PedalPink

We retrofitted a Gates Carbon Belt to our Co-Motion Primera two years ago and have been very happy with it. Everywhere we read, "the Carbon Drive lasts as much as 10 times longer" than a timing chain. We are nearing 10,000 miles with the belt.

My question is, how do we know when it is time to replace it? What indicates that the belt needs to be replaced or is about to fail? So far, we have not heard of anyone replacing their belts. (We have bought a replacement belt - just haven't installed it.)

Yesterday on a 200k brevet, we started the ride and during the first ascents heard a new noise when the captain stood up in the pedals. A random clack sound. It took us a couple miles to diagnose it as the timing belt slipping (we've never had any problems with it before). What helped us diagnose it was our pedals became out of phase. The eccentric was very tight so we don't think it had loosened. Does that mean the belt stretched - which we didn't think was possible. Could this be a sign?

Could the teeth on the belt being wearing down so it doesn't mesh as well with the rings?

Has anyone else replaced their belt, and what were the signs or reasons to do so.

TandemGeek

Ooooh.... Interesting question. I've been eagerly awaiting the early adopter / high mileage user reports on durability and wear.

Best bet lacking any feedback from someone here who has already done so would be to contact Gates directly with your question. That said...

For what it's worth, the only really detailed information I've found on evaluating wear and tear on the Gates Carbon Drive comes from the manual Gates produced for the Rohloff hub users. Not sure why there's not a similar manual out there for Gates Carbon Drives in general. I've included a link to the .pdf file.

https://www.carbondrivesystems.com/im...1271411518.pdf

You can search on "replace" as there are a number of references to when a belt should be replaced, to include making mention of a "ratchet event" being cause for replacement. However, since the Rohloff and single bike systems use a very small driven sprocket with a larger drive sprocket vs. tandems using two larger drive sprockets, I'm not sure if the same assumptions regarding presumed damage apply.

However, Section 3 / Page 30 of the manual goes into some detail on how to check the sprockets and belt for wear which seems a bit more useful. They show a gauge that can be used to check both the belt and sprockets for wear which would probably be the most reliable thing to do. Of course, what I found most interesting in reading the manual was the comment regarding sprocket wear. In short, it suggests that despite what intuition would suggest, the sprockets typically wear out before the belts and that the sprockets and belts should be replaced at the same time. Again, I don't know if that is unique to the drive belts or if would also be true of the sync belt application.

bobthib

Seems to me the easiest diagnostic is to mount the spare and hit the hills.

PedalPink

Wow, that manual for Rohloff hubs has lots more useful information than we received with our Gates system. And I'm not sure the news is good. There's a big $ difference between replacing the belt ($85) and replacing the sprocket and belt ($450) every x (still to be determined) miles. Once we did the field repair on the eccentric we completed the remainder of the 200km and had no additional problems (other than the usual vexing experience with the Garmin rerouting our brevet for a shorter distance - and a disqualification).

We were planning on testing the new replacement belt anyway. As a randonneur, we like to test our replacement parts in advance of needing to use them. We haven't yet been tested on our backup strategy and ingenuity on 600km rides when a critical component fails 200 miles from our start but it will happen someday in our cycling life. We usually joke about paying a towing service (available even in the smallest rural area) to haul our sorry selves (and tandem) back to the start.

We'll contact Gates this week and order the gauge to test the belt and sprockets, which I didn't know existed. Thanks for the link! We'll also try to determine our exact miles but we know we had >6,000 in 2010 plus a lot of miles in 2009. Again, we've been very happy with the Gates and specifically spec'd one for a second tandem we bought late last year. We are riding long to train for Paris-Brest-Paris this year and are a high mileage team.

We hope to hear from some other Gates owners who have faced the replacement issue. Did you replace just the belt ... or the entire system???

TandemGeek

So.... anything from Gates yet that you can share?

Inquiring minds, don't you know.

prathmann

Have well over 20,000 miles on our regular timing chain. So if the above is true then you should be able to go another 200,000 miles. Of course it does have that 'as much as' clause which could negate the whole thing.

If you get the gauge mentioned above or another method of checking for wear, please let us know what your experience is on real world durability.

PedalPink

Thanks for the nudge. We got busy last week and I hadn't contacted Gates. I did write them an email today with the inquiry. While on the website, I read this in their FAQ's:

"Normally tooth jump does not cause significant belt damage; however, strong riders who jump teeth at very high loads might consider replacing the belt. While this is very rare, these riders may need to set their belt tension above the standard recommendations to avoid this problem."

I have located the part number of the gauge but have yet to find where to order.

I will keep posting to this thread as we learn additional information. Thanks, much.

oldacura

I'll bet the real answer is "nobody knows". As an early adopter, you are the testers. Is this belt your timing belt (or the drive belt)? If it is the drive belt, the most likely early failure symptom would be for the belt to begin to slip on the smaller sprocket. If it is the timing belt, there are so many teeth in mesh (180 degree wrap) that it is very unlikely to slip. Timing belts in car engines will eventually break but give essentially no symptoms until that point. However, that is a rubber belt on aluminum pulleys. Are these pulleys aluminum or plastic? If aluminum, I think it would take a very long time to see any failure symptoms. If they are plastic, I would guess that the pulleys would wear out before the belt. Each pulley tooth meshes with way more belt teeth than any one belt tooth meshes with pulley teeth.

TandemGeek

I'm inclined to believe that the Rohloff operating demands are very different from single speed and VERY different from a tandem sync belt application, remebering that the Rohloff can generate some serious loads on that very small rear sprocket in its shorter climbing gears. A belt slip / ratcheting in that situation would be disconcerting.

As for the tandem sync belts, we had a couple belt slip / ratchet events with our very small diameter, experimental sprockets. In one instance the very high pulling load between the front & rear crank axles caused the eccentric to move and once the tension was reduced the belt began to slip. We retensioned the belt, used a bit more torque on the eccentric bolts and put a benchmark on the eccentric & eccentric shell to monitor eccentric rotation. The next time the belt slipped was under very, very heavy pedal loads on a very steep, short climb. After verifying the eccentric hadn't moved I was of the opinion that we probably induced enough frame deflection to cause the belt slip / ratchet.

Again, we were using 39t sprockets for our experimental system vs. the 70t (1st Gen) and 69t (2nd Gen) Gates sync drive sprockets: that's a world of difference in all regards. Well, that and our sprockets were CNC'd out of aluminum, not modeled out of polymers. In any event, our belt never gave any sign of being weaked or stressed.

Looking forward to hearing what the good folks at Gates have to offer.

merlinextraligh

We've had our Belt jump several times, so I was a bit concerned to see the info in the manual saying you should replace the belt when it skips. So the answer to the FAQ is a little bit more reassuring.

Even properly tensioned, we can pretty much make it skip if we try to. Up a steep pitch, out of the saddle in a big gear, say 53/13 and it's likely to skip on our bike no matter how tight.

The skipping seems to be much more related to torque, than power, so we've just learned to stay out of gears where we are really grinding. In a sprint, where you're turning a higher cadence, it's never skipped, even putting out more watts, than the situations where it skips at a lower wattage, but also lower cadence.

As for signs the Belt's ready to fail, if the failure is the result of putting the Belt on correctly, there can be diaganol scoring in the teeth, (which we found after our belt broke) As for signs its going to fail because of wear, I haven't seen that.

merlinextraligh

Uhm, no. Pushing down with a 1000 watt effort, out of the saddle, and the timing chain lets go, leads to a rather unfortunate collision between sensitive anatomical parts and the top tube.

Phantoj

If you think about it, the load in the belt from pedaling is going to be equal to the (tangential) force on the pedal times the ratio of the crank length to front sprocket radius... so a Rohloff setup would probably have about the same belt loads as a tandem timing chain.

Ritterview

While you are contacting Gates, be sure to inquire when their CenterTrack system, shown on half-bikes at NAHBS, will be available for tandem sync belts. It might be wise to hold off on getting a new belt until the CenterTrack is available.

Open Lateral

Over 7000 miles so far on our belt. It is the older 71 tooth pulley model. I have flipped the belt over a couple of times because it starts to creep and rub on the pulley guard. Right now the eccentric is about 1mm off center to offset this squeak.

PedalPink

We have had a response back from Gates:

"That is great to hear you have so many miles on the system. The life of the belt has too many variables to put a number on it. It is not uncommon for people to put the amount of miles on the system that you have. There is not a gauge specifically for tandem. The belt in the tandem application is an 8mm pitch, the drive systems use a belt that has an 11mm pitch. I would love to see some photos of your system to diagnose if I think your system is worn. The belt generally does not need to be replaced after a racheting event. The correct procedure is to increase the tension and check for reoccurrence. After I see photos we can decide if you need to purchase a replacement system. If we do decide to replace the system I would appreciate getting your old system back for testing. I like to take a look at the high mileage systems to determine if we can make more improvements."

So the good news is that a ratchet event alone does not require replacing the belt. We had done what Gates suggested (increase tension) and had no further problems on the 200k.

On close inspection as I took the photo's, I see a difference in the sprockets on the front and rear. The captain's sprockets are still squarish on the ends while the stoker's sprockets appear to have worn into sharp triangles (think teeth). I've looked at the system on our other tandem (with only 400-500 miles) and can definitely see a difference. The belt itself looks in good condition with no cracks although the blue has worn off (which I understand does not effect it).

Note - our system is also the original 71 and not the newer 69 model.

I'll post Gate's response. Meanwhile, wish us luck with the older system on Saturday's 300k in Kentucky. Miserable forecast (100% chance of rain).

TandemGeek

Thanks!

This is exciting...

Ritterview

Uh, don't forget to ask about the CenterTrack for tandems!

PedalPink

Gates has reviewed our photo's and is recommending that we replace the system due to the wear I noted on the stoker sprockets. We will be returning our old system to them for their inspection and evaluation. I hope to have more information to pass on after they've looked at it. I'm also asking them if we would have been able to get longer life from the system if we had rotated the sprockets from captain to stoker every 2,000 miles (like tire rotations on a car). My captain will be talking to Gates tomorrow, and he has lots of questions (including the CenterTrack).

We put another 115 miles on the system last Saturday ... far short of the 300k we'd planned. We went thru new brake pads during the first 65 miles due to the hills, rain and road grit. We had one spare set of pads but were uncomfortable riding an additional 126 miles with limited braking. It is still early in the season and we will have other opportunities to complete a 300k to qualify for Paris-Brest-Paris. A Louisville Bicycle Club tandem team did complete the 300k in the all day downpour - this is their first season of randonneuring and I'm so impressed by their ride.

merlinextraligh

This thread made me take a look at our system. I believe we have somewhere around 8000 miles on the rings. As you'll see from the Pics below the stoker ring is substantially more worn than the Captain's.

The stoker ring has enough wear that it is starting to reshape the teeth. The Captain's ring is not much more than the black surface starting to come off.

If it turns out that you need to buy new rings every couple of years at $400 plus, then that's going to be a problem for the system.

Captains



Captain's closeup



Stoker's



Stoker's closeup




[ hard to get the film plane square to the subject for sharp macro pics without taking the belt off, but these give the general idea]

oldacura

Merlin - Is it possible for you to swap captain / stoker pulleys or to flip them? If so, you should be able to get a lot more miles out of the pulleys. I'm surprised there is that much wear. I'm guessing it is from grit (?). Because there are so many teeth in mesh, it seems unlikely that your are close to skipping teeth. The wear pattern on the stoker's pulley (entirely on the clockwise side of the tooth) indicates that the captain is putting in the lion's share of the power.

merlinextraligh

I am think about swapping them. I don't think there is anyway to flipping them.

As for skiping, we already have that problem occassionally but I don't think it's related to the wear yet.

As for the power issue, I was thinking the same thing. My FTP is around 340watts, while my stoker's is 130watts so it makes sense.

Although it would seem to make sense that the stoker's ring is affected by both of our power, not just the stoker's, where the captain's ring is affected by only the Captain's.

oldacura

I'm thinking that if the stoker were stronger, the counterclockwise faces of the teeth would show wear. Since the captain is stronger, the clockwise faces of the stoker's pulley teeth are worn. Swapping them should double the life of the rings. The faces now showing wear should no longer be in contact after the swap. Maybe by the time the rings are worn out, it will be time to replace the belt as well.

TandemGeek

VERY interesting...

The sprocket wear / replacement cycle has definitely not been accounted for or discussed much in the Gates Carbon Drive marketing and lifecycle cost models.

Dean V

I can't help but think that this drive system is inferior to a chain and that its main attribute is the novelty/bling factor.

oldacura

The developers of this system would be (should be) very interested in your photos. This is probably a rare early adoptor data point. The only way I can think of to decrease the wear would be to change to a harder pulley material (like steel).
However, steel pulleys would be 3 times as heavy and likely more expensive to machine. If you do swap the rings, might you swap the belt end-for-end so you have a new wear surface on the belt as well as the pulleys?