I wasn't sure where to post this but as it has to do with frame geometry and handling, Framebuilders seems a reasonable shot at it.

I have an "all-road" do everything bike that I put together around an All-City Space Horse Disc frame. I love this bike! I use 42mm Soma Supple Vitesse tires on HPlusSon The Hydra 700c wheels. The head tube angle is 72 degrees.

The bike has pretty high trail with the tall tires and a 47mm offset fork (about 66 - 67 mm of trail). It can take a bit of effort to quickly steer around road debris, but otherwise the high trail hasn't bothered me. However, I now have a porteur rack (Soma Deluxe) that I plan to use occasionally with a heavy load (25+ pounds). I think the high trail could become an issue when used in that way. I had first thought of getting the fork bent to add more offset, but then had the idea that I could just get another front wheel in 650B. Since I have disc brakes this should be an easy swap to use either front wheel, putting the smaller one on whenever I put the porteur rack on. It looks like this would get me down to about 54mm of trail (since lowering the front will bring the head tube angle up to about 73 degrees).

Has anyone here ever tried this? I wonder if I will feel the need to adjust the seat or bars when using the smaller wheel (I hope not). Interestingly, my feet-to-seat-to-hands positioning will not be affected, but on a pure flat my position will be the same as on a slight downhill grade with the standard wheelset.

Thoughts?

Anyone have an opinion on this? Good idea? Bad idea? How much of a change to handling might I expect? The new front wheel will cost about $400 when all is said and done, and I'd like to get some feedback before putting that kind of money into it. Should I have posted this in a different sub-forum?

FWIW, the HPlusSon Hydra is only available in 700C, so I was going to go with a HED Belgium Plus rim in 650B. I will use the same hub and disc rotor as my 700C wheel (Hope Pro 4 hub and Hope floating rotor) so the caliper shouldn't require adjustment when swapping wheels. The Soma Supple Vitesse tires are also 700C only, so I plan to use a Compass Babyshoe Pass on the 650B. The Supple Vitesse use tubes, but I'll go tubeless with the Babyshoe Pass so I don't have to carry two different size tubes with me.

I had composed a reply but it either got lost in the either or I hit a wrong button and poof, it's gone...

Sure people have swapped out wheel sizes on bike s before. The current "craze" is to turn 1970s sort of touring bikes into sort of rando bikes with a 700c to 650b swap. Some work out well. But all seem to do both wheels.

There's no reason why only the front can't be done. But here's some thoughts. Expect to need to do some front caliper tweaking when changing out wheels. Sure you will hope for the same rotor/caliper relationship but the tolerances are so tiny. Even with the same wheel reinstalled there is a need to tweak sometimes. The seat set back (WRT the BB) changes as does the seat/bar relative heights. Some can be sensitive to these things, others seem blind to them. It would be quite noticed by me. As you move the seat back on it's rails to maintain the original set back you increase the reach to the bars too. Oh and the seat top angle changes.

Otherwise the steering will result in a bit steeper head angle and a reduction of trail. Whether you like this you won't know till you do the swap and any other fitting changes needed to get comfy on the bike. Only then will you be able to discern the handling changes. benefits or detractments.

If you could borrow a 650b ft wheel, it wouldn't need a disk even, to try it might give you a lot of information before spending the $. If you do this and before you start playing with seat/bars locations do document their current places. Just so you can go back should you want to without starting all over on the fit. Andy

What do you mean when you say both "I think it's kind of pointless" and "This is a huge positive with respect to handling"? The point of doing it is to have that positive impact on handling when carrying a relatively large from load.

I'm not sure it's really the trail in isolation that affects handling with a loaded rack. If you have a steep head angle and a 47mm rake, the load is going to be way out front of the tire contact patch. Contrast that to a 65mm rake fork that actually puts the tire contact patch under the center of the load. I suspect that with identical trail numbers, the fork with the longer rake is going to produce better handling.

I presume there are some people who change back and forth between a narrow road tire on 700c, & a wide cross tire on 650b.

However, you are using FAT 700c, and FAT 650b. If it was me, I'd plan on leaving the rack and 650b wheel on the bike 100% of the time. Or perhaps remove the rack for special events, but still leave the 650b wheel on. You could even get a shorter fork if you wish.

One issue is incompatible spares. However, you might be able to work around that. I put a 700c tube into my Bike Friday 20" (451) wheel to fix an unpatchable flat a while ago, and haven't changed back since.

I agree with others, find a cheap way to test your setup.

Check out a couple of local bike co-ops for a spare wheel and/or tire.

Quick Release?
https://www.ebay.com/itm/Weinmann-XM...e/282510787598

his idea is to decrease trail by increasing the head angle. It would be an interesting experiment, but I don't think I would want to invest money in it because my thought is it will not do what he wants.

Oh I meant there's no reason to swap back and forth, pick one and just go with that. The difference isn't massive enough that the headaches from two different wheel/tire/tube sizes is worth it. Also factor in caliper/rotor centering, saddle/bar adjustment, having an odd size orphan wheel just around your home, and it's kinda pointless IMO.

I'm not sure I'd consider a 25 pound load to be a heavy load, although some racks seem to have awfully light weight ratings.

Perhaps the first step is to simply install your rack on the bike as-is. How is the tire clearance?

That will give you a good baseline. You can make other adjustments such as wider handlebars, but most of the changes will likely give you minimal results.

my current road bike has high trail and I have a rando bag on it. It works fine, but I keep intending on building a different fork with more rake. It really depends on how sensitive you are to heavy steering. I'm not that sensitive. Sometimes I have 2 full water bottles in the front bag and that isn't bad enough that it stops me from doing it.

25 pounds can be a significant load when carried up high on a porteur rack - especially with very narrow bars and/or a long stem. It's much different than 25 pounds in low-riders.

I believe that a fork with longer rake is going to impart less torque to the handlebar due to road forces on the contact patch. I think it is very inconvenient and impractical to try to evaluate that at constant trail. The trail is not in isolation from head angle, offset, and wheel diameter. It depends on head angle, offset, and wheel diameter. The equation that shows this relationship is true and is dictated by physics. You can perform studies on the effect of head angle while holding trail constant, but you would need to at the same time adjust offset, or wheel diameter, or both. You can't isolate any of those four variables from the others. Same goes for wheel flop.

Whether this rake change results in better handling is a lot more complicated.

I don't think a discussion related to loading is as simple as "contact patch versus the added fork load." The load on the contact patch is affected by the total vertical load on the fork. This includes supporting the mass of the rest of the bike, the fork itself, the rider, all the added touring and gear loading, and the locations of the fork loads relative to the head axis, including the contact patch. It requires a proper free-body diagram. The effect on the rider steering input effort or flop control effort is related to torque balance around the head axis, so to see the effect of all these masses and their resulting torques, we need an equilibrium equation. With one term for each of the sources of torque we would be able to see what aspect of loading has the greatest contribution to steering force and the force feedback to the rider, under static and dynamic conditions.

That said, the contact patch will always be under the axle with the bike on a level surface and with the fork straight. The intercept of the head axis with the ground is what varies when you make a geometry change, so this changes the leverage the contact patch has when imparting torque to the fork assembly.

A front rack load above the steering axis (and the handlebar/body weight load?) imparts torque to the fork assembly, and its leverage is controlled by the perpendicular distance from the CG of the front bag to the steer axis. However, you can manipulate the bag position to reduce that distance, which will reduce the de-stabilizing effect of the front bag (inverted pendulum). Conversely, to lengthen the perpedicular distance to the CG of a system of front panniers, the greater will be the stabilizing force on the fork assembly (standard pendulum). Where is the set of masses and positions that represent "better handling?"

Should we evaluate it with the rider resting and cruising (rider weight resting on the hoods or tops and even pressing forward) or hauling butt (rider weight biased to saddle/pedals and even pulling up and back on the bars)? How do we consider the forces and their resultant when the fork is off-center? How does all of this affect the initiation of a turn, or the return to straight and vertical after a turn? Is the goal ("good handling?") to preserve the response of a good stage racer over the range of bicycle loading?

What is good handling in the first place? Since the front bag is above the steer axis you can manipulate the situation so that the tire contact point is under the axle at least while stationary and straight. While moving it will be different due to how the contact patch shape distorts due to rolling, which is the source of geometric trail. I would like to see ride reviews of your cited case versus having the CG of say a loaded front rack bag (i.e. Berthoud, Acorn or similar) ahead of the contact, over the contact, or behind the contact, then the effect with varied positioning of panniers as well. But if we don't understand "good handling" in some commonly defined way, of what value will it be? We can't even define something thought to be commonly known, like "twitchiness."

Sorry to be so persnickety, but some of these discussions don't make any sense to me.

From what I can tell, "good handling" is fairly subjective. I don't think it would be trivial to directly relate steering geometry to feel. But most people feel more comfortable with low trail in the presence a front load. Even then, a fairly wide range of trail gives the desired feel. All I know is that a high trail bike with a front load feels heavy to me, and is a bit annoying just wheeling the bike around.

A friend says he thinks that low trail works better with front loads because of reduced wheel flop. Which is the same no matter how you achieve low trail. I don't have a direct counter to that argument. So maybe the OP's idea will work. It's only money.

I think you essentially need to reduce trail to ease the effort of keeping the bike under good control when carrying a front load. I did this with a Trek 610, by having a custom fork made with an offset (aka rake) of about 66 mm, maintaining the original ride height. Nowadays Tom Matchak will build you a fork of this type. It resulted in a trail of around 35 mm. With an Acorn bag on the front it handles pretty easily. With the original fork the trail was around 50 mm, and the bars seemed to want to wrench out of my hands while riding with 15 or 20# in the bag.

One of Jan Heine's earlier articles about riders' opinions about trail indicated that for a 700c bike about 28c, the target value for trail with a light touring load (not overnight) should be around 40mm.

The requirement for brake reach became rather long, but in the Box of Old Brakes I found a Mafac Racer that is just long enough. The original Shimano 600-6207s could not have worked, in the front at least.