Help! my 'MTB' is too slow..

jamesg73

OK chaps (and ladies!) first post and it’s probably somewhat odd for the ‘normal’ folk!

I’m a moderately keen cyclist and commute daily across some less salubrious areas of London on my MTB to get to my office, and over the last 2 years have gradually gotten to the limit of my gearing, now spending the vast majority of time in top gear with nothing for downhill/ tailwind sections of road.

So, i need to increase my gearing, BUT – and it’s a biggie...

My current ride is as follows:
Unbranded 7005 alloy frame
Kinesis carbon forks with disc mounts
Pro-lite Allein rims
Michelin 26 x 1.1 run-r tyres
Clarkes skeletal 180mm hydraulic discs Fr & Re (utterly brilliant)
SRAM X7 re (long) mech
Shimano 442 crank arms on 105mm square bb
Blackspire 104/44 front chainring
SRAM 11-34 rear cassette
Plus pannier rack (unloaded) two bottle cages and lights etc it weighs in at 8.7Kg, so light! The weight is actually important as on average i climb 280 steps with the bike & a 5kg pannier on my shoulder every day...

Now as i said above, the 44- 11 gearing is too short for me now and i need to up the front ring size. I also have some slight knee issues so need to be careful with setup. I’m currently using SPDs set tight to hold my heels in, but a tighter Q factor on the crank would be a real plus. As such i want to go for a 50/34 compact crank but there are two problems.

1. The chainline is a lot further in on a roadie 50/34 than the standard mtb setup, although a bit of shimming at the rear hub should fix that.
2. My mtb frame is too wide for a standard 50/34 compact setup – the radius of a 50t chainring is circa 100mm, and at that distance from bb centreline, my frame is 100mm wide, whereas the ‘chainline’ for the compact setup on the 50t is 46mm – i.e. 4mm inside the frame width...

So, what to do?

I live in a flat so am restricted to one bike. I do the occasional trail blast and weekend off road (swapping rims) so just swapping for a road bike or buying a second bike isn’t an option.

I like the low inertia and nimbleness of the rims i have, so chopping to a 29er hybrid would be a last resort.

I’ve recently been considering changing the frame so was wondering if anyone has managed to cobble a 50/34 onto a mtb frame and if so, how!
P.S. The Pro-lite Aosta frame is looking good to me, but i’ve no idea if it will have the same problem with frame width at 100mm from bb centreline...

Oh and of course i’m not rich so can’t afford to go and buy xyz for silly money, otherwise i’d already have a Prolite Piemonte 2012 frame in the post..

I did say it was an odd one...

All sensible comments and advice are welcome. Thanks in advance...

supersonic

Not sure what you mean by the frame is '100mm wide'? Can you clarify this?

Are you regularly spinning out the 44/11 combo? At 120rpm that is nearly 40mph!

You should be able to get a 50/34 on there, but what is your front shifter and mech?

You might as well get rid of the cassette and go for a compact ratio as well.

YeehaaMcgee

supersonic wrote:

Are you regularly spinning out the 44/11 combo? At 120rpm that is nearly 40mph!

Sonic's right, but I'm going to go one further and suggest that maybe you need to pedal faster

Barteos

A 26x1.1 tyre has a relatively small diameter.

You can get some Schwalbe Kojak 26x2.00 tyres instead.
They are comfy, fast and they'll effectively increase you gear ratio.

bikaholic

My commuter bike is a Klein Attitude MTB running a road compact double 50/36T (110 BCD) on the front and a 9-speed, 32-11T cassette at the back.

The chainline just about works - if the smallest chainring were a 38T then it would not clear the chainstay and if the largest cog on the cassette were 34T then the chain would drop into a smaller cog when you back-pedal.

I've built many bikes with a similar setup.

I usually opt for square taper, Octalink or ISIS cranksets since chainline can be easily adjusted by the choice of BB spindle widths. There are also other mods, of course...

Cranksets come with a stated chainline for a given BB width - for the compact doubles that I have used in the past this was usually 43.5mm for a 110mm BB.

Standard chainline for the MTB versions of these cranksets is 47.5mm (to accomodate the wider dropout spacing on MTB frames). So, using a 118mm BB would give you the 47.5mm required chainline when fitting the compact road double onto a MTB frame.

Note that some crankset manufacturers measure chainline differently. Shimano's is centre of frame to the inside surface of the middle chainring on a triple; Truvativ's is centre of frame to the centre of the middle chainring on a triple. Thus, Shimano 47.5mm == Truvativ 48.5mm.

Since the compact double is shifted outboard when fitted onto a MTB frame, this also means that the MTB front derailleur will work if the stated max chainring size for the FD is 46T or greater (although, modding by grinding will help shift speed).

Also keep in mind that you will need atleast 2-3mm of clearance between chainring/chainstay and cranks/chainstays if you intend to run a compact double on a MTB frame. The clearance is to accommodate for any frame and/or crank flex when you are putting the power down.

supersonic

And you don't have to stick to the required chainline either - if you are predominantly in the higher gears, a longer front chainline is better anyway.

njee20

You can't fit a road chainset if you've got a 73mm BB shell, I'd check that before you go any further.

I'd be inclined to just stick a 48t chainring on your existing chainset. The shift may not be ideal, but it'll work, and give you a higher gear.

The Rookie

I use a 46t upfront on my commuter, but I rarely use more than 6th gear on the flat, 9th (smallest) is downhill only pushing 40mph plus.....no point changing the bike, you need to learn to pedal faster (it's more efficient anyway)

njee20

you need to learn to pedal faster (it's more efficient anyway)

Not that straight forward though is it. With 1.1" tyres at 100rpm (which is still fairly quick for a prolonged period) you'll be doing 29mph, following traffic/slightly downhill/tail wind etc, considering the low rolling resistance tyres that's not that fast, I'd want more.

Yes you can spin like a loon for short periods, but not for sustained hard riding, there's a reason road bikes don't have a 44t outer ring...

The Rookie

I wouldn't call 100rpm quick, I would say 120rpm is my average.

njee20

I wouldn't call 100rpm quick, I would say 120rpm is my average.

I'm prepared to wager it's not.

supersonic

You can't fit a road chainset if you've got a 73mm BB shell

Why not? if he is using ST bottom brackets, is no problem.

njee20

Aye, good point, forget about all this square taper malarkey :-)

The Rookie

njee20 wrote:

I wouldn't call 100rpm quick, I would say 120rpm is my average.

I'm prepared to wager it's not.

When I'm overseas on business I train in the Gym at the hotel and do 20 plus miles before dinner, the cadance meter usually records about 123-135rpm as my average.

How much did you want to wager?

njee20

I'll happily put down £5 that you don't average >120rpm for a real world ride of the OPs duration (unless the OP commutes 0.5 miles :? ), even less 20 miles.

Edit: and you make reasonable progress, rather than riding solely in the 22/34!

YeehaaMcgee

The Beginner wrote:

njee20 wrote:

I wouldn't call 100rpm quick, I would say 120rpm is my average.

I'm prepared to wager it's not.

When I'm overseas on business I train in the Gym at the hotel and do 20 plus miles before dinner, the cadance meter usually records about 123-135rpm as my average.

How much did you want to wager?

Go do it on a real bike outside, then get back to us.

The Rookie

I don't cheat!

No worries, though how we physically do this is beyond me, I don't have a cadance computer (or any other puter) on my bike.....

njee20

Indeed, I realise it's purely notional, but that is a really high cadence. Think about it, you're talking more than 2 revolutions per second. You'd be more efficient slowing down!

YeehaaMcgee

bear in mind that it's two complete revolutions, so your drive side crank will rotate a full two turns per second - not left foot, right foot.
If you see what I mean.

bennett_346

The Beginner wrote:

njee20 wrote:

I wouldn't call 100rpm quick, I would say 120rpm is my average.

I'm prepared to wager it's not.

When I'm overseas on business I train in the Gym at the hotel and do 20 plus miles before dinner, the cadance meter usually records about 123-135rpm as my average.

How much did you want to wager?

Sorry mate, i'm gonna side with them here. No way you average 2 full turns of the crank a second across your rides in the real world.

The Rookie

Still reckon I do when peddling in gear - ignoring pulling away and coasting to stops and the odd hill were my 1x9 leaves me pushing a 46/28 that's a little high, but where I have stable peddling.....

bennett_346

The Beginner wrote:

stable peddling.....

You sell horses?

The Rookie

No clearly just the stables.......

Back on topic, I have a 26x1.1 having a cicumference of 1.91m

100rpm is 1.76rps, 6012/hour, 24000 rotations of the rear wheel (44/11) per hour means covering 46Km, or just under 29mph.

If JamesG has GPS logs of him regularly exceeding that then I agree he could do with taller gearing, if he's not going over 25mph then I don't!

YeehaaMcgee

Who taught you maths? 100x60 = 6000. Not 6012.

njee20

And it's extremely plausible that he can sit over 30mph if he's got a bit of a tailwind, or a slight descent or whatever. Maybe he just doesn't want to be sitting in his absolute top gear, it's not really unreasonable! :roll:

If you do ride at 120rpm you should slow down, that's really not efficient.

The Rookie

YeehaaMcgee wrote:

Who taught you maths? 100x60 = 6000. Not 6012.

Just rounding it, the circumference data won't be accurate to 0.2%!

Lets see the data and the speeds, it's plausable, but he also says he IS in TOP gear in the first post.

I've tried 100 and find I can last longer at the same pedal load at circa 120.......maybe it's something to do with being as old as Cooldad!

cooldad

P1ss off. Compared to me Chris Hoy has chicken legs.

Although I must admit I'm very much a spinner, and love my granny dearly.

Chunkers1980

How can you round 60 x 100. It is an integer.

YeehaaMcgee

The Beginner wrote:

YeehaaMcgee wrote:

Who taught you maths? 100x60 = 6000. Not 6012.

Just rounding it, the circumference data won't be accurate to 0.2%!

What? That's not rounding.
100x60 is precisely 6000.

njee20

I've tried 100 and find I can last longer at the same pedal load at circa 120.......maybe it's something to do with being as old as Cooldad

On a gym bike though still? I definitely think you'd be surprised in the real world, that is very fast. If I do high cadence reps I do over 110rpm.

Agree on the 60x100 being 6000 though :?

The Rookie

Sorry, I meant rounding on my calc's, shouldn't have bothered going to rps, still it doesn't effect the basic end number, if the Op isn't pushing 29mph, he doesn't need to change gearing!