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I own a 2010 Trek District with a belt drive. I'm fairly happy with the setup, since after months of standing outside I just picked up the bike again and started riding without the usual chain maintenance.

However, the problem I currently have is that under large torque, so e.g. accelerating out of traffic lights makes the belt either slip or jump off the rear cog. Riding in traffic with cars close-by makes this a somewhat dangerous mishap, so I'm hoping I could fix this. Even when pedaling with high cadence the belt seems to be moving outwards all the time, and can be fixed by pedaling backwards one round or so.

The instruction manual and googled resources [broken link - try this trekdistrict discussion or this bike-manual link) claim that setting the correct tension and alignment to be easy, but I've found it to be quite a hard task. So, could someone with more bike mechanics skills and experience tell me how to do this or point me to a good resource?

The main questions are:

Is there an easy way to set the tension, without having a special tool for measuring the tension?

How accurate does the tension have to be in order not to slip under pressure?

How can I (easily) align the belt correctly for it not to travel right on the rear cog?

Thanks in advance for any answers! I could probably take it to the local shop, but I've been trying to handle all and any maintenance myself, since this is a hobby after all.

==== Edit ====

I don't have a picture of my own, but the rear dropouts look like the ones on this picture: Trek District Dropout

I've tried to adjust the tension and managed to get rid of the slippage, so the original problem is more or less solved. I've also tried and managed to tighten the belt too much, since it started to have a funny noise and the freewheel didn't seem to work correctly anymore. Now the tension seems about correct, but the belt still "travels right" (as in the image) and jumps off the cog, when accelerating.

I'm starting to think that the rear hub is not perfectly straight with regards to the belt, but I would think one would notice this when riding.

=== Edit 2 (2011-08-10) ===

After nearly getting hit by a bus, caused by the belt jumping off the rear cog, I decided to bite the bullet and take the bike back to the shop. Turns out I have strong fingers, since the problem was too much tension on the belt. The high tension made the rear wheel not fit perfectly in the rear dropouts. This made the wheel just the slightest bit misaligned (what's the opposite of straight?), which in combination with the tension made the belt to travel outwards when applying enough torque.

I got the thing fixed, and it now works as a charm. I'm unable to tell the difference between the earlier too tight belt from the correctly tensioned one, since the difference is very subtle. Thus, the moral of the story has to be that one should use the correct tools for the task at hand. And also that my mechanics skills are not quite as good as I was hoping :)

Anyway, thanks for all the suggestions here, but from now on I suggest all the belt-drive people like me either get the real tools, or support the local bike dealer for the no or low maintenance of the belt drive.

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    Cool! Belt drive! Can you post a picture of it? I am always amazed that my teeny tiny chain can handle the torque of my 280 lbs as I climb a steep hill out of the saddle. Can a belt do as well?
    – geoffc
    Commented Aug 2, 2011 at 13:53
  • Does your bike use rotating dropouts?
    – zenbike
    Commented Aug 2, 2011 at 14:13
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    Also, there are two styles of dropouts on the Trek belt drives. Can you post a closeup photo of your rear cog and dropout?
    – zenbike
    Commented Aug 2, 2011 at 14:23
  • I've added belt alignment info to my answer below. I hope it helps you.
    – zenbike
    Commented Aug 3, 2011 at 17:35
  • Get a buddy to help you. Hold the bike while he sits on it, and have him apply a fair amount of his weight to the pedal (right pedal at the 3-o clock position, or left at 9) while you site down at the belt. (It may help to remove the belt guard.) If the belt seems to curl out to one side then it's damaged. (You may need to try this 3-4 times, with different segments of the belt on top, if the damage is localized to one area.)
    – Daniel R Hicks
    Commented Aug 3, 2011 at 18:24

5 Answers 5

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As to measuring belt tension, the tool is your best option. If you own the bike, then you should own the tool.

Without it, however, place an S-bent spoke or similar wire in a hook over the center of the top of the belt. Add weight to the bottom hook. 10 pounds is the spec. Yes, this is a lot of weight for a bent spoke. Be creative. Or buy the tool.

Place a ruler next to the belt, at the point of the weight's pull. Write down the measurement.

Remove the weight. Measure again, in the same place. Subtract the first measurement from the second measurement. The difference should be between 5 and 12 millimeters. 10 millimeters is ideal.

The alignment is dependent on the type of dropout.

EDIT 1: The image below is a new tool from Gates, called the Eco Tension Tester, which does exactly as described above, in a more professional manner. Perhaps it will help describe the intent of the method I described.

NEW Gates Eco Tension Tester

With the dropout style you posted in your image, the alignment is actually set with two things.

  1. Wheel centering - the wheel needs to be equally centered between the chainstays. This is a do it by eye and feel job, although you can gauge your success by measuring from the rim's brake surface to the inside surface of your chainstay, assuming your wheel is properly dished over your hub.

  2. The second thing which affects the belt alignment on the pulley is your "chain" line. With the District, this alignment is set with the length of the bottom bracket spindle. The more perfect the chainline, the less likely you will have issues with the belt rolling off the cogs. Most bike shops have a tool like the Park Tool CLG-2 for measuring chainline accurately, although it is designed to work with a toothed cog.

Park CLG2

If you have consistent issues with the belt rolling off the cog, and your dropouts are evenly set up in the frame, and your wheel is centered, find an LBS to check your chainline.

You may need to swap the bottom bracket axle to get more perfect chainline, in order to solve this issue. Assuming the bike has not had the BB replaced by you, it may be wise to ask the shop where it was purchased whether it is the original BB, and whether they will help with any chain line issues under warranty status.

EDIT 2: It actually appears that belt "alignment" issues are directly related to belt tension with the Gates Carbon Drive belts. If the tension is correct, they track well. If it is too low, they will track toward the frame. Too high tracks away from the frame.

I hope that is helpful.

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    In other words, press down hard and look for 1/4-1/2" movement.
    – Daniel R Hicks
    Commented Aug 2, 2011 at 15:50
  • Pretty much. But the OP asked for a procedure, and the District's service manuals do specify the amount of pressure to use.
    – zenbike
    Commented Aug 2, 2011 at 15:59
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    And most people don't have a good feel for what 5-10 pounds of pressure feels like.
    – zenbike
    Commented Aug 2, 2011 at 16:09
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    No, an LBS will have the proper tool, as I recommended the OP get, in the first paragraph. The OP asked for a procedure that was accurate without the proper tool. Most specifications have a range of measurement which is acceptable. For the record, I tested today, and I got the same measurement by "pressing hard" as I got with 22 lbs of weight. By that light, "pressing hard would severely over tension the belt. "Pressing hard" by definition, means different things to different people, and that is why you have a specification for measurement.
    – zenbike
    Commented Aug 3, 2011 at 17:10
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    @zenbike Thanks for being specific. Hopefully what has become clear from the OP and answers is that "press hard" and "1/4 to 1/2" movement" is far too vague for belt drive bicycles and that the correct belt tension is critical to safety and proper function, and the margin or tolerance of correct tension is small. Thus belt drives bicycles are nothing like chains, as a chain can tolerate quite a difference in tension. I think the use of torque wrenches is a good parallel. If you have vast amounts of experience, you may not need them. Even then, to assure customers, shops will.
    – Jason S
    Commented Mar 11, 2012 at 22:53
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There is now an iPhone app to measure belt tension of a belt-drive.
Here is Gates blog post about it, and here it is on iTunes Preview
They say they are working on an Android app but this has to take into account the variety of microphones on Android phones. They will probably have to test and calibrate for each make and model of Android phone.

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    After using the iPhone app, I would use caution in trusting its measurements. I get wildly different measurements from the app, depending on factors which I am unable to control well enough to make it consistent. (Like the force at which it is "plucked".)
    – zenbike
    Commented Mar 5, 2012 at 7:11
  • @zenbike Thanks for the real world experience. I don't have a belt drive so haven't used the app. I posted because I read about it when considering a belt drive.
    – Jason S
    Commented Mar 11, 2012 at 22:45
  • My experience matches @zenbike's. Additionally, after a recent update, the app crashes as soon as I try to launch it on my iPhone, so it's totally useless for me now.
    – jayhendren
    Commented Mar 28, 2016 at 21:27
  • In my experience the app is the only easily available way. I actually bought a tool, only to notice after the fact that it only works for longer chainstays than the one on my folding bicycle. The app is crap, but after spending half an hour with it you get a feeling for how to pluck and which measurements you should discard and which are ok. I probably spent 2-3 hours when I got the bike to set up the tension correctly (multiple tries, test rides, etc., over a few weeks).
    – Nobody
    Commented May 12 at 15:11
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Three months ago I bought an Avanti Blade 8C (belt-drive, Shimano Nexus 8-speed) and have had problems with noise in the drive train.

Belt tension appeared to be associated with the problem. I downloaded the Gates Carbon Drive iPhone app and adjusted the tension so it vibrates at approx. 50 Hz and the noises have (almost entirely) gone away. This value falls in the middle of the range for internally-geared hubs (45–55 Hz) recommended by the app.

So it looks like belt tension is very important in this setup.

Today (four days later) I rode the bike to work and it is very noisy again. I checked the tension again using the iPhone app and it is now shown to be very low. This is puzzling because the rear axle is held tight with 16 mm nuts. There are also screws that provide fine control over the horizontal position of the rear axle and I used them last week to set the belt tension.

Regarding the accuracy of the iPhone app, I get consistent reads on multiple attempts if I spin the cranks and flex the belt first. The strength of plucking will have very little effect on the frequency of vibration (like with a guitar).

Sorry, I take it all back! I am totally confused about the noises my bike makes and they may not be caused by belt tension problems.

Furthermore, I am getting inconsistent results trying to use the Gates Carbon iPhone app. I can’t get a consistent ‘twang’.

The noisy bike saga continues. A belt-driven bike is not supposed to sound like a chorus of cicadas!

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  • One would expect the belt to stretch a bit when new. But presumably that stretching would have been pretty much all done in 3 months of regular riding. But note that temperature will affect tension as well. ("Chorus of cicadas" sounds like friction.)
    – Daniel R Hicks
    Commented Mar 7, 2012 at 3:16
  • @DanielRHicks I doubt the belt stretched, unless it is damaged. That is one of the major reasons to go with a belt. No stretch. Sounds like the iPhone App is inconsistent though, also judging by zenbike's comment to my answer.
    – Jason S
    Commented Mar 11, 2012 at 23:13
  • The bike has noisy days sounding like this and quiet days (basically silent). Each morning I am not sure which it will be. Back to the bike shop!
    – pharsicle
    Commented Mar 14, 2012 at 11:36
  • To end the story: the bike is now as good as silent. The final fix was that the local Gates Carbon Drive rep replaced the rear pulley and the retaining ring. I suspect the original assembler (Avanti here in Australia?) put the wrong retaining ring on. Shimano here in Australia would not help because belt-drive is ‘non-standard’ for Nexus hubs (or something).
    – pharsicle
    Commented Jun 12, 2012 at 0:47
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A general rule for flat belts is to twist the belt midway between the front and rear belt drives, it should twist to 90 degrees with moderate two finger effort, if it goes past 90, then it is too loose, if you cannot make 90 without great effort, then it is too tight.

This method depends on how strong your fingers are and the width and thickness of the belt, the more you use this method the better you get at judging what is correct for any given belt.

Another method is to make small tension adjustments until the slippage stops.

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  • This is interesting. Do you have any source/advice online to confirm it?
    – Mac
    Commented Aug 3, 2011 at 23:32
  • Just my own experience with using it on industrial equipment to cars and motorcycles. Invented it myself since I did not always have the tension tools or specs, has work quite well for me over the years, I am the source on this one.
    – Moab
    Commented Aug 4, 2011 at 0:23
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    Gates, who manufacture belts used on bicycles, recommend NOT twisting the belt, due to risk of damage and then failure at high load. There are other warnings in their belt handling instructions document.
    – Jason S
    Commented Oct 6, 2011 at 2:39
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    It says "excessive" twisting, 90 degrees on the longest point to point is not excessive. Any manufacturer wishes to overstate precautions to prevent failure of their product, this is where common sense comes in. They said the same thing about automotive cam belts, which I never had a failure in 20 years due to mishandling.
    – Moab
    Commented Oct 7, 2011 at 23:39
  • The definition of excessive is vague, but since it is referring to damaging the carbon fibers they use in their belts, I would err on the side of excessive caution.
    – zenbike
    Commented Dec 14, 2011 at 8:10
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When a belt doesn't stay on a pulley it's either because the pulley are misaligned or the belt is damaged. You should check the wheel/axle alignment by checking the space between the tire and the seat and chain (er, belt) stays -- make sure the tire's centered between the stays. If not that then likely the belt has become damaged.

(When a belt is damaged, with the cords along one edge broken, it doesn't stay straight under tension but curls outward on the damaged side, and tends to creep off the pulley towards the damaged side.)

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