Carbonello

A couple years ago, when Gates belts came out, and even recently, people mentioned that Gates belts are only available in one length. I even spec'd my Santana an inch longer (to 28.5 inches) in order to use the belt. That doesn't seem to be the case now. I was looking on the Gates web page:

- Gates Carbon Drive Belt System

Bob

Dean V

Look a bit more closely. There is only one belt length of 2.0m/250 teeth that is suitable for a timing belt. The others are much shorter.

Bezalel

Is the Tandem belt simply a modified Poly Chain GT Carbon? These are available in a number lengths.

https://www.gates.com/~/media/Files/G...17595_2011.pdf

twocicle

Gates provide 2 timing ring sizes 69t and 74t. The latter, bigger ring works with the default Santana BB-bottom tube-BB spacing and a standard 2000mm/250t timing belt. No need to custom spec a Santana frame just to use the 69t ring, unless you have the cash and your stoker greatly appreciates the extra room. Did Santana not explain this?

TandemGeek

Actually, there are four different size tandem timing rings out there, the aforementioned 69t 2nd generation belt for 28.5" frames, the 70t 1st generation for 28.5" frames that was a very tight fit, the 74t for Santana's frames, and a recently introduced 66t developed to support Cannondale's large-size (29") stoker compartments last year.

And, depending on when our friend's Santana was ordered, there was only the 70t/69t timing rings for the 28.5" frames on the market. The Santana spec' 74t didn't come out until a year or two after the original timing belt was introduced so the only option may have been a longer than standard length frame.

TandemGeek

Same technology, but the GTs are 12mm wide vs. 10mm for the carbondrive so a different, wider sprocket is required. That said, we have friend who has been adapting the Gates belts to work as a tandem sync drive since 2004, well before Gates collaborated with Co-Motion to develop the tandem-specific Carbondrive belts & sprockets. You can read more about the GT belts in a 2010 blog entry I wrote as we began doing some tests with a very compact GT belt drive at the following link:

Product Development: Timing Belt vs Chain | The TandemGeek's Blog

The 33t sprockets ultimately proved to be too small to be practical and we opted to stick with chain vs. going to the wider spindles that would have been needed for a larger timing ring.

Timing Belt: Update #6 (Finale) | The TandemGeek's Blog

twocicle

Gates may have made a few 71t (not 70) in the Mudport CDC style but I don't recall that size ever in the CenterTrack (CDX) variety. It's hard enough to source the "standard" sizes without needing to look for oddball components.

Current CenterTrack CDX 5 bolt 130bcd ring availability is posted on the updated Gates web pages:

Tandem: 74, 69 ,66 (- Gates Carbon Drive Belt System).
Other(?): 70, 60, 55, 50 (- Gates Carbon Drive Belt System). BTW, only the 55t and 50t on this page look beefy enough to use as tandem timing rings

Doing some rough calculations based on the new size diff of the CoMo 29" being 1/2" longer and 3 belt teeth less than the 69t, Mark your 30" frame seems to be approx at the 50t (69-19) size while using the standard 2000mm belt. Of course, being off the estimate by even 1t can make or break the fit since most eccentrics have very limited amount of uptake.

Anyone have an accurate formula to calculate possible stoker lengths using the "other" rings I posted above and the 2000mm belt?

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Also one minor note to TG's older belt experiment regarding chainstay - timing ring clearance, Calfee and other mfr have adjusted for this clearance need on their frames.

Carbonello

Exactly. Back in 2005 I spec'd the Santana to have the same rear length as my previous Cannondale. They added about an inch. It worked out to be the correct length for belt drive with 69 tooth belt rings when they became available. When I replaced the stolen Santana with a new Santana this year, I had them use the same design drawings as the 2005 bike. I added couplers.

TandemGeek

70t was a typo, but you mean what I know. The 71t was the original spec and it made the belts too hard to mount on the 28.5" frames, hence the 2nd gen 69t.

As for the tandem vs. non-tandem sprockets, bear in mind that the tandem sync belt uses an 8mm pitch whereas all of the right side drive belts use an 11mm pitch, making them incompatible.

LelandJT

Tandemgeek, great blog. I read all your belt posts as well as many others. You are as fastidious as me when it comes to setup so you're the perfect person to ask this: Why is your eccentric in the "up" position? As a mechanic on Santana tours I've seen many bikes like this and when I point it out the owner always says they didn't realize. I flip it to the down position and lower the saddle & raise the stoker stem a corresponding amount. The idea being to lower center of gravity and minimize reach to the ground. Santana designs frames with the intention of eccentrics being run this way so I always assumed it was a mistake when they are "up" but I'm betting you'll have an interesting reason why.

DubT

I do not know why TG runs his in the up position but I run mine that way for a couple of reasons: 1. If for some reasons the eccentric loosens, I believe that the pedaling forces will help keep the belt/chain tight. 2. I prefer additional pedal clearance when cornering.

twocicle

Ah nuts. Thanks for correcting my oversight about the pitch diff.

As for eccentric position, I run mine in the down-forward position mostly for some of the same reasons as Leland... lower center of gravity and minimize reach to the ground. I would find having the captain's seat extra high would be a real pain for the stop/starts and be too different than the ride heights I'm used to from singles.

The main reason is that with the eccentric in the lower position it helps me use a slightly smaller frame size (54cm) than my singles (56 current Spec. Tarmac and 58 previous Trek) and still hit my exact setup measurement requirements and not needing to add a chunk of stem spacers. Our tandem has plenty of pedal (Speedplay Zero) clearance too (higher BBs than singles), so no need to think about pedals at all, even given my preference for leaning the tandem at fairly high angles in corners.

LelandJT

I'm so used to gravity oriented, full suspension MTBs with super low BBs so it's just a given that I can't pedal when leaning. However, those low BB bikes corner INCREDIBLY and feel so stable and balanced. I love that Santana specs a lower BB than other brands and with all our prototypes I'm always asking if it can go lower. Last week someone stuck a note on my tool board that says, "Lower and slacker please," since that's become my standard response when returning from a test ride.

Back to the belts: Tandemgeek, what number of teeth do you think would be the ideal belt ring size to balance weight and frame accomodation with durability? I'm gonna try hounding Gates about making a shorter belt. I like 32t or 34t rings when using a chain so I'm tempted to shoot for the equivalent size.

twocicle

I should go get the specs for both our Calfee and my single (Specialized Tarmac SL4) and compare those. My impression is that the tandem has a much higher BB already and even though I use 175mm cranks on that, there is almost no way for me to clip a pedal. Even on that single Tarmac SL4, I did a criterium race this year and was pegging +34mph through a high speed 90 degree corner and pedaling all the while. The tandem seems to have plenty of clearance, so boosting the BB/eccentric shell is definitely not needed on our frame.

TandemGeek

Short answer: It lets me ride with my saddle a 1/2" higher than it would in the "down" position and that gives my wife's handlebars a little more separation from my saddle / posterior. That allows her computer/GPS to be where it should be -- just left of center -- and visible when we ride.

FWIW, you'll be happy to hear that it's in the down position on our triplet and off-road tandem, but I do that on those tandems to lower my riding position relative to the head tube, i.e., it's a bike fitting choice. Frankly, up or down is truly a preference; there's no right or wrong way to set it if you understand what the subtle trade-offs are. 'twocycle' has IDs some of the reasons and I've had it both ways on all of my tandems at one point or another as I tweaked things to find our right answer for eccentric position.

It's a subtle thing that Steve P. at Gates pointed out to me when, like you, I thought I'd broken the code on fitting the 2000mm belt to our 30" crank span by using the drive-side sprockets as timing rings. I was truly crushed as I really wanted to give the belt another chance on our Calfee, this time with proper-sized sprockets.

For me, it's 60t @ 8mm pitch since that would let us use the 2000mm belt on our 30" crank span. Seriously though, that's probably about as small as you'd want to go with the belt drive IMHO.

twocicle

I'm curious what the reasoning might be for Gates having 8mm pitch for tandems, versus 11mm pitch for single drives?

Given a standard BB spacing of 724mm/28.5", I wonder what size ring would be needed if a 1452mm/11mm pitch belt were used?

Thomas Knoll

The circumference of the resulting "rings" would be: 1452mm - 724mm * 2 = 4mm. Or about 1/3 of a tooth.

Dean V

Which is very small indeed.

Bezalel

I think it's a matter of reusing molds from existing belts. I don't see any existing 11mm pitch belts long enough to be adapted for tandem use. (The longest 11mm pitch belt I see is 1705mm.)

LelandJT

Help math people! Using the 1452mm 132t belt with 46t rings, how far apart will the BB axles be? This is the smallest ring I can find that fits a road crank and longest belt besides the looong tandem belt. Another option would be making 104mm BCD 4 arm spiders so I can use the 39t sprocket. How far would this place the BBs apart?

Dean V

473mm for 46t and 511.5mm for 39t.

vrooom3440

11 mm pitch
39T of sprocket circumference == 429 mm of belt wrapping around
1452 - 429 == 1023 or 511.5 mm between centers
Or about 20 inches