Carbon Timing Belt Part 2 (oh snap!)
 

We now have the distinction of being the first team to break a Gates Carbon Timing belt. Last saturday's ride hammering out of the saddle on a short, but steep incline (around 15%) in the big ring, the belt lets go. Fortunately we were going slow enough at the time that we just came to a stop with no problem (except I banged the heck out of my knee on the handlebars, suddenly having zero resistence when your standing on the pedal with full force is not a good thing.)


Apparently, the belt is fairly sensitive to proper installation. The instructions make it clear that you should not "roll" the belt on the rings like you would normally install a chain. Rather you need to move the eccentric to where you can place it over the ring, and then tighten the eccentric. I did this as instructed on the first installation.

However, on the way to a group ride, in the process of getting the bike out of the car, the belt came off as I was getting the bike out of the car. (combination of the pedals going backward, and the belt being pushed by stuff in the car, as I pulled the bike out.

With limited time, and not wanting to loosen and retighten the eccentric, I carefully rolled the belt back on. It didn't take any force to do so. Thus I though the caution in the instruction had to be overstated.
I was wrong.

At the point where the belt broke ou can see very small line of diagonal scores across the teeth where the belt rolled on. Apparently, the twisting of the belt where it rolls on can lead to failure, and in our case did.

In spite of confessing my sin, Co-Motion is replacing the belt under warranty, which I take to be a standup gesture, given that they could have used my user error to deny any warranty defect.

The incident does give me a bit of pause about the whole belt idea. (It pains me to admit this knowing the rath of Mtbke I am sure to have heaped upon me.) Obviously I won't be rollong the new one on, and we'll see how the new one goes.

Back off on the steroids. Good thing it was only your knee that got banged!

How did your stoker fair? If you bonked your knee, did she come out unscathed?

^ not as big a problem for the stoker. When the timing belt (or chain) lets go, the captain's pedals are in free fall. The stoker's pedals are still connected to the drivetrain, so she still has resistence on the pedals.

Had I thought about it...even a little, I would have figured that out.

It seems as though the belt is non-field repairable. Did your stoker power you home?

merlinextraligh, Thanks for being the official / unofficial carbon belt tester for this forum. Hopefully the second belt will last longer. I hope the usual critic has refilled his medication. If not, get braced for a scolding!

You rolled it on one time and that caused it to fail? I had no idea these things were that fragile.

^ there appears to be a bit of a disconnect, given that the belts supposedly last 4-10 times longer than a chain.

Yet, rolling them on is a known cause of failure. I'm told we're the first to report breaking one on a tandem, but the rolling on issue has been a problem with single speed belt drives.

Belt drives are starting to become more common, with Trek specing them on a new commuter, among others. So I can't believe they are that prone to failure in normal use.

We've done our share of testing new products; some failed some made the grade . . .
Among some of the products we tested: tires (failures/blowoffs), a new-type fork (broke after 'only' 15,000 miles), saddles, rack (bad design), a carbon timing belt (prototype), a new sub-100gram eccentic (failed), etc.
Have broken a couple timing chains and we know the feeling: W-h-o-a!!! . . . Captain's feet do a super fast spin! In each instance captain steered/coasted while stoker kicked in the afterburner to get us home.
Thanx for posting the results and glad no-one was really hurt.
As usual, Co-Motion comes through with super-service
Pedal on TWOgether!
Rudy and Kay/zonatandem

Bummer about the belt, glad you'll survive. Some of the belts I used when prototyping belt drives in model cars back in the 80's were very efficient but fragile. The best belts, performance wise, had short lives. Not so much snapping but rather going from a cog belt to a smooth belt right now.

We popped the drive rings in a Hugi hub on the MTB tandem, big effort, slight climb, slow speed. It was not fun having both sets of cranks go into neutral. The noise and feel through the frame was bad too.

On the MTB tandem, each time we drag our timing chain or chainring on logs or rocks, I keep thinking how it would be nice to have a driveshaft and gear setup like a car rear axle. Use a splined BB spindle to couple the gears, and stuff the shaft inside the frames lower tube. Seal the tube and run the thing wet.

PK

From your description it seems that the belt came off fairly easily. Wouldn't you say that the belt "rolled-off"? If that is the case then the belt could have been damaged when it rolled off or when you rolled it on. Either way it does not seem like that alone should have damaged a belt... but it did.

I'm glad you are ok and that you posted this.
I was considering going with the belt drive on my next tandem.
Now I am thinking not. Safety is always the most important thing when cycling.
Oh and why were you in the big ring on a 15% hill?
I would not be doing that even on my single.

^ 1. I wouldn't necessarily rule the belt out. the failure apparently occurred as a result of me failing to follow directions. Also in hindsight I don't think it was completely without warning. I was hearing a noise, sort of a clunking sound, that I thought was coming from the captain's bb. In hindsight I think it was where the belt was already starting to break. (its not there now with the timing chain.)

2. We've broken a timing chain before, so it's not totally unique to the belt.

3. As for being in the big ring, this is like a 100 yard climb (not 100 yards vertical, 100 yards total) We bogged down more than I would have liked, and were muscling it out when it broke, but we've done that little section in the big ring a bunch of times before.

To me, the verdict is still out on the belt. If the next one fails or wears out following the instructions, then there's a problem. If it turns out it truely was my installation, then its just a cautionary tale that you do need to follow the specific instructions.

Other than the belt snapping, do you prefer it over a chain and was it worth the extra cost?
How much does a replacement belt cost?

Well, somebody had to be first (and I'd bet money that you're really not the first). If nothing else, thanks for being the first team whom I'm aware of that was willing or able to share your experience, as I'm sure it will go a long way towards driving home the need by others who own tandems with the Gates Carbon Drive to use care when handling the belts.

Just some miscellaneous thoughts that come to mind...

- A Gates carbon belt is probably best described as inelastic, far more-so that even a chain

- The carbon cords that are at the core of the Gates carbon belts are analogous to carbon seat posts and handlebars: if you handle them properly they will remain incredibly strong.

- However, if you bend, twist or stretch the fibers beyond their design limits they will crack -- in much the same way that a carbon seat post will crack when a clamp is over-torqued -- and once the carbon and resins that hold them together debond the cords lose most all of their strength.

- Like all carbon bits and uber-light or exotic materials, buyers really do need to read and heed all of the manufacturers handling instructions. Note that most of the high-end stem, seat post clamp, seat post and handlebar manufacturers have taken to putting maximum torque values on their components for this very reason.

- Gates seems to be well aware of what will damage the carbon cords in its belts:
http://www.carbondrivesystems.com/im...1239728288.pdf

This makes me a bit worried, because I'm pretty sure that I've also not been following the instructions for our Gates carbon belt perfectly.

Even when the eccentric BB is in the most rear-ward position, there is still not enough slack to install the belt without turning the cranks a little to help it on, and so I have done this a few times. It requires very minimal force with the cranks, but it is certainly not possible to install it without this assistance. The BB spacing should be perfect for the belt because we have a stock-sized 2008 Co-Motion Speedster.

Can anyone who has this kind of set-up confirm that it is possible to get the belt on by following the method described in the instructions, or does anyone have any idea how I could do something different to make it possible to do so? Thanks.

Sorry to hear about your incident! My wife and I just upgraded from our Cannondale to a Comotion Machiatto w/the belt drive. The belt drive feels more responsive on accelerations than the timing chain, and certainly quieter. The bike is pretty amazing overall.

However, our new tandem is a whole lot more fancy than what we were riding, so it's probably hard to compare apples to apples. Good to know about the potential issues with the timing belt - thanks, we'll exercise caution.

The inquiring mind wants to know: how much are these belts? Is the Gates carbon fiber
the only option here? Timing belts are available in fractional to multi horsepower ratings
for constant service, not sure how that translates to a strong team standing on the cranks
on a steep hill. I suspect your team could go upwards of 2 kw for a very short time.

sch, the answer to all of your questions is probably somewhere in this lengthy thread:
Carbon Timing Belt System

Installed the new belt today. I think I have the answer to your problem. First, put the ecentric as far back as possible. Second install the rear ring, and put the belt on it. Third, slip the captain's ring over the spider of the crank, but don't bolt it on. Fourth, put the belt on the ring while its unbolted. The fact that you can move the ring around will give you the play to put the belt on without rolling it on. Then, push the ring in place to bolt it on,with the belt already on the ring.

I hope that makes sense.

That makes sense, thanks. I have previously tried removing the whole crank, spider, and ring to install the belt onto that before re-mounting the crank. At least this only required messing with one bolt instead of the five that are required to take the ring off and replace it. However, I was unable to re-mount the crank when it had the tension of the belt pulling it out of position (and I did try doing so for several minutes). Hopefully removing just the ring instead of the whole crank will make this work better, but it is certainly not an easy solution.

A far better solution would be for them to make the belt 5-10 mm longer - I have LOTS more room to take up the extra length using the eccentric BB that I don't need with the current belt length. Maybe the belt length is designed to initially be this way because the belt will stretch a little over time, and then I will need to use this extra tensioning and the belt will go on easily without removing the ring. However, in our first 2,500 km of use, I haven't seen any noticeable stretching. Plus, I can't imagine that it would match the teeth on the rings that well if it was to stretch by that much, so I'm really not anticipating much stretching to happen.

Anyway, enough moaning about bad design. I guess the only way for me to avoid a repeat event like yours is to remove and replace the timing ring whenever putting the belt on. Thanks a lot for this tip, I'll let you know how I get on when we pack the bike up again to fly to Canada in two weeks.

Given the tooling costs to change the drum mold they probably use to lay these things up, I'm guessing that ain't gonna happen any time too soon unless they're selling belt drives at a much faster clip than I could imagine. It would be easier for Co-Motion to simply take 2mm out of their boom tubes on newer bikes.

Seems it would be simpler to make different diameter rings?

Perhaps... But, to be candid, of all the 'innovations' to come along for tandems the belt drive is right up there with the superlight Topolino wheels: damn expensive, 100% proprietary, of questionable value but really cool nonetheless.

I 'took one for the team' wit the Topolino wheels and will continue to use them when conditions warrant but don't feel I've been left out by not being able to fit the belt drive to our tandems.

I assume the timing pulleys are molded. A new belt mold or new pulley molds would be very expensive. They would have to sell a bunch of these things to justify either mold.