller end of the cassette then having the chainline slightly more outboard won’t be a problem anyway.

I use MTB cranks (spacers as standard) and deraillers on my road tourer with no issues.

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don’t road frames have a shorter TT to allow for drop bars ?

may be better with a cheap mtb frame for flat bars

This, for a hypothetical 6ft bloke you’d want a 56cm road frame with drops, or a 60cm with flat bars, but then the headtube will be really high making the position a bit odd, and the top tube will make stopping at trafic light uncomfortable!

If he want’s a flat bared commuter you’d be best served by a cheep disk braked XC MTB, then re-rim the wheels (of get some 135mm disk cx wheels) with road rims which easily fit in the frame.