Let me start off by saying that it is NICE to be back from Interbike, and to be spending time on developing some new products again. I had to take a week off from the blog just due to exhaustion and a huge workload, but I'm back at it and I will be getting to some reader questions that have come in over the last couple weeks. Keep the questions coming!!
Through e-mail and at the Interbike tradeshow, I've gotten a few people asking if it is possible to optimize a cross country or all mountain type suspension for use with a single sprocket.
The question usually goes something like this; "if you replaced your triple ring up front with a single sprocket, would that be better or worse for overall suspension performance"?
A couple of months ago I answered a few questions on suspension for Alan Muldoon, of MBR. MBR is a UK mountain biking magazine, one of the best for those of you who have not seen it, and Muldoon is one of the most technically savvy editors that I have ever met. The man is on it. The questions that I answered were published in part in the Summer issue, and also online.
One of the questions covered the question of single sprockets on all mountain bikes almost exactly. Here is what I wrote:
Question: With massive variations in chain angle in relation to the pivot, how can any manufacturer claim to have optimum anything?
DW: This is an awesome question! There are so many variables that come into play here, I love it. I think that I could write a hundred pages on this subject, but I’ll do my best to keep this short.
Bicycles have evolved in an interesting way over time. Over the first hundred or so years, the measurements of wheel diameters, bottom bracket heights, top tube lengths, etc. had just been empirically “figured out” by tinkerers. The geometry that we ride today in the grand scheme of things is not all that far removed from what people were on long ago. Sure, small changes make big differences in feel, but we are talking about really small changes at this point. (No adult is riding 10 inch rear wheels on their mountain bike.) Of course, this empirical geometry development was really just a systematic chain of choices based on what worked for human ergonomics. What felt right lived on, the Darwinian selection of cycling geometry if you will.
As bikes got more complex, gears were added, derailleurs, shifters, this original ergonomic heritage lived on. Drivetrain companies figured out through testing that a 22T front sprocket worked out well for climbing with an 11-32T and later 11-34T cassette. A 32T front sprocket felt good on flat ground with that same cassette range. A 44T front sprocket gave enough push for descending and getting up to speeds that would scare even the hairiest of men.
Then suspension came, and a whole new era of Darwinian selection began for cycling. Some designs were far ahead of their time, some were downright comical. A set of physical elements never before encountered were at work against cycle suspensions. Suspensions bobbed when riders pedalled hard, climbed hills, and did all kinds of other crazy things. Forces like gravity and anti-squat were acting on the suspension systems, yet few if any people in the world understood the how’s and why’s of why suspensions reacted the way they did.
One thing remained the same through this entire time. Drivetrains still used 3 sprockets, 22, 32, 44. It’s my opinion that the cycling public got extremely lucky here, as the variable front chainline is the savior of cycling suspensions.
I will go out on a limb here and say that without the variable front chainline of the front derailleur 22, 32, 44 (or 48 whatever), the suspension bicycle would have died in its infancy. Sounds crazy, I know, but hear me out.
This is going to take just a little bit of physics, but its not going to be hard to follow I swear. Earlier, I mentioned gravity and anti-squat. You can’t have anti-squat without gravity. I’ll explain both briefly here.
Everyone reading this knows the feel of gravity. It’s acting on everyone on the planet Earth right now. Gravitational force is pulling you directly downward at the earth, right toward the core of the planet. When you ride your bike on flat ground, gravity is pulling you toward the earth at a 90 degree angle to the ground. When you ride your bike uphill, say a 10 degree incline, gravity is pulling you toward the earth at a 100 degree angle to the ground. Gravity is still pulling straight through the center of the earth, but the angle of the ground has changed in relation to gravitational force. When you ride your bike downhill, say on a 10 degree decline, gravity is pulling you toward the earth at an 80 degree angle to the ground.
You might be asking yourself, "What exactly is “anti-squat”? Anti squat is a force that balances the effects of mass transfer on the suspension, giving the best possible bump compliance, while at the same time providing excellent energy efficiency. There are two forces that combine to create anti-squat; chain pull and driving force. Chain pull force is multiplied through your rear cogs and wheel as a lever creating driving force. Because of this leverage, driving force is always the greater than chain pull force, but both are significant. If you hear someone talking about “chain pull force” without mentioning “driving force” in the next sentence, there is a good chance that they have a bridge to sell you somewhere.
OK, now for the tie in! The amount of anti-squat that a suspension can develop is based on (among other things) the angle of the ground that the bike is riding on and the angle of the chainline. It just so happens that as a bike is climbing a hill, the amount of anti-squat drops because the direction of gravity in relation to the bike changes. What this means is that if you are pedalling along in your 32-18 on flat ground and have just the right amount of anti-squat, then start to climb a steep hill, say 15 degrees or so, the amount of anti-squat is going to lessen. It just so happens that moving the chainline downward, say like if you selected your 22T cog, increases anti-squat. In an Apollo 13 like turn of events, people actually use their 22T cog when they climb hills as steep as 15 degrees (you basically have to). The two changing anti-squat amounts balance out, leaving the rider with very similar riding characteristics while climbing in the granny and riding on the flat in the middle ring. Amazing, huh? As you may have guessed, the same goes for descending with a larger ring.
Because of this, chainline variability made some very poorly designed suspension bikes that would have otherwise been unrideable at least reasonably useful enough that people eventually tinkered away and arrived at bikes that performed well enough for suspension to become a reality for the masses.
Variable front chainlines are ALWAYS going to be a good thing for mountain bikers who ride their bikes on variable terrains. Without them, suspension bikes might still be considered a bad idea, and I would most likely be riding motocross.
I design for optimization in the middle ring in the flats and light climbs, granny for the big climbs, and big ring for the descents. There is a lot of overlap there.