In road biking, titanium and steel have a legendary reputation for “compliance” and comfort. Why? What does that mean? Why can’t aluminum or carbon frames provide this? The answers may be simpler than you think.
TL; DR: Smaller tubing geometry allows steel and titanium to optimize directional flex, providing better tracking, cornering, and endurance.
First, what do we mean by compliance, and why not just call it flex? The simple answer is marketing. Flex sounds bad (we’ve all ridden flexy wheelsets), while compliance sounds good! Flex can be both good and bad, so generally compliance is used to mean desirable flex. We want our bikes to flex in certain directions in a controlled, defined distance, and anything outside of that is bad. This is best felt on a hard tail or road bike, where this is no suspension. However, the flex that frames provide is not in the vertical direction. Even steel hard tails (where compliance is the main feature) have very little vertical flex. The “double diamond” tube structure of nearly all frames prevents any meaningful vertical compliance. The difference, therefore, between a stiff and compliant frame comes down to lateral and torsional flex.
Lateral compliance has two benefits: line tracking and off camber cornering. Imagine you’re flying into a rock garden at speed. You have to pick the smoothest line through the section to maintain momentum. A very stiff frame will be very precise, and the front wheel will track exactly where you steer it. But do you really want that? Would it not be better to have some compliance, laterally, for your wheels? This gives leeway in your line choice. Think about it, allowing your tire to deflect left and right will cause the tire to find the smoothest track- imagine a fish swimming in rapids, snaking its way through the smoothest choice. The fastest reaction time in a human ever recorded is about 1/10 of a second. The average rider’s reaction time is probably closer to 1/5 of a second. If you’re riding through this rock garden at 20mph, thats nine meters a second. In 1/5 of a second, you travel nearly 2 meters. By the time you feel a rock you’re already two meters past it. Reacting to every rock and rut is simply not possible when moving that fast. Having some lateral suspension gives you the same benefit as having vertical suspension- it allows the bike to conform to the terrain faster than a human can react, keeping the chassis stable, increasing traction, and allowing tires to move around obstacles instead of getting hung up on them. Rough terrain is not two dimensional, its three dimensional with obstacles coming at your tires from multiple directions, not just from below.
The second benefit is off camber cornering. An “off camber” turn is any turn that isn’t dramatically bermed, where your side knobs are engaged and the bike experiences meaningful / lean / . Suspension is vertical, to allow the tire to move up and down to conform to the terrain. In a turn, your bike is far from vertical. It leans over, and now your suspension no longer travels vertically. Braking bumps, roots, and rocks no longer hit your tires at the optimum angle for your suspension to work. In the late ’00s, when carbon rims were entering the mountain bike market, reviewers and riders found themselves slower than on their aluminum rims because the first generation of high end carbon hoops were so dang stiff that 26″ wheels couldn’t conform to the ground. Lateral compliance allows for a dramatically leaned over bike to work with the suspension to increase grip and decrease deflection & feedback to the rider. Finally, this reduces rider fatigue, along with the increase in performance.
How much flex is too much? In the 90s, when mountain bikes were still in their infancy, there were some really flexy frames out there. Single wall aluminum wheelsets were also super flexy. To put it bluntly, those bikes rode like noodles. It was terrible. Its one thing to have some forgiveness in line choice, its another thing to have your handlebars file away your steering input under “suggestions”. Flex in a frame has to be tuned. Optimized. Controlled. Too much flex not only decreases performance, it can hurt pedaling efficiency and increase wear & tear on components. So how is flex tuned and controlled? The simple answer is tubing geometry.
For bikes, the #1 factor influencing frame flex is local tubing geometry. A larger diameter tube will flex less; its pretty intuitive. There are large differences between materials (steel, aluminum, titanium, carbon fiber), but different materials are tied to different tubing sizing because of their various innate characteristics. This is why steel, the stiffest of any material, produces the most compliant, flexiest frames. Steel is very dense and heavy, so weight constraints require very small tubing. Aluminum is brittle and soft, so it requires relatively large tubing compared to steel, even though its effectively lighter and much less dense. Titanium sits right between these to materials in density, allowing for the perfect goldilocks zone of performance. Steel frames often feel amazing in rough, naturally cut trail settings, but are too vague and dead for high speed berms, pumpy trails, and bike parts where riders want more pop out of their bikes. Titanium is light enough to have larger tubing, but strong enough to keep the tube diameter smaller than aluminum, finding the perfect balance between the two materials. This allows for a compliant, softer ride (especially when leaned over) but preserves some pop for more flowy, shaped trails.