Originally Posted by
Spoonrobot
There's no theory, just experience, measurements and math. Headtubes and steerers are almost 60% greater in diameter, rectangular downtubes/chaninstays and toptubes that are almost the exact same measurement between mtbs and gravel bikes. If you're bored one day, take a measuring tape or caliper to REI or your LBS or whatever and measure the tubes. The wall thickness for almost all aluminum tubing used in bikes is the same - the area of the tube is what matters and where stiffness/compliance comes from. My gravel bike has a 52mmx48mm downtube. This is enormous and is slightly larger than my hardtail from the same year. It's ridiculous.
Bigger tires and lower pressures aren't magic, especially once speed increases. There's no way to make a production bike with an oversized 44mm+ steerer, headtube and fork have the same compliance as a similar bike with regular 1 1/8" components. A big driver for road plus was to get overbuilt rigid mountain bikes posing as gravel bikes back to a comfort level of the previous generation of converted cross bikes/early gravel bikes. When a frame is built stiff enough to act as a pivot point for a suspension fork but is actually a road bike you need 47mm+ tires to get anything approaching acceptable ride quality. And even then it's only in the same area and often not as compliant or comfortable.
There's an old saw about if you're really so into frame/fork compliance ride your bike without tires and see how it matters. The reality is, ride your bike with 23mm tires at 5% sag and see how much it matters. It matters a bunch and the current generation of gravel bikes are a big step backward in terms of ride quality, regardless of whatever marketing and brand iconography we've all internalized.
Keep in mind a big part of the brief for frame/fork design of production bikes is passing european/ISO fatigue testing. It's part of the reason standard tubing went away during the steel years, why 1 1/8" steerers happened and why oversized headtubes are appearing on any bike meant for non-paved riding.
All quite interesting and something that I continue to watch from afar and wonder about. Coming from 80s steel road bikes with traditional tubing and having a gravel bike with steel OS tubing as well as a road bike with steel OS tubing, i see the 44mm head tubes and huge tubing of so many bikes and wonder at what point are the returns diminished. I get that aluminum tubes need to be larger than steel due to material difference, but current steel gravel frames with a huge front end(44mm head tube, gussets, etc) and the similar sized carbon and aluminum frames just seem like overkill. Given the amount of suspension now built into some frames(futureshock stem, isospeed frame, or various rear suspension mechanisms on the seat stay), it seems like frames are overly stiff to the point of needed suspension to then be designed in.
I am 235# and my OS steel frame gravel bike hasnt ever felt noodly. Even though it hasnt felt noodly, would a 44mm head tube and OOS tubing feel better? Or would it be too stiff? Thats something that would be frame and bike specific and would only be answered with extensive riding, which isnt realistic without buying the frame/bike.
My current steel frame design(still subject to change many more times, im sure) is a mix of OS and OOS tubing to hopefully make a slightly stiffer frame than what I have, which still being far smoother than what I would otherwise get if buying something already made.
An OS seat tube, head tube, and top tube matched with an OOS downtube. That downtube would hopefully help offset the oval chainstays i will use since oval bent stay will naturally flex laterally more than the round stays that are crimped on my current bike.
The only settle is that I will use a carbon fork with straight steerer so thatll limit me to a Woundup or Ritchey...and the Woundup has a longer steerer which i will need, so i guess ill go with that. At the same time, to get a different carbon fork would require a 44mm head tube and giant tapered steerer which is all probably more stiffness than I need.
I ride gravel solo and a 15mph average on a 30+ mile route is a really good day. I wont set records and wont ever snap cranks due to Herculean power. Pretty sure a front end and bottom bracket can eventually be too stiff for my benefit.
Anyways, interesting points you bring up. I didnt realize there is someone else who sees the tubing diameters of some frames right now and questions the claims of comfort.