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Old 05-05-14, 10:43 AM   #1
bakerjw
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Different length tandem cranks

I'm looking for some options on getting different length tandem cranks without breaking the bank. DaVinci Designs has some available but they cost more than our tandem does.

The issue is that I can pedal fine with 170 or 175mm crank arms. My wife has an inseam of 27.5" which most calculators determine works best with a 155mm crank.

Any ideas?
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Old 05-05-14, 11:38 AM   #2
mstyer
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Depending on the cranks that you're using, it might be possible to have them shortened.

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Old 05-05-14, 11:37 PM   #3
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My wife is a 'tall' 4 ft and 10 1/2 inches and uses 170 mm cranks with no ill effects.
And, she has been riding those length cranks on 5 different tandems we've owned since 1975.
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Old 05-06-14, 12:17 AM   #4
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One cheap(ish) option is to go with a single-side drive setup, using one of the chainrings on the right side as a timing ring. Then you can put some kid's cranks on the rear (just get a model that has bolt-on rings, not riveted-on so that you can play with the sizing), and get the right length bottom bracket axle to make the chainline similar on the two cranks.
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Old 05-06-14, 02:07 AM   #5
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St John St Cycles/Thorn in the UK have 155mm stoker cranks, 110/74 BCD 5-bolt in square taper for $154 right now. The Thorn brand is perfectly good and SJS are reliable and long established. These are not really flash hi-tech cranks but they are solid and look nice on the bike. The 74 BCD is standard for a road triple inner ring. The most common 110 BCD rings these days are 34T and 50T 'compact' road rings, which are not horribly expensive in say Tiagra if you shop around, or your current rings may transfer directly.

Only thing ya gotta watch is that SJS do not always pick the most economical postage option when sending stuff overseas.

Lots of tandems have 38T timing/sync rings. A 110 BCD 38T ring might be pretty hard to find if you currently have a 130 BCD rear sync crank. Tandems East and other specialty shops have options I imagine, if you call. SJS sell a very spendy Middleburn. My Fuji uses 39T timing rings, so I needed a 39T 110BCD chainring to set up the kiddy-crank - I just got a TA Zephyr from Wiggle for about $50 - it is a lovely ring but not exactly cheap!

You may be able to sell your old cranks for a decent price to offset the costs.

Let us all know what happens, and post a picture too!
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Old 05-06-14, 04:09 AM   #6
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Quote:
Originally Posted by Chris_W View Post
One cheap(ish) option is to go with a single-side drive setup, using one of the chainrings on the right side as a timing ring. Then you can put some kid's cranks on the rear (just get a model that has bolt-on rings, not riveted-on so that you can play with the sizing), and get the right length bottom bracket axle to make the chainline similar on the two cranks.
BMX component maker Sinz makes a crank for both square taper and ISIS bottom brackets in increments of 5 mm from about 130 mm to 180 mm. I use them on the rear of my single side drive tandem that is set up for my kids to ride on the back. They used to be available on eBay in all sizes from a couple of dealers, but the selection is smaller now--I'm wondering if they are no longer made. There is also a 155 mm triple crank made by Origin8 that sells for $45.
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Old 05-06-14, 04:42 AM   #7
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Can't you just install Crank Arm Shorteners?
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Old 05-13-14, 11:21 AM   #8
bakerjw
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I ended up upgrading to DaVinci custom cranks. They cost more than the complete tandem did. They gave options for 150 or 160 so I opted to go with a ratio of crank length to inseam length.

170mm x mm
------ = -----
31.5" 27.5"

Solving for x gives 148.4 mm so I went with 150mm cranks. I'll post more when I get them in hand and installed.
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Old 05-14-14, 08:39 PM   #9
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Your wife will love the shorter cranks!! My wife rides 130's at an inseam of 30.5 inches due to a knee injury. You'll probably be faster because it will reduce the dead spot in her pedal stroke as she comes over the top.

One other note, we had to raise her seat height to maintain the same amount of extension. She has a better sight line for looking forward now and our seats are set an approximately even height. She's 5'8 ish and I'm right at 6 foot with 170's and a 34.5 inseam. I do think that you'll probably notice when she shifts her body weight a little more now, but the positive trade off is that she'll pedal more smoothly.

I'll be interested to hear how you both adapt to the new cranks.
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