Howdy Escapees! 
I could use some input from geometry nerds, excuse me, aficionados. I just finished design work on a new road+ frame with our factory and will have a prototype in hand around August. I would like to play around making geometry tweaks with the prototype prior to going to production, which for us is still in relatively small batches, so the commitment isn’t massive but still non-trivial. The notion of having multiple prototypes created is a big investment for us. I would like to see how far I can push testing changes with a single prototype test platform.
Changes in headtube angle (HTA) are pretty easy using angle adjust headsets, which can provide 1 and 2 degrees of changes in HTA, and both increases and decreases in HTA. Seat tube angle (STA) is also not that difficult to change using a combination of straight and setback posts. Again, angle can be adjusted to increase and decrease STA. Add in saddle fore and aft adjustments and I can become Lachlan Morton pretty quickly.
I’m less worried about changes revolving around the fork (e.g. trail, fork offset, axle to crown, etc.) as I’m pretty set on the ENVE fork we will use, so I consider this a fixed variable. Stack and reach is also pretty easy to adjust and keep consistent as other changes are made using a combination of stem length and spacers. All good with these things.
What has my brain in knots is the interplay between reach and front center. Obviously I can’t change front center of the frame. I can increase reach, which will move my weight more forward and will result in a fit difference, but what is the impact on handling? And how much does this change mimic an increase in front center? Is there a tight or loose relationship between going from something extreme like a 50mm stem to a 120mm stem and increasing the front center by 70mm? Yes the weight distribution is different but when increasing stem length and not front center, the wheel is still in the same place. Whereas if I were to increase front center the wheel is actually in a different place. Is this difference meaningful to the point of relegating the test invalid? If invalid for a 70mm change, is there an approximate amount or range where one could conduct a viable test?
Hopefully my questioning make sense and I’m not taking crazy pills.
Interesting concept. I thought about it and think changing the reach on the frame could be possible with use of a S&S connectors. I can visualize using multiples to effect different TT lengths. Wonky, no doubt. For the front-center, my memory is a tad hazy but I recall GT had a rigid fork which had horizontal drops. The concept was that you could adjust the front-center/wheelbase for a more relaxed handling when wanted.
You could have such a frame and fork built for what you are trying to do here. But it’s likely best done in steel and would not to have the same feel as a carbon frame and fork. But you would get to play with the geo a bit.
1 Like
Stem length will change weight distribution, bit not wheelbase. Stem length also changes steering response: for a given lateral hand movement, a short stem will give more steerer tube rotation than a long stem.
4 Likes
yes, but it’s not what one may expect. A longer stem can actually end up with more weight on the saddle/rear wheel (except for standing). Steer tube rotation does differs greatly, too, but sometimes less thoigh depending on whether its a short vs long stem paired with a drop bar or flat/riser.
1 Like
I can increase reach, which will move my weight more forward and will result in a fit difference, but what is the impact on handling?
It sounds like in this scenario you are retaining a fairly steep HTA while increasing the FC, so if the stem length stayed the same, I imagine the handling would suffer a bit in a “road +” application. Probably would feel a bit non-linear in terms of the input/change of direction. Shorter stem and bar reach would help that a lot, though.
And how much does this change mimic an increase in front center?
IMO, it doesn’t mimic a longer FC - in fact maybe the opposite? Because essentially you would just be moving the rider closer to the front axle of you keep the FC the same and increased the reach w a longer stem. Whereas if you just increased FC and keep the rider in the same place in space, you would be moving the rider further from the front axle. I just don’t think there is much to feel there, at least with any certainty to where it would affect the geometry design. But, I definitely don’t think increasing reach will “mimic” a longer FC.
Zooming out, I think “Road+” can mean a lot of different things; depending on the tire clearance, you will probably see your customers end up with some stark differences in handling based on their tire and fit choices. I do think you’re on the right track with considering a longer front center for this type of bike.
Keep us updated!
1 Like
I’d think that the bigger the tyre that will fit, the longer the front centre should be, both for toe clearance, and to make sketchy dirt descents less traumatic. At one end you have the “I like quick handling, must-keep-wheelbase-short for road” types, and on the other there are people who like stability and dislike overlap - the two aren’t likely to agree on geometry at all. And then there’s fit, with multiple theories for racing, endurance/recreational, and gravel. There’s no way to satisfy everyone, so IMHO you may as well build what you personally like…
1 Like
IMHO conflating cockpit geo with frame geo is a non sequitur.
4 Likes
@Stochastic and @Scott_Anderson I think you are both hitting on what I worried about, you can’t really mimic FC increase by adjusting reach. By doing this, you’re only changing cockpit/fit geo and not frame/ride/handling geo. There are some changes, e.g. shorter vs longer stem will impact quickness of steering due to lever differences from the bars, but this is not mimicking a change in FC.
@Bruce_Lee Wild idea your notion of using frame couplers and different front halves to test different FC. Pretty fun experiment actually! Maybe one day…
@Stochastic In my case road+ is defined by me for this bike as fitting 38mm and 35mm with fenders. The intent is to stay squarely on the road bike end of the spectrum and not drift into the gravel space (yes I know it wasn’t all that long ago we thought 38mm tires were gravel tires but the game has evolved 
) . Please we already make a drop bar MTB adventure bike thing and I want to create a clearly differentiated product.
Geometry is a bit of an experiment with a more progressive-for-a-road-bike geometry with a steeper STA and slacker HTA. I’m keeping the stack a bit higher in hopes of preserving the all-day comfort needed for the more endurance-oriented riding I do and the audience the bike is targeting (or for people who just want a comfortable bike to ride whenever and for however long they want to ride).
Prototype arrives in early August, so I will have to wait patiently until then to start testing and experimenting, just not experimenting with FC changes.