Does mountain bike geometry cross over?

This may be a repeat or noob question around here but as a mountain biker primarily, I never get to talk road geo with roadies. Given modern day MTB with the long low slack geometry, I get why steep seattube angles work to help people keep their front wheel down on steep climbs but I’ve never bought into the claim of increased power. That being said there do seem to be a few studies that demonstrate increased power with steeper STA. I tried to find evidence to demonstrate increased efficiency or power with a more traditional road sta and couldn’t find it. Given how far mtb geometry has come in the last 20 years and the vastly increased speeds that have gone along with it I’m always surprised that road has seen so little change. Is this due to UCI rules maintaining traditions? Is it a compromise for handling over pure power? Or is a road sta actually better for delivering more power?

I think the answer lies in upper body position which is very different between the two sports.

Combine this with the available range of hip angulation and you have two different sets of useable seat tube angles.

Take this to the extremes and you have the TT / tri bike at one end and the downhill MTB at the other.

Actually road bike geometry has changed dramatically in the last 20 years, so your basic premise doesn’t hold.

I’m not sure that’s true and your examples don’t seem to support your argument: the geos on those two bikes look very similar to me, allowing for the the later bike being a larger frame size and the earlier bike being faster backwards.

I’d guess the STAs are about 74, the HTAs about 73, chainstays about 410 and FCs a bit under 600.

I think you missed the head tube size and configuration, the sloping/wavy rear triangle, the curved fork, and the seat tube/top tube intersection, and only a couple of tubes are actually straight in the modern, compared to all but one in the older one–and, this, was just a random example, not meant to prove any “rule” but simply to refute your premise–many modern frames are much more pronounced in their differences than what I showed here, with compact, curved geometries, very radical headset/cockpit configurations and so forth. I have bikes from both “eras” in both types (MTB and road), and, again, both geometries have evolved significantly, while of course you will always find exceptions.

A large aspect of geometry is also about conditions of the surface the bike will be traveling over. Mtb is a more rough variable surface with the rider constantly standing or hovering over over the saddle. So geo has to take into account how you approach obstacles from a standing position as well vs road where you are primarily seated the entire time. So geo has been optimized around most efficient position there which also includes aerodynamics and drag not just power.

I would disagree that road geometry has changed significantly; we still have seat and head tube angles around 73 degrees etc. Whereas mountain bikes have gone from 73 deg STA to 76 degrees or more, and head tube angles are now in the 65-66 degree range, even for some XC oriented bikes.

I’m referring more to the sta and hta than the specific shape of the tubes, if you read the post. More specifically, I’m curious if the premise that a steeper sta allows a rider to generate more power is something that roadies subscribe to?

I totally understand why mtb has gone the direction it has with sta and hta, but when they start saying that a steep sta allows them to be more efficient and generate more power, I don’t believe it as that hasn’t been my experience. Unfortunately some of them have found some data to back that up.

This is to give a taste for how far mtb has moved the needle with regards to geometry.

There’s no benefit to road bikes of a slacker head tube; road bikes aren’t descending 3:1 slopes through a rock garden. For road handling, you want to be over the front hub and bringing it through corners. Shorter wheelbases are good descending technical mountains with hairpins but also moving around in bunches and avoiding potholes that you see at the last second.

There’s a case for steeper seat tube angles; triathlon has gone that way to “open the hip angle,” and allow for easier transition to the run. The UCI has a rule to keep the nose of the saddle behind the BB, so that is a restriction in STA.

You have to remember, road geo has been evolving slowly for over a century, so it’s been pretty much dialed in by now. The big progression in road bike position has been how much knowledge of aerodynamics has advanced since the 80s. MTB on the other hand is still relatively new, and as recently as the 90s racers were still using their road position as the starting point for their MTB position.

Haha I am not sure a geometron with a dual crown to an old yeti is necessarily a fair comparision. For your geometry though picture a rider sitting on that their upper body is almost vertical up and down. They definitely can generate alot of power through the pedals similar to doing a squat. But imagine them going at over 20mph that body is gonna be acting as a sail slowing them down. TT or triathlon you can minimize and help that by you bar selection. But if your running drop bars it would be hard to and nobody wants a group ride with everybody on tt bars. Aero is where the advances are coming from on the road mtb it’s still geo and suspe sion that are really driving it aero is way less of a concern due to average speeds. Other than in dh but the uci banned the skin suits there

1 Like

None of those factors are relevant to the actual geometry.

I think you have mistaken the geometry of a frame with its shape: in your example the shapes of the tubes have changed, yes, but their geometrical functions have not, or at least not that I can see.

A good case in point is the wavy fork: The fork crown will be pretty much the same distance from the axle as the older fork and the axle position will be offset (raked) by an amount that suits the HTA. What happens between these two points has no effect whatever on the geometry.

1 Like

Some thougths from a MTB perspective here;

  • Steep seat tube angles come as a necessity as the reach became longer. To keep the top tube length short enough. From that we found out that steeper seat tube angles also makes pedaling in steep terrain a lot better. As a long time user of long reach frames, my point of view is that a too steep SA is not something i want for flat and undulating terrain. But as my trails have long accents and descents, it is a good compromise.

  • On the road/gravel bike you dont have to compromise in the same way, since the postion on the bike is the same both up and down.

  • I do love the slack and long geo of modern mtb’s, but i am not a fan on gravelbikes. If you ride in a rough terrain that needs a slack head angle and long bike, i would choose a light 29’er hardtail. Or even a fullsuss, if the roads are really rough.

1 Like