What’s an idler?

Well, I went out for a beautiful spring ride in the country over the Easter weekend.  It was gorgeous!  I even had to take off the neoprene cockpit cover to stay cool.  Sorry Kevin!  I guess I’ll have to change my wardrobe to match the weather soon.

Anyway, I was on the final stretch of my journey home, starting off at a stop light, and the chain suddenly seized up in the middle of the intersection.  I had to put my neanderthal arms out and row to the far sidewalk.  That must have looked, well, awkward!  And no, I don’t have video of that.

I found that the chain was broken at the back wheel – torn apart actually, but I couldn’t get the rest of the chain to pull free at the front.  Looking under the seat to investigate, I found this…


Gah!!! What’s that?

It’s called an idler sprocket.  Not coming from a recumbent background, these are unfamiliar to me.   Apparently they help direct the chain coming from the rear wheel cassette to the chainrings at the front with as little friction as possible.  Well, there’s a little friction now.  You can see that the chain is now sideways and wedged between the idler and the bottom of the chain trough.


There is a small machine screw on either end of the idler axle.  They come in through the velomobile shell and hold the axle in place.  I unscrewed those and the whole assembly pulled out – with a bit of coaxing.

Here are the idler assembly parts…


The idler axle has a flange on one end to keep it from pulling through the velomobile shell.  That insignificant looking rectangular piece of metal welded to the axle actually performs many duties: it keeps the sprocket centered, it keeps the chain return tube and derailleur cable from rubbing on the idler, and also prevents the axle from spinning and unscrewing those little machine screws coming through the velomobile shell.  That important looking tube at the top is just a spacer for the other side of the sprocket.

There is no flange on the other end of the axle because the idler and spacer must be slid on.  This is also the reason that I had some trouble getting the axle assembly out – it had burrowed a bit into the velomobile shell on that side.  Not so good.

Here’s what happened to the plastic toothed idler sprocket…


I know, pretty gruesome isn’t it!

Lesson Learned – The idler sprocket is an essential part of the drive train.  If it fails, you’d better hope there’s a pub nearby.  I’m replacing mine with a strong precision idler from Terracycle.  I’m also looking into a new axle assembly which keeps the axle from eating into the velomobile shell.  It would also make it super-easy to take the idler out for inspection or replacement.  While writing this, I’ve even started thinking of carrying a spare.  I just haven’t decided – spare idler or spare Guinness… Hmmm…!

11 thoughts on “What’s an idler?

  1. I should have taken photos of Stuart and Gregg trying to get the velo into the car. It sort of fit – went from the front passenger seat (folded down) through the back (thank goodness for the hatchback). That was gruesome 🙂 next time I’ll remember to snap some pics!!!

  2. Wow! My thoughts would be a bearing failure which would seize the idler pulley and cause the axle to eat into the shell. And then the idler locked up and chain broke. I ordered the optional Terracycle Idler and will be rushing out to the shop and check it now. It looked good last time I oiled the chain. Back soon….

    • The idler bearing seems to be fine. I don’t know how the chain got sideways under the sprocket. Everything else appears to be OK but I’ll have to get it put back together with the new idler and give it a turn to know for sure.

  3. Hi, I’m not an experienced velomobile-technologist, but could it be that there was too much dirt into the idler, which lifted the chain from the idler? It’s recommended to oil the chain every 800km, but I’m starting to think that the full chain system has to be cleaned more often, esp in winter time with all that dirt. In my Mango, all the dirt is collected by the idler for some reason….I can feel the idler getting too dirty when some weird resonance in the chain occurs at high chain speed.

    • That’s a really good point! There is a drainage trough that cuts through one of our bike paths which is usually just water but filled up with mud over the winter. I scooped a bunch of that mud in through the foot holes one day and some of it ended up in the chain trough. I let it dry overnight then vacuumed it all out. I re-oiled the chain which looked fine but, I know that my bike oil gets awfully thick in the cold of winter. It could certainly hold a lot more dirt in the idler itself. Thanks very much for sharing your experience! I’ll be cleaning more often and listening for the dirty resonance in future rides!

    • On my Strada, there wasn’t a chain guard but a plastic chain tube is attached to the idler shaft and runs toward the chainrings for a short distance to protect my right leg from grease. It was removed at the time of the photo.

    • Thanks for sending the link. I’ve actually had an eye on one of those ever since I had to take mine apart! I bought my racekap from Wim and you’re right – he’s great to deal with. I like so many of his products that I think I’ll wait till my wish list is long enough and my pocketbook is deep enough, then I’ll make an order. Cheers!

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