Product Development: Timing Belt vs Chain

Updated/Edited: I think I was half asleep when I originally posted this.

As foreshadowed in last week’s “I Want Another Tandem” blog entry, we received a new sync drive in Friday’s UPS shipment that we’re beta testing for our good friend Bob Thompson.  Yes, it uses a Gates Carbon belt; however, it’s not the Co-Motion / Gates Carbon Drive system.  Below is a photo of our Calfee with Bob’s sync-drive installed just before we headed out for our shake-down ride on Saturday afternoon.

A little background is probably called for here.  Bob first developed and began using a belt-driven sync drive on his Seven tandem over six (6) years ago.  He and his lovely wife Jan have logged over 30k miles on several different variations of his belt drives since that time.  I first learned of it back in March of 2005 when another friend from Florida, Steve K., made mention of the belt in a thread he started at BikeForums.net entitled: Timing Belt Instead of Chain.  I was a bit skeptical,  but over time all of my original concerns have proven to be unfounded.

We first met Bob and Jan a year later at the 2006 Georgia Tandem Rally in Americus, Georgia where I was able to put my eyes and hands on the belt drive: it was slick, just as Steve K. had suggested. However, I still had some concerns, most of which Bob addressed during our Friday ride at GTR ’06: he’d really done his homework and at that time already had several thousand miles on the pre-carbon Gates belt and a pair of custom belt sprockets his machinist had made to Bob’s spec that bolted up to a standard set of 74 BCD cranks.  Bob worked with and sourced his belts from the folks at Gates for his design and, as their belt technology evolved, Bob adopted the versions that seemed best-suited to the tandem’s sync drive. These included the Gates PowerGrip GT2 and, more recently, the original 12mm wide, 8 pitch version of the Gates PolyChain GT Carbon belt. The latter preceded the bicycle-specific 10mm wide Gates CarbonDrive belts and asymmetrical aluminum sprockets.

So, make no mistake about it: Bob was doing extensive research, prototyping and field work on his own nickel with his own tandem well-before the Carbon Drive system debuted in 2007 with its more narrow version of their high-modulus synchronous PolyChain GT Carbon belt and lightweight, patented sprockets with mud ports.

Fast Forward: May 14, 2010:

So, how did we end up with this slick system on our Calfee ‘test mule’?  Before the start of Friday’s ride at the 2010 Georgia Tandem Rally, Bob asked me if I’d be interested in doing some testing with his latest version of the Gates belt-based sync drive and, as you’d expect, I said sure.  He just needed to know the center-to-center length of the front & rear cranks on our longer-than-average Calfee  to determine which length belt and what size sprockets to spec.  I provided those to Bob in mid-June and the hardware arrived on August 20th.  I should note, we were ideal candidates for the testing since our tandem was already fitted with daVinci’s cranks, I do all of my own bicycle maintenance, and can typically solve all kinds of tandem assembly and adjustment challenges.  Yes, the system is designed around the daVinci’s spiderless tandem cranksets( & discontinued White Industries cranksets)… which we’ve used exclusively since 1998.  The photo below shows the daVinci standard chain-drive timing sprockets with chain sitting next to the custom-machined, Gates GT profile compatible sprockets and Gates Polychain GT Carbon belt.

As to why Bob continues to develop his system… it’s pretty straight forward: not everyone has a tandem with a 28.5″ long stoker compartment / boom tube!   At present, I believe this remains the only standard size frame dimension supported by the 2000mm length version of Gates Carbon Drive belt mated with the original 71t, or newer,69t timing sprockets.   Conversely, the 8 pitch, 12mm wide Gates Poly Chain GT Carbon belts are offered in seven (7) different lengths that would lend themselves to a tandem sync drive when coupled with custom-made, Gates GT-profile sprockets, i.e., 1600mm, 1760mm, 1792mm, 2000mm, 2200mm, 2240mm, and 2400mm.  The trick is getting the right length belt & sprocket size for a given boom tube length.  In our case, and with a 30″ stoker compartment / boom tube, Bob spec’d a pair of 33t sprockets and a 1792mm belt.  Bob’s test objective with the very compact sizing was to see just how small a sprocket could be used without creating any issues with belt durability and life.

Before diving into the details of the swap-over, let me give you a quick and dirty run down on my  first impressions:

Pro’s:

• Quiet, I mean REALLY quiet
• Smooth / very low drag
• Excellent power transfer & positive engagement… no chain slop
• Net weight (370g) 170g less than the already lightweight daVinci chain-based (540g) sync drive
  • Chain = 390g vs. GT Carbon belt @ 90g
• Very compact, minimalist appearance – will certainly draw lots of attention and questions
• Compatible with daVinci’s super-light, timeless alloy square taper cranks… our preferred cranks since 1998.
• Replacement belt widely available with MSRP of $79 that can typically be found for less than MSRP.
• Sprockets don’t appear to be items that will ever need replacement due to routine wear & tear.
• Very low  maintenance

Con’s:

 

Gates Carbon Drive Sprocket

• Poly Chain sprockets with their beefy flanges & solid core are heavier (260g /pair) than the daVinci chain sprockets (140g /pair) as well as the Gates aluminum Carbon Drive pulleys (195g /pair) with mud ports and asymmetrical, inner flange-less design.
• Not compatible with narrow bottom brackets due to interference with chain stays
• May require asymmetric spindles to minimize Q-factor impact of wider axles needed to gain BB shell / chain stay clearance
  

  Gates Poly Chain Sprocket (with bearing, which is removed for this application)

• Very compact, minimalist appearance – will certainly draw lots of attention and questions
• Field repair of a broken or lost belt could prove to be problematic, particularly on tour.
• 33t ‘sprockets’ could accelerate belt wear given higher peak loads on fewer sprocket teeth
• Requires use of  daVinci (or an old set of White Industries) cross-over tandem cranks, which remain square-taper only… a good thing in my book.
• Each different length tandem stoker compartment / boom tube requires a different sprocket/belt analysis, component specification and custom-made sprocket size.

 

Neutral / TBD:

• The 8MGT-1792-12 PolyChain GT Carbon Belt is 2mm wider (12mm) than CarbonDrive tandem sync belt (10mm), but also shorter (1792mm vs 2000mm) and as a result is about 15 grams lighter.  I need to double-check the thickness.  There are likely some trade-offs on both designs that I’ll explore in future updates as they reveal themselves.

OK, here’s the  installation, shake-down and current status report.

We did a 17mi shake down ride on Saturday (long day working in the yard + 95F doesn’t make for a lot of energy this late in the day) and the belt/sprocket system was flawless.  Very quiet, so quiet that both Debbie and I are now aware of other drive train noises we never noticed before!  Very smooth, very linear power delivery with no wind-up or surging when climbing out of the saddle, etc…  Even though the front & rear sprockets are not 100% parallel, they’re within Gates’ design limit of 1/16″ per linear foot  for run-out alignment and there’s no noticeable drag or noise.  Also, no worries about derailing the belt given the inboard side of the sprocket has a flange vs. the Gates sprockets Carbon Drive system.  On Sunday we did a ~40mi ride in East Cobb County with our friends Roger & Eve (who hosted and led the ride) and Mike and Nancy.  Roger calls the ride East Cobb’s 6-gap ride, noting there were two climbs with grades that topped 18% and another two that topped 14%.  Again, no issues noted at any point on the ride.

So, at least at present, we’re cautiously optimistic the system will work quite well.  However, the 33t sprocket strikes me as over-reaching a bit as I’m concerned the higher tension from the slower-moving, harder-pulling belt will reduce the belt’s life compared to the 69t now spec’d by Co-Motion.  There is apparently a 59t / 6″ sprocket offered for the 12mm wide Poly Chain GT Carbon — which is coincidentally about the same size as daVinci’s 34t chain rings — that might prove to be the sweet spot, but this is why we test; yes?

Trade-offs:  The additional 2mm of belt width coupled with the dual flange sprockets drives a need to add ~5mm of left-crank off-set to clear our Calfee’s left chain stay vs. the chain drive. Of course, the larger diameter, aluminum Carbon Drive sprockets also create a similar  fitment challenge. I would think the  systems should fit on a tandem with typical , stock width 118mm bottom brackets.  Conversely, the GT Carbon belt with its extra 2mm of belt area which may partially off-set the potentially shorter life of the belt due to more frequent and higher peak loads of the small sprocket.

Installation took a little time and parts swapping, noting we were running 108mm front bottom bracket (BB) with 111mm in the rear.  Chainstay / rail clearance was an issue, as well as front pulley clearance given the very over-sized frame dimensions of the typical Calfee.

Thankfully, I use Phil Wood BB’s which allow for right or left off-set vis-a-vis the two adjusting cups that provide +/- 5mm of adjustment.  I also had a spare stainless steel 119mm Phil Wood BB with +5 off-set sitting in my parts bin that I used to solve the chainstay clearance problem. Although the +5 off-set is supposed to be on the right side, I simply flipped the BB around so the +5 is now on the left side and provided the needed clearance (see photo). Debbie has not been bothered by the 8mm increase in Q-factor, so that’s a good thing.

With the 119 + 5 on the rear of the bike, I used the original ti-mag 111mm rear BB to replace my 108mm front ti-mag BB and that gave me most of the extra clearance I needed.  Off-setting the eccentric by a few mm provided the rest.  The front pulley is still about 4mm closer to the centerline of the frame compared to the rear pulley but, as noted, it doesn’t seem to be an issue with the chamfered dual flanges to keep the belt centered and it’s well within the limits outlined in Gates’ engineering specs for the Carbon GT belts.

I’ll probably go ahead and procure some new Ti-Mag BB’s with +5 left off-set for the long-haul this winter if all goes well with the evaluation process, and as I said… I’m cautiously optimistic the system will work just fine.  I’m also encouraged to see the 8MGT-1792-12 Gates GT belts are very affordable, far more cost-effective than the belt marketed for tandems by Gates and even the least expensive, quality chains.

Again, more to follow as we proceed with our analysis and feed back info to Bob.