Cranks-Part III

[dnovo]

Posting another unusual crank: Mavic SSC crankset ("Road 631" on the box.) Part of a complete Mavic group including the Mavic pedals shown (which will work with the current Look cleats -- thank god or I would really be in trouble) NOS circa 1990 as mounted on my NOS circa 1990 Lemond Team Z frameset (picts of the bike in the Custom gallery.)

[zap]

Really nice cranks with probably the lowest Q factor out their. I have a full mavic zap groupo on my Look Monoblade and mostly Mavic groupo on our tandem including 3 mavic 631 spiders :cool:

By the way, the fully rebuildable Mavic rear derailleur works great with Dura Ace 9 speed shifter, cogs and chain.

[jbay]

zap wrote:
Really nice cranks with probably the lowest Q factor out their. I have a full mavic zap groupo on my Look Monoblade and mostly Mavic groupo on our tandem including 3 mavic 631 spiders

Zap,

Are you using single side drive on your tandem or are these cranks heliocoiled? Mavic never made any cranks for crossover drive tandems.

-- John

[BigMac]

If "Zap" is using 3 spiders on a tandem, it must be a cross drive, ie. left-side synchronize drive. This crank would not really work witha single side setup due to it being a racing double (130 BCD) and the unique 'ring attachment which prohibits attachment of synchronizing ring outboard of largest drivering position. I suppose one could set it up with a single drive chainring, use the normal outer as a synch ring and limit gear selection to rear cluster...hey that would actually work with a Rohloff Speedhub setup, no? Nice low Q to boot.

Given that "Zap" has chosen a traditional cross drive setup using road cranks, I suspect he/she simply used Loctite thread compound on left side and front right pedals. Using double-sided mtb clipless or most mtb caged/platform pedals its a pretty easy task. Single-sided road pedals of course require axle swapping which frankly makes the added expense of a tandem crankset worth the added investment. If using Loctite, just recheck that pedals are securely attached before each ride. The added hassles and cost of Helicoil obliviates any savings of using std. road cranks.

Frankly, while the Mavic 631 and its model 635 stablemate (110/74 mtb triple of similar design) are much lower q than Campy, Shimano or Suntour of that era -- today's cranks are often even higher q, especially the Shimano models -- the Ritchey Road Logic and Mountain Logic of same era was 7-8mm lower q than Mavic. Ritchey also used a more traditional ring attachment setup allowing a far easier single-side tandem drivtrain setup.

One should also be careful with the early generation 631 cranks, they were notorious for fractures at spider-arm junction. This was corrected within a year of the units introduction and is easily corrected on the early models with a diligent usage of a round file. Look at inside transition at spider-srm, if the transition features a sharp edge, this is an area prone to stress riser which will lead to said failure. Round edge with 1/2" dia round file.

FWIW: I really liked the accompaying Mavic BB. In an era which saw British, Italian, French and even the occasional Swiss BB threading, none of which were completely compatible (Fr-Eng were similar) Mavic designed a BB which literally removed the 1st few threads of the BB shell and could be fitted into any frame, regardless of origin or OEM threading. It was nicely sealed and very durable as well.

Ride on!

[jbay]

BigMac very correctly pointed out:
If "Zap" is using 3 spiders on a tandem, it must be a cross drive, ie. left-side synchronize drive.


Thank you, BigMac. I was apparently doting when I asked my question.


This crank would not really work witha single side setup due to it being a racing double (130 BCD) and the unique 'ring attachment which prohibits attachment of synchronizing ring outboard of largest drivering position.


I was thinking that you could use the triple adapter and mount the timing ring (with spacers) on the inner ring on the rear crank. However, I also forgot that the original 631s limited the size of the smallest 'big' chainring to 49T, so this wouldn't work well with the front crank. Silly me!


I suppose one could set it up with a single drive chainring, use the normal outer as a synch ring and limit gear selection to rear cluster...hey that would actually work with a Rohloff Speedhub setup, no? Nice low Q to boot.


Nice idea alright and one of the nice features of using a Rohloff on a tandem. However, thinking about it a little more, the 631s probably still present at least one obstacle for running single side drive - namely, the length of the spigot that the chainrings sit on. The two chainrings would need to be spaced apart so that two chains can clear each other. This would mean that the inner chainring, be it the timing ring or the drive ring, would not be sitting on that spigot. There are probably ways and means around that though...


Given that "Zap" has chosen a traditional cross drive setup using road cranks, I suspect he/she simply used Loctite thread compound on left side and front right pedals. Using double-sided mtb clipless or most mtb caged/platform pedals its a pretty easy task. Single-sided road pedals of course require axle swapping which frankly makes the added expense of a tandem crankset worth the added investment. If using Loctite, just recheck that pedals are securely attached before each ride. The added hassles and cost of Helicoil obliviates any savings of using std. road cranks.


I suspect that you are correct. Using Heliocoils also increases the risk of crank breakage, and nothing is worth that IMO.

I was interrupted during my original reply and cut it short, as I was going to add that I have what I believe are the only genuine, bona fide 631 tandem (crossever) cranks in existence. Peter White (of the eponymous Peter White Cycles) was kind and resourceful enough to track down three unthreaded 631 cranks for me (two spiders and one non-drive crank) and get them threaded in a machine shop in the correct orientation for crossover tandem use.

The missus and I got many miles of enjoyment from their utility and beauty before a crash (which was out of our control) bent one of them. What a pity!


Frankly, while the Mavic 631 and its model 635 stablemate (110/74 mtb triple of similar design) are much lower q than Campy, Shimano or Suntour of that era -- today's cranks are often even higher q, especially the Shimano models -- the Ritchey Road Logic and Mountain Logic of same era was 7-8mm lower q than Mavic. Ritchey also used a more traditional ring attachment setup allowing a far easier single-side tandem drivtrain setup.


Are you sure about the q-factor figures, BigMac? I'm a q-factor minimalist (as it sounds like you are!) and currently have the Ritcheys on two tandems (in minimal q-factor mode, while retaining proper drive chainline) and the q-factor figures are as near the Mavic as dammit. The same holds true (for me) with Ritcheys and Mavics on solo bikes.



One should also be careful with the early generation 631 cranks, they were notorious for fractures at spider-arm junction. [...]


I had a 637 (I believe that this was the model number of the 110/74 triple) crack in this area. Another pity...



FWIW: I really liked the accompaying Mavic BB. [...]


Yes, it's very nice alright and I have a stash of them. That said, I have a friend who broke a spindle in one, so that tempered my enthusiasm for them a little.

-- John

[BigMac]

jbay:

Truth be told, I'm not a tandem guy so I don't have a great deal of experience with said drivetrains. I however beleive a 3/32" pitch chain should offer sufficient clearence using a standard road crank setup like the 631 with outer synchronize and inner ring drive chains. If not, Shimano does offer a 1mm thick spacer/washer machined specifically to fit ring bolts that should provide the additional clearence as needed. I'm sure your local hardware store would probably have such an item readily in stock for a few pennies as well.

The q for the 631 setup of a modern 43.5mm chainline is typically 135mm. The Ritchey Road Logic is typically 127mm. Both of these are significantly better than the current Record/Chorus which will net 148mm on average or the DA which is an incredible 155mm. Heck, the Ritchey and Mavic triples -- thank you for correcting Mavic model number as I honestly did not recall exact number other than 630-something -- are lower q than the Shimano double and equal to Campy double. I'm a definite low q advocate, unfortunately it appears not a single current crank vendor sees this as a viable concern or entity. The Ritchey version I am referring to was from mid-late 80's, it was produced by Sugino based on Tom's design. Cold-forged in silver only with large black script "RitcheyLogic" logo on each arm. Lines were sharp and angular, a bit like SR/NR w/o the cutouts or like the last generation of DA cranks.

Ride on!

[jbay]

BigMac, my fellow low-q factor fan,

For a non-tandem guy, you're holding your own on this tandem drivetrain stuff! Given the desire, I too am sure that you can can single-side drive to work with the 631s, but I probably won't be trying it any time soon.

Regarding the exact q-factor numbers, I will have to take your word for them at the moment, as I'm getting ready to move and most of my bikes are stored away. Regardless, your point about the q-factor of the old _triple_ Ritchey and Mavic cranks being lower than or equal to current _doubles_ is both valid and head-scratchingly troublesome!

The Ritcheys were fantastic cranks and if that overly large logo bothers you - like it did me - a little paint stripper will soon take care of it!

On yer bike,
John

[dnovo]

Thanks for the advice on the stress riser issue, Big Mac. I will pop the crank off and take a look -- and will grab that file if needed. Dave N.

[zap]

My wife and I ordered our tandem equiped with (mostly) Mavic Groupo from CoMotion back in 1992. It's possible that they had it helicoiled but if I recall, CoMotion did have them machined. CoMotion sold several tandems in '91-'92 with Mavic groupo. Six years ago we (I) had a pedal come loose by several threads on one crank and it did strip the pedal threads in the crank when we hammered up a hill. Had my mechanic helicoil that one but we are still using the cranks (knock on wood) and they survived a pretty decent crash 2 years ago.

We spec'd the tandem with Phil Wood bottom brackets because I didn't care to have the bb shell machined to Mavic's specs. Original bottom brackets are still in use as well.

Our setup is a cross drive with 48 tooth timing rings (again, original). It's big but improves efficiency and as you can see, lasts a long time. On the drive side we are using 39/55 combo, 12/25 rear. We did replace the 55 Mavic ring 3-4 years ago with a Dura Ace 55 TT ring so that it would match the Mavic spider arms.

An adaptor was available to run a small 3rd ring on the 631 cranks and we did get one from a friend 2 years ago. Haven't used it yet but figured we would later when we are old and slow and want to climb up to the Blue Ridge Parkway.

The following are some q's I found*.

Mavic 631 road 134mm
Mavic 637 mtb 134mm
Campy Record 150mm
Shimano 7410 149mm
Ritchey Logic Road 142mm
Ritchey Logic MTB 145mm

*Source "The Ultimate Bicycle Source 1994"

Lower Q is better. Shimano has their head in the sand in this regard.

[bubba]

Looks cool.Boat anchor heavy.