#twentysevenfive is dead and Modern Forward Geometry

In 2017, I chanced across an interesting article #twentysevenfive is dead in the MTBR forum, which leads me to delve further in the R&D of Modern Bike Geometries.

Here’s a great starting article on Forward Geometry from Peter Verdone.  In it, he described:

This isn’t a revolutionary development. It’s evolutionary. Mountain bikes have been evolving and changing as we break from what we thought we knew into what really works or just works better. In the early days of mountain bike development, bike designs were informed by road bikes and their shape reflected that. Then, mountain bikes were essentially road bikes with fat tires and flat bars. Today, we arrived at a true dirt bike, after 30+ years of racing and engineering, data and competition lead the charge for change. Still, we need to continue to move past what we think we know to get to someplace better. In 10 years, things will look still different but we have made a lot of progress for the right reasons most recently.

So, you’ve been asking…what the hell is Forward Geometry?

This is my vision after putting considerable time into the subject. The use of this design strategy is informed by and depends on the use of a dropper seatpost. Without that, it doesn’t work as well as it could. As no modern offroad bike is used without a dropper seatpost, this is fine. Even urban applications require drop seats. 150mm droppers should be at least standard but 185mm preferred. A post with 125mm of drop or less is a considerable problem. 100mm is almost useless. Any bike used on dirt must be spec’d with a dropper.

The bike interacts with the ground by way of the tires. The rider interacts with the bike by way of the pedals, hand grips, and, sometimes, the saddle. The center of mass (COM) in this system determines how forces are distributed.

For a bike that is used in the dirt (MTB or Hybrid), we want the front wheel as far ahead of the rider, COM, as we can get (in general).  The center of mass sits somewhere over and ahead of the bottom bracket. The further ahead the front wheel is from the COM, the less a sharp edge or quick bump will disrupt the COM as the front wheel goes over it. The angle of attack is much lower so the chances of going over the bars is also much lower. This means that the bike rolls over the ground instead of getting interrupted by it. This translates into less loss of speed and control and safety. The rear wheel matters little from this perspective.

The rear wheel of the bike will be close to the COM, but not too close. The rear wheel and the COM are coupled in a more constrained way than the front wheel. If the rear wheel moves too close to the COM, the bike will plow or wheelie when seated climbing. If it’s too far away, the front wheel becomes extremely hard to lift and climbing traction suffers. The rear wheel should end up just as close as it can be to the COM while still allowing for comfortable stable seated climbing in most places. Rider testing is very important here as the rider’s height and weight will dramatically change the rear end. A short light rider can have a bike with a much closer relationship between the crank spindle and the rear contact than a taller heavier rider can have. Obviously, chain stay length should be changing through a size range from any manufacturer.

The bottom bracket of the bike should be as low as possible for maximum control climbing and descending. Pedal strikes will happen, sometimes often. What we don’t want is pedal strikes that are disruptive or to happen unexpectedly. If the bottom bracket is low, climbing and descending are improved, the bike is stickier. If the bottom bracket is high, standing up onto the rear wheel becomes easier.

The placement of the rider on the bike now is the issue. The fit upon the bike is dictated by the arm strength and arm endurance of the rider. A bike that will be raced for two hours will have handgrips far more forward and lower than a bike that will be ridden all day. The more forward and low the grips, the better the bike will climb. Period. Moving the hand grips rearward will take weight off the arms when seated but will decrease climbing performance and shift weight rearward. The grips should be low so that out of the saddle pedaling ergonomics are optimized. The bars can move upward some from that depending on back pain or how aggressive the bike is going down the hill, but it won’t be very much. The change in front wheel placement allows a much more forward position for descending that on old designs.

Let’s assume that the hand grips are optimally placed ahead of the pedals so that the rider is able to climb hard and isn’t running out of arm or feeling too much back pain. As said earlier, we want the front wheel far ahead of the COM. The first thing we do is slide the steerer tube up against the handlebars. For extreme customs, a 35mm stem will do this. For a more open fit, a 50mm stem will do this. but that’s not all you can do. For a mountain bike we can change the head a bit, 65 degrees is about as much head angle as I’ve found good before things get worse, and that assumes a stout fork. For hybrids, 67 may be more reasonable. For the Concorde I ended up with about 820mm of front center. For riding in the most challenging trails in Marin, that’s about the limit for length (with a 29er wheel) that doesn’t get me into too much trouble in the tight stuff. Overly slack head angles become a real problem due to fork sliders binding when used in normal terrain and should be reserved for super high speed bikes hitting rocks hard and downhill. Slack is a bad thing unless it’s applied correctly. Personally, I’d love to have a 67 degree head angle on my bike but I’m making a compromise there to get the wheel up front more. This whole ‘slack’ thing people talk about is totally uninformed.

So, our hand grips are set, we place the seat. Most of us know what kind of cockpit works for us (how much we can stretch out) and we use that. Essentially, we want the arms at about 90 degrees from the torso when seated on the bike. Knees bend about 150 degrees at full extention. I’ve found that a real seat angle (since modern mountain bikes will have bent seat tubes, center of bb to center of seat rail clamp) of about 74 to 74.5 degrees is a good place to start with and steeper for more aggressive performance oriented bikes. Young racer types and stretch out more and hold more weight for longer. This steeper seat position also helps getting out of the saddle as we do a lot offroad.

Now the rear wheel can be placed. Often, testing is required for this as I haven’d done a formal study on the locations of the mass to the wheels. Like I said, good climbing without plowing. 420-425mm of chainstay is pretty good. Getting shorter than that will probably turn into problems unless running a very steep seat angle or the rider is small. If you aren’t climbing on the bike, you don’t have to worry as much about going too short.

What are the advantages?

A modern bike like this is ridden differently than old fashioned bikes. Jumping right on to one, it will feel very different, even strange. It takes some time to get used to. It will feel long as hell for the first ride. The steering may feel slow. You won’t get around the corner you thought you would. You may feel off balance. It may still strange on the 5th ride. Slowly, as you change how you work the bike and learn what to do, it will feel right. Remember, you are changing years of learned response. It will feel strange at first but once you try going backward, you’ll immediately ‘get it’. From then on, you wont go back. It’s awesome.

The action on the bike takes place out of the saddle with it dropped out of the way. Instead of having the handlebars high and back to allow the rider to negotiate getting their ass behind the saddle, the handlebars are low and forward and the riders ass floats above the saddle. The rider is standing on the pedals, like on a DH bike and bending knees like a jockey to smooth out the flow. With a 170mm dropper, not much matters about where the rear wheel is or what it’s doing.The bike will be solid and stable at speed.

Cornering confidence goes up as more weight can be put on the front wheel safely. Generally, the front wheel has a lot of traction with the shift in bias. This is a popular misconception about the longer bikes. You actually end up with more weight over the front wheels, not less.

Climbing! The advantages are instantly obvious; bio-mechanics that allow for real power to be delivered to the bike in a comfortable way. Seated, the front wheel has weight on it allowing it to track well but in a way that doesn’t turn into a disaster when hitting rock or root hard. The low, forward bars feel good normal to gravity when the bike is tilted up with the surface of the climb so it feels right. Out of the saddle climbing is an expression of pure power.

I like to describe the philosophy as riding a downhill bike in a cross country position.

What are the disadvantages?

A forward bike is going to feel worst riding on flat terrain. It’s going to feel like there is too much weight on the arms unless you are hammering hard full XC. That just goes with the territory and XC racers would be the ones to notice this the least. The good thing is, for most real mountain biking, you go up and you go down with not much in between. This isn’t a deal breaker and a bike can certainly be made that feels good on flat, taking some advantage of a wheel shift, it’s just that you won’t be getting the serious magic that happens elsewhere as a cost.

Very tight, very steep downhill switchbacks are an issue that I notice. I’m talking seriously steep, super tight, hairpin switchbacks. Not the casual or normal hard corner. I’m talking steep and tight, hang off the back of the bike steep. Anyway, this gives rise to the pro-enduro style of nose pivoting corners that you see a lot of in the movies. This is just a function of the length of the bike and how things need to get done. If you aren’t all gnar and this concerns you, don’t be concerned. It’s an issue for folks determined to go hard and they’ll find a way. If you couldn’t ride that section before, you probably still can’t ride it.

Jumping and liftoff. Being set forward, it’s not easier to lift off the ground. It takes a bit more work. Once in the air, you’re a bit forward. This isn’t the set-up for dirt jumpers or freestylers. This setup is for hauling ass and crushing rocks.

In the end, your arms are the biggest disadvantage to all this. If they were stronger.. if they could last longer…your bike would go faster.


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