roverpig is correct – I just found the email from Cy at Cotic. Copied and pasted the appropriate bit below:
Question:
iv) Can you educate me on Bottom Bracket height (there are no doubt myths around this). I’ve see a lot of emphasis placed on the benefits of low BBs lately.
Me:
This is getting to be one of my hobby-horses because we are now being accused of having BB’s that are high when 3 years ago the original Rocket was consider ‘slammed’ and pretty extreme. Yes, there are bikes that are going lower on the BB, and by quite some margin, but it’s a matter of what and where they’re being used, and how their suspension works. On groomed bike parks or even steep, rough ground where gravity keeps you going, a low BB is fine. Generally a lower BB makes cornering easier so it’s worth aiming as low as you can. However, I (and I suspect most people) like to pedal across rough ground, and particularly around here there are rocky, knobbly climbs which need some ground clearance to enable you to climb them. And I don’t buy the whole ‘you could walk faster’ crap. I never have, but especially around here and also on flatter, rootier terrain where you need to pedal to keep your speed up, walking is definitely not faster, it’s much slower. And I don’t like walking, I like riding! I have a prototype at the moment which is 5mm lower on the BB than the current MAX bikes and I can REALLY tell the difference. On my benchmark climb I catch my pedals more and often get stopped by a mis-timed pedal stroke. I tried it with the team lads and they immediately noticed as well, as they like to pedal and keep speed across really quite rough terrain. Low BB isn’t the be all and end all. Like anything with MTB, there’s a huge variety of terrain and rider out there and we pitch our frame layouts according to what we want to ride and what works for us. We hope in striking that compromise there will be other people who see the same as we do. The other thing to consider is that static BB heights are only half the story. Our bikes are quite progressive so they’re designed to run with 30% sag without feeling too soft or blowing through their travel. And because of the progressive nature of the spring curve that means they sit down a fair way from the static height with rider on board. Some of these super low BB bikes might need 20 or 25% sag dialed in to give them support if they have a more linear spring rate. For example, although it’s 140mm travel, the RocketMAX sags 50mm at the rear wheel when setup correctly. At 25% sag it would only sag 39mm at the rear wheel (the frame rate being progressive means it’s not a directly proportional relationship), which would lift the BB about 7mm for the same fork sag. It’s not recent experience, bit I know from riding Orange and Specialized bikes in the past that I have needed to run them stiffer with less sag than I can get away with on the droplink setup.