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On my Nishiki Colorado Comp MTB, the chainstay crosses above the chain instead of going directly to the center of the crank. I didn't think much about it until I recently put a larger chainring on, increasing from a 30 tooth to a 36 tooth. I wanted the larger chainring for a relatively flat 180 mile gravel ride I will be going on soon, so I could have a higher gear ratio. In general, I plan to use the bike mostly on flatish trails, pulling a child trailer, riding on essentially no technical features. When I ride with the 30 tooth chainring, I constantly feel like I max out too quickly, unable to ride as fast as I would like.

With the 36 tooth chainring, the chain now rubs directly on the chainstay (when in the middle gears). I tried putting a piece of paper lamination with electrical tape, but after a relatively flat 26 mile ride the electrical tape was cut and the laminate is wearing through. Here are photos illustrating. image of chainstay image of where chain is rubbing

I am considering buying helicopter surface guard tape (8 mil, 1"x30' for $24) and putting one or two sheets on the chainstay. I'm not sure how long this will last though with the chain rubbing directly on it at times (depending on which gear the back derailer is set to).

My other thought is to put back on the 30 tooth front chainring, and just deal with a lower max speed. I recognize that the chain rubbing on the frame will damage both the frame and the chain. Note that the frame is aluminum and not carbon fiber, so frame damage is bad but not as bad as for carbon fiber. What do you think or recommend?

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    Rubbing all the time (or all the time in certain gears)? Or rattling against it when freewheeling?
    – Chris H
    Commented Jul 27 at 10:30
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    Tape won't help, consider the amount of force your weight contributes to chain rise. It'll cut the tape, your frame and whatever is in it's path.
    – some dumb guy
    Commented Jul 27 at 12:56
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    @Criggie I don’t think you’re realistically doing that with an aluminum frame. And for OP, you really don’t want any frame damage. The chain sawing through your frame is a big deal (to say the least) regardless of frame material. It may be possible to attach a custom hardened steel guard, but the noise, vibration, and friction would be horrendous.
    – MaplePanda
    Commented Jul 28 at 2:28
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    @MaplePanda agreed - this frame is just not suitable for this combination. Maybe an enormous chainring would let the chain go over the chainstay when in the lowest gears, but that's going to have other issues and would be sending good money after bad. OP should consider a different bike for this particular ride.
    – Criggie ♦
    Commented Jul 28 at 9:45
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    Have you ever been tempted to buy another bicycle? Just one more would be OK, wouldn't it?
    – Andrew Morton
    Commented Jul 29 at 17:47

4 Answers 4

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Even with the helicopter tape, I wouldn’t keep the 36T chainring. I would try a 34T or even a 32T before I reverted back to the original 30T ring.

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I don't think any solution taping or glueing something onto the chainstay will work, this is the tension side of the chain, at very best, you will suffer high friction losses and have to do regular maintenance

You will have to either drop the chainring size down to a small enough size the chain clears the chain stay or install an idler pulley that the chain runs across, keeping it clear of the chain stay.

Another option would be to go to a cassette with a smaller small cog. Cassettes with 10 and even 9 tooth cogs are available. 10 is equivalent to going from a 30 to 33 chain ring in terms of gear-inches, and 30-9 gives you a higher gear than 36-11. These options won't be cheap upgrades, personally I would not recommend them on a $US700ish bike.

If those options don't work for you, you can always learn to spin faster :)

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    Idler pulley won't work properly. It's a derailleur system and it's the tension side. On recumbents it only works because they are so long.
    – Michael
    Commented Jul 27 at 10:00
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    you can always learn to spin faster I'd say given the bike simply isn't designed for larger chainrings, the OP "plan[s] to use the bike mostly on flatish trails, pulling a child trailer", and a 30-11 gear combination on a 650b MTB at a mere 80 rpm results in close to 20 mph/30 kph, which is awfully fast for pulling a trailer with a MTB, just learning to spin faster should certainly be considered.
    – Andrew Henle
    Commented Jul 27 at 11:22
  • An idler wheel that pushes the chain up or down away from the stay would also have to have horizontal float, for when the chain moves sideways with gear changes. And you're right about the tension side - the chain will cut through anything it rubs over.
    – Criggie ♦
    Commented Jul 27 at 22:02
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    An idler that is above the chain ring would not need to float. e,g, as you see with high pivot MTB's. (Not suggesting its particularly practical in this)
    – mattnz
    Commented Jul 28 at 2:07
  • How about a chain retention device - but the cog type ones, certainly not a friction one - mounted on the underside of the chainstay? Very ghetto but if it works it could make the bike even more awesome. Would need a slighter linger chain but that's peanuts.
    – Vorac
    Commented Jul 29 at 22:20
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In which rear sprockets is it rubbing? If it's only rubbing in the biggest ones the obvious solution/compromise is to get a smaller cassette.

Otherwise you have to go back to a smaller chainring.

A 32t chainring and a 10t sprocket would get you all the way to ~45km/h (at 100rpm cadence). 41km/h with a 11t sprocket.

I can't think of any other option, unless you find a way to offset rear axle or bottom bracket downwards.

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    OP might need to spreadsheet the gears that work without rubbing, and then calculate the Gear-Inches for each permutation and decide based on that. The high gearing (11 tooth rear) will likely be fine on all the chainrings - its specifically how low a low-gear does OP need ?
    – Criggie ♦
    Commented Jul 27 at 22:00
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I see that's a quick release hub. You could have a new wheel built with the Sturmey Archer 3-speed hub that takes your cassette and disc brake.

SRAM also used to make a similar system.

It is a replacement for having a triple chainring so you would have your normal range on the cassette in the middle gear as well as additional high and low ranges.

It would add about a kilo weight to the bike and has a cost implication but would solve your problem.

This link demonstrates the gear range: better high gear with the 30t ring than you had with the 36 by some margin!

https://kstoerz.com/gearcalc/compare/?link=1&&unit=0&lbl=1&tsid1=1&cogs1=11,12,14,16,18,21,26,32,50&rings1=30&ighid1=2&tsid2=1&cogs2=11,12,14,16,18,21,26,32,50&rings2=36

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    I have a system like that on a recumbent and it works great.
    – Willeke ♦
    Commented Jul 27 at 15:26

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