Been reading the posts about crank wobble with interest. It's something we've spent a lot of time thinking about. For our Rene Herse cranks, we went beyond what is standard in the industry to ensure they run true: The cranks are machined in special custom-made fixtures (one for each crank length, since we use different forgings and the arm thickness also varies with crank length) to make sure the spider arms don't deflect during machining. We're confident the arms run as true as they can, and far better than most cranks.
How do you get a wobble then? Every part has a tolerance – meaning it's close to the value specified in the design, but not quite there. You can reduce the tolerances – which increases the cost – but you cannot eliminate them. On a crank, four factors are the main culprits when there is a wobble: 1) the square taper on the spindle. This is the most important one, because the radius of the spindle is so small. Even a tiny wobble gets translated into a significant wobble at the chainring. Unfortunately, it's also one of the hardest to machine, so it's not uncommon to have issues there. See below how to address this. 2) the crank itself. The Rene Herse crank has to be machined to very high tolerances, because the small BCD translates imperfections into a larger wobble at the ring than you get with a larger BCD. Thanks to the tight manufacturing tolerances, there is no problem here, unless the spider gets bent if the bike falls over. 3) the interface between crank and chainring. Make sure there is no metal shavings, etc., trapped between chainring and spider. All interfaces of the Rene Herse cranks are machined at the same time, to eliminate issues that one chainring tab might be out of alignment with the others. 4) the chainring. As you machine the chainring from a flat sheet, inbuilt stresses are released. If you've worked with wood, you know this – as you carve away, the wood may crack or warp. With aluminum, this is less of an issue, but it still exists. Basically, even a perfectly flat sheet of aluminum will look – very slightly – like a potato chip after machining. If you are lucky, all the tolerances cancel out, and the crank runs super-true. If you are unlucky, the tolerances stack up, and you get a noticeable wobble, even though each part is fine by itself. What can you do in that case? a) Make sure everything is assembled correctly. Grease the spindle to make sure the crank seats perfectly. Tighten the crank bolt enough to seat the crank properly. When in doubt, use more torque. We've yet to see a busted Rene Herse crank or broken crank bolt... (When cranks came loose, it's usually because the bolts weren't tightened enough during the installation.) b) Rotate the cranks 90°, 180° or 270° on the spindle. That way, you can find where the tolerances cancel out. c) Just as the arms can get bent in a fall, you can also true them. Spin the crank and use a marker like a truing stand - you'll get a line where the chainring has runout. Then put a piece of wood on the nearest chainring bolt and hit it with a hammer. You need a good tap to bend the arm. We don't usually recommend this, as you need to be a good mechanic, otherwise, you'll probably make the problem worse. This allows you to adjust for all the tolerances and make the rings run true. If you take off the crank, mark the orientation on the BB spindle, so you mount it the same way. d) The old tool for truing the chainrings (basically a lever with a slot) doesn't work on Rene Herse cranks. The rings are machined from ultra-hard 7075 aluminum for wear resistance. 7075 cannot be bent. It springs back. If you bend it too far, it'll snap – in theory. In practice, you'll have to bend it almost 180° before it breaks. The final question is how much wobble is acceptable. I'd say 3 mm is too much, but if you get 1.5 mm, you'll never notice while riding. Your frame flexes far more than that. That is also why you can't judge the chainring wobble while looking down as you ride. You need to spin the cranks backwards, so there is no load on the chain... Reducing tolerances is mostly a question of money. A jet engine costs upward of $ 10 million because of the extremely tight tolerances. There is a Japanese jet engine parts maker, Gokiso, who makes bicycle hubs. They cost a whopping $ 5000 for a set. I've seen them, and they do spin truer than your average hubs, but I doubt I'd notice the difference on the road. Jan Heine Founder Compass Cycles www.compasscycle.com -- You received this message because you are subscribed to the Google Groups "RBW Owners Bunch" group. To unsubscribe from this group and stop receiving emails from it, send an email to rbw-owners-bunch+unsubscr...@googlegroups.com. To post to this group, send email to rbw-owners-bunch@googlegroups.com. Visit this group at https://groups.google.com/group/rbw-owners-bunch. For more options, visit https://groups.google.com/d/optout.
