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Gates claims their belts do not stretch over their life, so theoretically speaking you should not have to adjust the tension (which appears finicky).

I'm interested in anyone with real world experience. Do you ever have to adjust the tension of the belt, apart from when you have to move the back wheel for another reason, such as to change a tyre?

Gates, who make the belts have a FAQ. I can't get a direct link, but if you go to http://www.carbondrivesystems.com/support.php under the question "What are the benefits to the Carbon Drive systems?" this is part of their answer.

No Stretch – The Carbon Drive belt does not stretch over time like a chain. This means you don’t have to re-tension the belt, or replace the belt due to stretch. This also means no excessive wear on pulleys due to stretch.

Does this claim hold up in practice?

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Have you had problems with yours? My partner's bike has not required any adjustment at all in about six months of daily commuting. – DC_CARR Aug 23 '11 at 20:30
    
Many car engines use "timing belts" which are fundamentally the same concept. They last 70-100 thousand miles. In much more difficult conditions. – Daniel R Hicks Sep 10 '13 at 11:28
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Timing chains in car engines also last many thousands of miles without replacement or adjustment, but it would be wrong to conclude that therefore bike chains can do the same. – armb Mar 27 '15 at 18:57
up vote 4 down vote accepted

Once the system is set up correctly you will not need to tension it again until it is time to replace the system. There will be a slight drop in tension over the life of the system but it is not enough to require adjusting the tension. When performing a tire change using a bike with sliding vertical drop outs or an eccentric bottom bracket it is not necessary to re-tension the system, just pop the wheel in and out as you would with a chain.

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Aluminium alloys have a large coefficient of expansion (the grow with increase in temperature) and this could explain your summer and winter experiences. Do not own a belt drive but do maintain large aluminium aircraft with control cables. when are cables replaced or are adjusted it is carried out after aircraft temperature has stabilised and cable tension is corrected for temperature ie lower tension if aircraft cool & higher tension. This is due to the length of the aircraft and it's cable runs changing enough to make significant differences in cable tensions.

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Do you think the length of a chainstay (about 40 cm) is going to expand enough to change the belt's tension so its outside of recommended tolerances ? Aluminium has a thermal coefficient of about 23 and steel is 11 to 13, but I lack the maths to figure out the difference of 40 cm of aluminum at -20 and +50 degrees C. Want to give it a go ? – Criggie Mar 26 at 11:32
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An interesting topic, but needs some hard numbers. E.g. is the expansion linear with regards to temperature and pressure? Aircraft will be subject to much greater temperature and pressure ranges than bicycles. The type of aluminium alloy used may be different. I have a steel frame. In this case we're interested in the change in length of the chainstay - a linear expansion rather than volumetric. Larger sprockets may result in less belt tension change for the same chainstay length change. Sprockets may also expand. From @Criggie, will the change be within tolerance? A lot to consider! – Jason S Mar 27 at 20:36
    
This should probably be a comment to my answer, rather than an answer in and of itself? – jayhendren Mar 28 at 21:20

I have been riding a Gates CDC (not Centertrack) belt drive for almost a year now. Gates claims that tension adjustments are not necessary over the lifetime of the belt drive. My experience does not support this claim.

One of Gates' claims is that the belt does not stretch enough to warrant tension adjustment after a break-in period, as you would on a brake or shifter cable. I found this to be true.

However, I have noticed that the tension of the belt changes significantly with temperature (and possibly humidity, though I don't have enough data to test that hypothesis). I have ridden my bike in various weather conditions and temperatures ranging from over 90℉/32°C to under 0℉/-17°C. I use the Gates iPhone app to check my belt's tension. I have noticed that, if the belt is tensioned properly at about room temperature, then the tension in the belt decreases below the recommended tension range when the ambient temperature drops below about 20℉/-6°C. I have noticed problems with the belt drive when tension is too high or too low, including excessive noise, skipping, and misalignment leading to dropping the belt from the sprockets. Therefore, I found it necessary to retension the belt slightly in the beginning of the winter and again at the end of winter.

I'm not certain yet if the weather-related change in tension is caused by frame shrinkage (I suspect this is the case - my frame is aluminium) or by another factor.

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This means, as long as you do not winter riding, you do not need to adjust it. Which is a valuable information. – user375251 Apr 14 '15 at 9:16
    
Does the belt change in length with varying temperature/humidity, or is it the bike frame, or both? – Criggie Mar 26 at 12:34
    
I don't have enough definitive, emperical data to answer your question. My suspicion is that the change in tension is due to the frame growing & shrinking with temperature changes. I live in a dry climate where humidity doesn't change much, so I haven't noticed any changes correlated with humidity, but I suspect humidity would not make a difference. – jayhendren Mar 28 at 21:22

I have a Gates belt drive. After getting tension right (using the iPhone app) initially I did not touch it for a year and was surprised to find it had not changed. For me at least, no stretch.

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