Author Topic: Anyone got experience with 'illegal' chainring/sprocket combo on Rohloff tandem?  (Read 1013 times)

Teaboy

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We've got 46 x 17 teeth on our Thorn 26" wheeled tandem.  We're both 60 - so not of Olympian athlete strength.  We have good cardiovascular fitness, but not at strong as we once were.

We find that when riding with luggage we routinely are in gears 6 - 8 on the flat.  We seldom use gears 13 or 14 but frequently use 1 and 2.  We think we'd like a couple of lower gears, but to achieve this we would need to 'break the rules' by using, say, a 40 x 19 combination.  Rohloff say 42x17 is the lowest chainwheel/sprocket ratio that can be used without invalidating the warranty.

Does anyone have any experience of using a lower than recommended ratio?  If so, I love to read about it.

Donerol

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46 x 17 gives gears 19.6", 22.2", 25.3".... 103.2"
42 x 17 gives gears 17.9", 20.3", 23.1" .... 94.2"
40 x 19 gives gears 15.3", 17.3", 19.7" .... 80.3"

So the suggested but 'forbidden' combination does indeed give two more lower gears.

Leaving aside the matter of the warranty, I'd guess a lightweight, light spinning solo rider could use 40x19 without problems but two riders on a tandem, even like yourselves, will be putting out more torque.  I hope someone else can come along with more useful experience for you.

Mike Ayling

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46 x 17 gives gears 19.6", 22.2", 25.3".... 103.2"
42 x 17 gives gears 17.9", 20.3", 23.1" .... 94.2"
40 x 19 gives gears 15.3", 17.3", 19.7" .... 80.3"

So the suggested but 'forbidden' combination does indeed give two more lower gears.

Leaving aside the matter of the warranty, I'd guess a lightweight, light spinning solo rider could use 40x19 without problems but two riders on a tandem, even like yourselves, will be putting out more torque.  I hope someone else can come along with more useful experience for you.

We are a tandem team with combined age of 142 and combined weight of 125kg.

Mary has never ridden a single and only took to tandem riding about five years ago.

We selected the minimum 42 X 17 for our Thorn tandem when we bought it in December 2012

We have used gear  #1 on occasion but not with any regularity, getting down to #4 more regularly.

We find the #14 very easy to spin out on even slight down hills.

YMMV

Mike

Andre Jute

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We think we'd like a couple of lower gears, but to achieve this we would need to 'break the rules' by using, say, a 40 x 19 combination.  Rohloff say 42x17 is the lowest chainwheel/sprocket ratio that can be used without invalidating the warranty.

42x17 = 2.470 torque multiplier
40x19 = 2.105 torque multiplier

You're for the firing squad, Teaboy. I don't actually think you'd break the box, but you'd lose your warranty. You may want to read the par in my post at
http://thorncyclesforum.co.uk/index.php?topic=4412.msg86100#msg86100
about how strong the Rohloff is suspected to be, compared to the permitted inputs. But that's not an argument for throwing away your warranty before you explored all other avenues.

I were you, I'd write first to Thorn (as the initial provider of warranty service) and ask if they're willing to make an exception for you and your wife, given that you're a genteel pair of spinners, absolutely guaranteed not to be anabolically pumped up mashers. If they decline to take the risk, write to Rohloff and ask them. Then you can think again. That gives you two bites at the cherry of keeping your warranty, plus a reserved opportunity to take your chances.

Furthermore, if it comes to taking your chances, you might want to consider approaching the 40x19 stepwise and economically by first replacing the 46T chainring by a 40T and keeping the 17T sprocket, which already gives you a less radical ratio of 2.353 and a more agreeable gearing spread.

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Another approach is to go back to first principles:

The two nastiest, whiningest (actually the sound is more like a tired sighing) gears in the Rohloff box are 7 and 8, where you now spend a lot of time. The most efficient ratio in the Rohloff box is gear 11, and it is also perfectly quiet, so it is smart to choose your gearing to do most of your riding in gear 11.

If you post your bike's tire size and your cadence (pedalling rate as rpm), I can work up a table to show which sprockets you need to be in gear 11 on the flat. That would leave 10 gears below, and three overdrives for downhills, so you'd be coasting sometimes on the downhills but that's the lesser evil, especially for a tandem.

Teaboy

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Well, I wimped out of voiding my warranty and replaced the 46 tooth with a 42.  It's made a significant difference.  We've just returned from a holiday in Brittany and rode in gears 9-11 on the flat, and sometimes the heady heights of 12,13 and even 14 on slight downhills or if there was a following wind.  More importantly, we were never defeated by any (on road) hills!