merlinextraligh

So the left crank arm on the captain’s crank cracked on our Dragonfly.




not really blaming Lightning, I’m pretty sure it happened when I dropped the bike getting it off the rack, and the pedal struck the ground first. Prior to that we did get 7 years of use out of it.

Problem is now, I’m having a hard time finding just the Captains crank. Lightning is checking their inventory, but based our conversation, I think I’m looking at a whole new crank, and several months wait.

And the alternatives, all appear to be heavier, and would require changing both cranksets. So I think Lightning cranks have a nice weight advantage, but given that they’re on the far end of the weight/durability curve, you pay both an initial price in the purchase, and potentially down the road on durability, and parts availability.

Chilepines

FSA SL-K carbon cranks are readily available for $975. I'd be surprised if they are much heavier than your Lightnings. I'm not sure of compatibility with your Gates belt drive, though.

merlinextraligh

That might end up being the answer. However, since I posted, Tim at Lightning has offered to fabricate a new left arm to replace the broken one.

it will take awhile, but I think I’m going to wait and stick with the lLightnings for a couple of reasons.

The Lightning cranks are absurdly light. According to published weights the entire Lightning crankset, captain, stoker and bb’s comes in at 832 grams. The SLK’s are a surprisingly porky 1795 grams. So I just can’t bring myself to add over 2 pounds to what was pretty much a complete weight weenie build.

We have the SLK’s on another tandem, and they’re fine, however I don’t particularly like the wide Q factor, and the new bolt pattern would require new rings and belt for the timing belt belt.

merlinextraligh

In fairness to Lightning, I think it’s likely that heavier crank would not have cracked the way this one did. However, imho, I think that’s part of the trade off when you go this far out on making something extremely light.

We’re a larger team and have raced the bike competitively, so we’ve put a substantial load on the cranks, and they worked fine till I banged a pedal into the driveway.

Also, the fact that Lightning is working with us to fix it makes me fee better about them.

Thus, I’d still recommend Lightning if you want a very light build. Just recognizing, the trade off includes not only the initial cost, but also, likely some degree in durability.

Chilepines

I'm going to guess that the Lightning weight does not include chainrings, and possibly not spiders, given their design. And that the SL-K includes rings and spiders, given that they are sold as a complete set. I'll pull the front right crank arm off this weekend to see what it weighs without ring or spider.

reburns

I wonder if it could be repaired. I’ve heard that Calfee will repair CF frames. Maybe some CF specialty shops as well. Might not save $ or time, but could be worth a couple of calls.

Chilepines

FSA SL-K crank arms are 192 gm each. Full tandem crankset w/o BB is 1244 gm

merlinextraligh

Source? My past experience with FSA ius that they are notorious for lying about their weights.

Co-Motion on their website puts the weight much higher than that. https://co-motion.com/collections/st...-tandem-cranks

Chilepines

Source was me weighing mine. Although I did not include the spindles and presumably Co-Motion included them.

bwebel

How did you weigh the SL-K cranks without the spindle? Aren't the spindles attached?

Chilepines

Spindle is attached to one arm, but removable from both. So I just pulled the front right arm - they are all identical except for pedal thread direction. The spiders are easily removable with the right lockring tool - and I had already switched out the 50/34 on a spider they came with to 46/30 direct mount rings.

merlinextraligh

Our Lightning captain’s crank, including spindle and spider comes in at 382 grams.

So virtually the same weight as just the arms of the SLK. I would imagine, the spindle and spider together would have to add at least 150 grams.

merlinextraligh

based upon the 382 gram weight of captain’s crank, I’d bet 832 is within 50 grams or so without chainrings or bbs, given that the stoker’s s crank would only add one spider to the weight of the captain’s.

However, Lightning’s website claims that 832 with bb’s which I don’t see, if the weight of our captain’s crank is representative.

Chilepines

My SL-K crank spindle is on the bike but I weighed the spider at 67 grams. I have a White Industries crank with nearly identical spindle which weighs 100 gm. So the extra weight of the SL-K compared to the Lightning is 170 gm for each crank set, so 340 gm for the set. Way less of a difference than the 1 kg difference we started at!

Chilepines

I've never been able to figure out how that Lightning spindle works. Is the notched spindle aluminum? It looks very beefy compared to most current spindles. Does the threaded rod get tightened from the outside of the crank arm? Is the threaded piece steel? I'd think it has to be but then the weight would be difficult to achieve.

TooMany

I have the Lightning Cranks on my tandem. The threaded bolt is tightened from the non-drive side (left side of the bike, gets a bit weird on a crossover tandem) with a long 6mm allen key - you can see the whole that the allen key goes into in Merlin's original picture. Campy ultra-torque (or whatever they call it now) cranksets are constructed in a similar way, but secured with a 10mm (I think) instead, so the hole in the crank is much larger. The half spindles are aluminum and the bolt that joins them is titanium.

Chris_W

Research Hirth Joint, here's the Wikipedia article. The bolts on some cranks like this are titanium to save weight instead of using steel. The axle is definitely alumnum.

twocicle

Not to hijack this Lightning crank thread, but I am curious what the q-factor is for the "newer" EVO386 based SL-K tandem set. I'm guessing it's not super narrow given that the set supports 68 and 73mm BBs. Can you report?

Also, with the removable spindles, I would hope for other spindle lengths available as well since EVO386 is suppose to work with many other (non-BSA) BBs too.

If 46/30 suits your purpose, then great. What is your typical rear cassette with those chainrings?

merlinextraligh

Just to follow up. Lightning is sending us a new crank arm for $300, given that it is out of warranty. Not cheap, but cheaper than other options.

Chilepines

Finally got around to checking - I got 175 mm Q factor by my measurement on the current FSA SL-K tandem crank.