Should I go for shorter cranks on my grasshopper?

[gadgetmind]

I started 'bent riding a few years ago on a HPV Spirit, but last year bought a HPV Grasshopper FX and sold the Spirit. I've done about 1200 miles on the 'hopper, and am generally happy with it, but I'm often finding need for lower gearing. I'm not sure what my bottom gear is - I've got the standard sram dual driver 3x8 with what looks like a 46 tooth front chainwheel. Any ideas?

Anyway, I've recently climbed both Holme Moss in Derbyshire (steady but long) and Fleet Moss in the Yorkshire Dales (high and brutal) and both needed much more pushing than my knees can sustainable take. I tried putting a 39 tooth chainwheel on the front, but 1) The chain skipped like crazy so is obviously stretched, 2) The chain guides/guards either side of the chainwheel wouldn't fit the smaller one as the chain dropped in too deeply.

Bikefix have quoted me for new cassette, chain, chain tubes and front chainwheel, but they're trying to steer me towards a full 38 tooth chainset with 155mm cranks. I have read about shorter cranks, but I'm six foot tall with 34" inside leg, and have the leg muscles of a tyrannosaurus (and matching weedy arms, but let's not go there) and am not sure it's for me. Is 155mm worth a go or should I wait for them to get a 170mm chainset in stock?

Oh, and are the chain guides available anywhere else? Buying a new chainset when all I need is chainwheel and guides seems OTT.

All feedback welcome!

 

[arallsopp]

I run an SMGTe with a standard configuration (single chainwheel up front, 4?T, SRAM dual drive at the back, 8 speed cassette), and am amazed to hear you're bottoming out.

Either these bikes aren't as similar as I think, or my math is out.

Assuming you're running a 20" nominal at back, and there are 46 teeth up front, I make that:

• 3.5 mph in low/low (46 x 0.73 / 34) at 60RPM.
• 40.6 mph in high/high (46 x 1.36 / 11) at 120RPM.

I'm not sure I could keep the bike upright below 3mph, and 60RPM is still a healthy cadence for a climb. Am I missing something?

Given the top end is a bit big, you could drop to a 38T up front. This would drop you below 3mph for the climbs, and take you to 33mph on the descents at 120.

Some say smaller cranks reduce leverage and encourage a faster cadence. Generally its considered wise to step down the teeth whilst you're shortening the lever, so maybe that 38T is a good idea. I'm not sure this is the problem though.

Lots of fun to be had here: http://www.sheldonbrown.com/harris/gears/

Andy.

 

[arallsopp]

Sorry, what are "chain guides"?

If the front is skipping, the chain (and cassette) are probably both worn. Putting a new ring up front will only exacerbate the issue. Is your rear cassette the 11-34 one?

 

[gadgetmind]

I'm not often seeing 3mph on climbs but 4/5 mph is all too common.

What you're missing is the creaks from the seat and my knees as I try and get the bike up some of the sustained 20% grades with 25% sections and some 30% hairpins thrown in to keep life interesting.

Maybe I just need to get fitter?

 

[gadgetmind]

By "chain guides" I mean the disks that are bolted onto either side of the chainwheel. What's the real name?

Dunno what my rear cassette is but ISTR 11-32.

Yup, I've got a quote for doing the whole lot as it's obviously all stretched. I bought the bike new but with visibly low mileage so am surprised it's all worn like this in 1200 miles.

 

[arallsopp]

By "chain guides" I mean the disks that are bolted onto either side of the chainwheel. What's the real name?

Mmm... No. Dunno. Chain guides it is, then.

I bought the bike new but with visibly low mileage so am surprised it's all worn like this in 1200 miles.

Me too. I've got over 10,000 rainy miles on the SMGTe and (although rider, drive, and seat all creak like fury) there still seems to be a fair amount of wear left. Do you have a derailleur mast on the boom up front? If so, a stock compact, bar end shifter, and creative routing might see you right for a similar cost.

 

[squeaker]

I'm currently running a 36T Q-ring with 152mm cranks and 11-30 8-speed cassette on my DD equipped 'Hopper. Gives a range of 17 to 83 gear inches (GI) with a 35mm section rear tyre. Possibly a touch too low geared (a 39T might have been better) but the 17" gear does occasionally get used when I'm tired and the bikes heavily laden. I have spun up to 120rpm (=30mph) on a slight downhill when in a real hurry, but usually I free-wheel once speed gets over 25mph. (I'm 5ft11in with 62yr old legs...)


Assuming you have the standard 40mm Marathon racers fitted, your standard gear is about 22 to 108GI with the 46T front (and an 11-30 - what rear cassette do you have?), and would be 18 to 88GI with the 38T. The standard crank on my 'Hopper was a 170mm with 130mm PCD. If you do go for a shorter crank, I would lower the gearing proportionally. SB's calculator is a good start.

As for getting the 38T to work: sometimes the crank spider can foul the chain (although the original crank off my 'Hopper is working OK with a standard 38T ring on my Roadrat), also the chain tube could be causing problems as the angle to the chain ring will have changed a lot. I suspect that you don't need to run with the chain guards, as the run to the powerside idler is at a constant angle, but I've not tried that as the 152 cranks came with their own guards. Do check the chain length for wear (scroll down) though - it must be really shagged if it skips on a chainwheel!

If you do go the new shorter cranks route, do bear in mind the chain ring PCD, as IME 38T is a small as you can go with 130mm (and pushing it, 39T has less chance of the chain hitting the spider).

IMHO you should only go to shorter cranks if you want to spin faster than is comfortable at present. Personally I really like shorter cranks on 'bents, but....

And finally, if you do find some chain ring guards that work with a 38T on 130mm PCD, do let me know, as I could do with one to stop the chain leaping off on the Roadrat (if I don't keep pressure on during upward changes in the high gears).

HTH

 

[byegad]

The effect of shorter cranks is to gear the whole system up. So shorter cranks need lower gearing overall. It's to do with foot speed and distance your foot travels.

I'd suggest a new chain, smaller chainring and new cassette will do the job, the reason you got skipping is the chain is worn and the cassette will be too.

 

[Fiona N]

When I had a Speedmachine, I found that on steep terrain with low gears I got problems 'turning the pedals over'. The cranks seemed to get stuck at the far forwards position (sort of 3 'o' clock). Talking to other 'bent riders convinced me that shorter cranks were the answer and, despite the relative increase in the gearing, the 160's did make pedalling much smoother. The thing is, I'm only 5'6" with a 31" inside leg and even so I found the short cranks felt quite odd for a few miles and that was only going 170 to 160 mm.

Eventually I gave up with the Speedmachine as biomechanically we never got on. Basically, they're made for people with longer legs and having to pull the boom in to its shortest position meant that the position of the BB was too low relative to the seat position for me so it was never very efficient. Certainly the power I could get out of it never came close to my Windcheetah where the position's much more efficient.

 

[gadgetmind]

Thanks all. I think that with my long legs, I'm better off sticking to standard crank length, or maybe just shaving 5-10mm off rather than 15mm.

However, bikefix say they can't get the chainwheel and, errr, guides separately, so maybe I should go for the full chainset and then flog the 155mm cranks?

Or as I'm told the bolt pattern is the same, I could just play "pick and mix" to see what's best for me.

 

[gadgetmind]

Yikes, I've just spotted that bikefix said *152* mm cranks. Wow, that's short, perhaps far too short.

 

[Riding in Circles]

Shorter cranks are good for the knees on people with shorter legs, at your height you should not be going below 165mm cranks, have you considered a Schlumpf Mountain Drive?

 

[gadgetmind]

Shorter cranks are good for the knees on people with shorter legs, at your height you should not be going below 165mm cranks

Thanks for the confirmation. I'll stick to 170mm.

have you considered a Schlumpf Mountain Drive?

I would if I needed the current top end, but do I really need to be able to pedal lazily at 40mph+ ?