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Does the front derailleur have limits to which size chain rings can be used?

I have a groovy old 1988 Schwinn Prelude and for the most part it does just what I need. However there are some steep sections of excellent paths nearby that are a bit to much. Fitness is decidedly a factor ;)

I currently have a 2x6, I assume stock parts but I cannot be sure. Chain rings are 52-40 and cassette is 14-28. The aggressive old style. There aren't many options for changing the cassette, but I was thinking perhaps I could put a compact or even a gravel/mtb chain ring set on.

I use exclusively the 40t ring. I can count on one hand the number of times I've used the 52t. I mostly commute and go for leisure rides, so the racing set up is impractical. Can I simply switch the chain rings out for a 42-34 for example? Or will the front derailleur be unable to function properly?

The derailleur is mounted with a bolt on band so it is maybe possible to lower it on the seat tube slightly to accommodate the new rings.

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  • Depends on the derailer and the bike. In most cases the front derailer can be loosened and slid up or down the seat tube to adjust to minor changes in chainring size. But there is also the issue of whether, when this adjustment is made, the derailer will still work OK with the smaller chainrings. And there is also the matter of the curvature of the derailer's chain guide -- a significant change in ring radius could mean the shape of the guide no longer works. So you really have to evaluate it on a case-by-case basis. – Daniel R Hicks Apr 6 '20 at 22:24
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As Criggie says front derailleurs have a maximum chainring difference and min and max tooth counts. the more modern a derailleur and shifter is the more important this is. With an old friction shifter you can probably push this quite a lot.

The bigger issue is the 'bolt circle diameter' on the chainrings and crank. It's probably wide enough to preclude a ring much smaller than 40 teeth, if you want a smaller ring you'll need to find an alternative crank.

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A front derailleur has a quoted capacity, which is the maximum tooth-difference it can reliably shift. The real capacity might be a little or a lot higher.

The actual numbers will depend a lot on the available positions on the bracket too - how high/low it can go. Band-on mechs would have more space than Braze-on mount mechs.

In your position, I would simply ride in the 40 tooth chainring and leave it at that. If you want some climbing gears (a smaller grannie ring) then that also depends on the spider you have, and its Bolt Circle Diameter (or BCD)

Additionally, if your bike has chainrings that are rivetted on, the whole thing needs swapping, which starts to get expensive. A donor MTB might be a good source for parts.

It is likely your old schwinn has a square taper bottom bracket, any scrappy cheap MTB could offer up a suitable crankset. The gotchu might be if the new smaller crankset is a triple, for which your bike's double shifter might not work, and that the chainline will be a little more to the right.

However as you observe, we've got more time on our hands now, so give it a go. The worst that can happen is you need to return to original specs, so the only thing that might mess you up is shortening the chain... leave that till dead-last when you're happy the shifting is working okay.


Or if the 40 tooth is all you need, consider removing the 52 tooth chainring and ride a homebrew 1x setup. You can even remove the left shifter and cabling, though the front derailleur might be required as a chain-keeper.

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    FWIW, that bike has friction shifters and is from the era when there was no such thing as a "triple" front derailluer. An older triple MTB square crank would probably work acceptably. If you really want a wide gear range and keep the bike vintage looking, you can get 46/30 square taper cranks. These are a great upgrade for older bikes and older riders and not that expensive. somafabshop.com/shop/product/… – Fred the Magic Wonder Dog Apr 7 '20 at 15:44
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I am not that familiar with components of this age, but there are some general principles that apply. The information here is complementary to Criggie's post.

Chances are very good that you can't switch out the chainrings on your current crankset for ones that small - you may hear this size called "sub-compact" if you were shopping for current-generation cranks.

Cranks have a minimum chainring size that's determined by their bolt circle diameter. If your crank has a 110mm BCD, which is comparable to a lot of modern compact cranks, then it will not be able to take smaller than a 33 tooth chainring (some cranks with this BCD can accept 32t rings, but they usually are special cases; alternatively, oval chainrings exist that have effective gearing of 32t, but no smaller). You would need to find the crank's BCD. It's possible it may have two separate BCDs if there are two sets of mounting points for the chainrings. Given your current setup, it's even possible your BCD is 130mm, which means that 38t is the smallest ring you could possibly mount. (I said that because 53/39 and 53/42 are fairly common setups for 130mm BCD cranks, and your reported chainring sizes are close.)

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  • OP here. Great advice team thanks as always – Ben Apr 8 '20 at 2:33

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