What is wheel "dish"? How wide should a tandem hub be? What about the new Shimano tandem hubs? Should the new standard be 145 or 160?
What is dish? Because early racing bikes didn't have derailleurs, 120mm dropout spacing predated the 5-speed freewheel by a few decades. In order to stuff 3, 4 and eventually 5 cogs into a "standard" frame, manufacturers compromised wheel integrity by offsetting the hub flanges and creating a "di shed" wheel. On a dished wheel the spokes on the right side have 2 strikes against them: (1) diminished bracing angle to resist side loads and (2) higher tension causes right-side spokes to shoulder a majority of the wheel's load and torque. Contrary to popular belief it is wheel dish, and not Murphy's law, that causes freewheel-side spokes to break most often. By the mid '60s it had been determined that cramming a sixth cog within 120mm single-bike frame was totally unworkable--the proverbial straw that broke the camel's back.
The situation for tandems was, of course, worse. Those of us who used a tandem for heavy touring and long distance competition (double centuries) carried spare spokes, a freewheel tool and a 12" wrench. Until the mid-70s a strong tandem team could expect to endure nearly as many broken spokes as flat tires!
Because I was determined to "cure" a tandem's chronic wheel problems, in 1974 I worked with a fledgling hub builder named Phil Wood to co-develop a "final wheel solution" in anticipation of an as-yet unnamed line of reliable tandems.
Phil's original modular rear hubs featured either a 50mm or 60mm steel "barrel" containing the axle and bearings. Hubs were assembled by gluing an aluminum flange to each end of the barrel. To combat the dreaded effect of dish Phil's earliest effort was a very special hub with 24 spoke holes on the threaded freewheel flange and only 12 spoke holes on the unthreaded left flange. Phil and I both realized an even stronger wheel could be built by combining his 60mm barrel with a pair of the 24-spoke flanges--of course the resulting wheel would be 20mm too wide for existing frames and have 12 too many spokes for existing rims!
When our mutual friend Spence Wolf (of Cupertino Bike Shop) convinced Super Champion to custom drill a few rims, I immediately commissioned the first tandem frames with 140mm spacing. History has proven these early 140mm tandem wheels met my goal of being just as reliable as the narrower wheels found on single bikes.
In the late '70s, when customers demanded 6 speed freewheels, we had Phil assemble the same flanges onto his narrower 50mm barrel. While the resulting wheel was still 140mm and dishless, the narrower flange-to-flange spacing reduced the spoke bracing angle and wheel problems increased. When 7 cogs became available a few years later, we co-designed a series of Japanese tandem hubs with only 2mm of dish. While 1/10th of an inch of offset may not seem like much, the resulting wheel was over 10% weaker. By the early '90s customers wanted 8 cogs. We did some testing and discovered a 140mm wheel with eight cogs would never meet our standards for reliability.
To provide 8 cogs instead of 7 without affecting reliability, it would have been necessary to add 5mm to each side of the hub. After considering 150mm, we instead decided on 160mm to allow 8 cogs AND to recapture the strength we had lost through the two earlier revisions. For the past four years all of Santana's premium tandems have featured 160mm spacing.
Santana did not expect everyone to immediately jump on our 160mm bandwagon. After all, for a decade after Santana introduced the 140mm standard most builders stubbornly continued to build tandems with 120mm and 126mm spacing. I can safely predict another decade will pass before some of our competitors figure out 160mm spacing.
In the meantime, after both Trek and Burley discovered the failure rate of 8 cogs in 140 spacing was way too high, they convinced Shimano to stretch an existing (and inexpensive) 135mm ATB hub by 10mm to 145mm. Since Shimano is the bike industry's standard maker, other tandem builders quickly signed on.
What's wrong with 8sp/145mm? Because the resulting wheel is considerably weaker than 7sp/140mm, this "new standard" is a giant step backwards!
Is there a way to predict wheel strength? Sure, we developed a reliable wheel strength index here at Santana a few years ago. First: to determine the wheel's bracing angle measure the distance from the right flange to the centerline of the rim and frame. The best hubs from Phil and Edco have 30mm for right-side bracing. An average 6-7 speed hub has about 25mm and the worst 8sp hubs have only 20mm. Second, determine the dish or offset between the centerline of the rim and the centerline between the hub flanges. If the "dish" is zero all spokes share the same load. The best hubs from Phil and Edco are totally dishless. Suzue and Sansin 140mm 7sp tandem hubs have about 2mm dish. Most hubs found on tandems have 4-5mm of dish and the very worst hubs for a tandem (Dura Ace 8sp) have over 7mm of dish. Santana's index multiplies the right side bracing distance times degree of dish to determine an approximation of a wheel's relative reliability:
HUBS USED BY SANTANA
Phil Wood 5sp fwl 140 (Santana O.E. 1976-79): 100
Phil Wood 6sp fwl 140 (Santana O.E. 1980-84): 83
Suzue 7sp 140 fwl (Santana O.E. 1984-92): 72
Campagnolo 7sp 140 fwl (Santana 1990-91): 68
Edco 8sp 160 (Santana O.E. 1992-current): 100
HUBS USED BY OTHERS
Shimano 7sp 140 (Trek 1993): 63
Hugi 8sp 140 (Trek & Ibis): 56
Shimano old XT 7sp 135 (Fisher tandems): 55
Shimano XT/XTR 8sp 135 (current ATB singles): 40
Shimano Dura Ace 8sp 130 (current road singles): 36
SHIMANO'S NEW STANDARD
Shimano's new 8sp 145 (new tandem standard?): 59
Shimano's newer 8sp 160 (Santana O.E. 1997-?): 93
When Shimano informed us about the 145mm hubs they were planning to build for Burley and others, we were ecstatic--at long last Shimano was agreeing to build a tandem-specific hub threaded for disk/drum brake. Those other builders, anxious to agree on a standard that could be wedged into a current 140mm frame, never bothered to ask Shimano for a symmetric version with greater flange spacing. As soon as Santana explained the reasons behind the 160mm standard we adopted back in 1992, Shimano's engineers quickly grasped the advantages and agreed to produce a 160mm version of their new hub. After we received our early samples, we got Shimano to agree to further improve the 160mm version--which delayed introduction of this new hub until later this season.
A few weeks ago at the Taipei show the first 9-speed cassettes were shown. In a year or two, when these new cogsets become de rigueur, the 145mm "new tandem standard" will necessarily become obsolete. Because Santana's 1992 160mm standard, like our 1976 140mm standard, has enough extra strength to easily accommodate an extra cog or two, this proven dimension is the wise choice for today's strength AND tomorrow's compatibility.
The only good thing about the new 145mm hubs (in my opinion) is that if you simply must convert your old tandem frame to 8-speed, 145 is slightly better than 140. You can certainly stretch a steel frame by 5mm without fear of failure. Do NOT stretch an aluminum frame (a Santana AL frame is already 160mm). In addition to the now-available Shimano hub, Hope and Sachs have recently announced 145mm hubs.
If you are ordering a new custom frame, ask your builder to use 160mm spacing (prepare yourself for the sinister rumor about ill-defined chainline problems--tandem customers have been getting this run-around since 1976). In addition to the 160mm dishless 8-speed tandem hubs from Edco, Phil Wood and Shimano, two additional U.S. manufacturers will introduce 160mm tandem hubs in the next few weeks.
Bill McCready; chief hub-nutBack to Tech Tips!