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Surly Big Dummy Idler Kit

5 reviews
SKU: IDK185.1

We have run out of stock for this item.

The Big Dummy Idler Kit is the whole package. With both a Power and a Return idler to control the chain in both directions, pedal efficiency is greatly increased and noise is nearly eliminated. When these idlers are in control, gone are the days of dinged paint with every bump and a muddy chain due to low ground clearance. Make your Big Dummy one step closer to maintenance free with T-Cycle's Big Dummy Idler Kit!


1. What does this kit do?

This kit controls both the power and return chains on a Big Dummy, preventing them from flopping up and down and swaying into things. If you ride on bumpy roads, or offroad, having the extra chain control is really nice.

2. I don't understand the choices. What do I need?

The reason for asking about what type of rear wheel gearing you have is because

  1. derailleur and internally geared hubs have a very different return chainpath, and
  2. the idlers can be used for tensioning the chain on an internally geared hub.

The internally geared hub version is simpler, with fewer parts, so it costs less. The deraileur version has a different chainpath designed to make sure the return chain is up and away from the ground and from your kickstand.

3. How does it install?

A CNC machined clamp attaches to the diagonal tube coming down from the seat. It is machined specifically for this kit and fits the tube perfectly. The mount has an 8mm threaded hole in it that takes the stainless idler bolt. The idler(s) go on the bolt, and are free to move side to side to track your shifts. On the internally geared rear hub version, you slide the mount up and down the tube to set the chain tension.


  • Installation consists of installing the idler mount and adjusting the idler position. If you have an internally geared rear hub, you can use the idlers to set the chain tension. On the derailleur version. you can move the idler mount up and down to get the chain clearance you want for your kickstand.
  • The idler mount attaches with two m5 bolts. Use a 4mm Allen wrench to take the clamp apart, then reassemble it on the frame. Tighten the bolts loosely, so the mount stays in place but you can move it by hand if you want to.
  • Put the power side chains the cog, and put the return (lower) chain on the grey urethane bed. On the internally geared idler, the power side chain goes under, and the return side chain goes over. On the derailleur version, both the power and return chains go on top of their idlers.
  • Adjust the mount up and down to get the desired chain tension/chain path, and tighten the clamp bolts the rest of the way. You're done!


  • At T-Cycle, we make all sorts of specialized drivetrain kits for unusual bikes (and trikes, and quads, and all sorts of things). We are Xtracycle aficionados ourselves, and we and a good number of our Xtracycling friends had chains that bounced around too much sometimes. We take care of problems like that on recumbents all the time, so we designed solutions for the Free Radical and the Big Dummy.
  • If your chain is rubbing on your wheel or fender, or bouncing around onto your kickstand or even the ground sometimes, these kits are for you. They turn those long, out of control chains into nice, under control chains. Gain ground clearance and peace of mind no matter what crazy place you find yourself.
  • The attachment clamp is CNC machined 6061 T6 Aluminum with stainless clamp bolts. The clamp is machined specifically for this kit, so it fits the Big Dummy diagonal tube exactly.
  • The Internally Geared rear hub version of the kit is designed to let you set the chain tension by sliding the idler mount up and down. It uses a combined Over/Under idler, with a power side and a return side idler in one unit. The power side chain is carried on a 7075 AL cog, and the return side is carried on a tough urethane chainbed. The idler rides on an 8mm stainless bolt.
  • The rear derailleur drivetrain version has two idlers, one for the power side, and one for the return side. The ride on a common shaft, and rotate in opposite directions. The power side idler carries the chain on a 7075 AL cog, and the return side idler carries the chain on a urethane chainbed. The 8mm stainless bolt that carries the idler is longer than the other kit and includes a stainless steel chainkeeper to make sure the chains stay on the idlers where they're supposed to.
  • The idlers are very low maintenance. The bearings are top quality ABEC 7 rated. The idler materials are abrasion , knock, and bang resistant and are designed to take a lot of abuse.
  • This kit is specifically designed for the Big Dummy cargo bikes. The mount is machined for a 1-1/8 inch round tube. If you have something other than a Big Dummy, and would like any of the parts of this kit in a different configuration, let us know.

Customer Reviews

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Customer Reviews
5.0 Based on 5 Reviews
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Filter Reviews:
A T-Cycle Customer
Tobe O.
United States United States
Very pleased!

Super fast shipping, packaged very well, and installed easily! What more could one ask for?!

Michael L.
United States United States
My Mistake

I was excited to purchase this product for my newly acquired Surly cargo bike - an improvement for the washboard gravel roads I ride. In my haste, I failed to recognize that it will not work with my Surly Big Fat Dummy. My fault. Looks like a great product, but unfortunately I cannot use it.

Derek B.
United States United States
Worked great with mid-drive motor.

I use my big dummy with a BBSHD mid-drive motor as my year-round bike. The idler helped to keep my chain clean in the winter when it would typically get slushy. The idler also fixed a chain line issue I was having with the chain rubbing on my rear fender in my bottom two gears.

A T-Cycle Customer
Merritt C.
United States United States
Well made, and looks great.

Sturdier than I expected. It's designed well, and the machining/build quality is top notch. Plus, it looks great! My only issue is that my heavy duty chain rubs a tiny bit on the guides, but I don't believe this is something I can fault the product or T-Cycle for.

Freddy K. Aron
This is a game changer!

I have a common issue, I like putting the widest tires on my Surly Big Dummy, and take it on single tracks. The issue I had was the crazy chain flop, sometimes bad enough for the chain swing horizontally and momentarily catching on a tire lug of my magic mary’s. While a little tricky to get all the angles right in the installation, once done I cannot believe Surly doesn’t do this at the factory! This thing is the 1# modification I have done, and that is saying a lot! Get this thing!!’