I figured that I would lead this post with an image of my old 2005 CBR600RR after I busted it up against a car on BoFax. Play rough, pay the cost. The forks needed to be swapped. Few people get to break a set of sport bike forks between triples and live to talk about it.
I’m digging in more for the 32″ front wheeled AR project.
It’s not really a fork problem. More a tire problem. I was thinking about an issue, that the Maxxis 686-61 Aspen tires are expected in the US late in 2025. I’d like to get to building a bike that will use this within the next 6 weeks. How do I start on a bike that might not be usable if outside forces choose to change direction.
I came on a possible solution; make two forks. One for the theoretical 32″ wheel and another for an existing 29″ wheel. This isn’t my usual modus operandi but in this case it could be the most sensible solution.
Additionally, it will make possible back to back testing with the minimum of changes possible. A great testing opportunity. More, if the 32″ wheels turn out to be garbage, the bike will still be usable with 29″.
Now I need to model the extended 29″ fork and see if I can stomach making such a creature. I’m not a fan of forks made longer than needed but this special case drives special considerations.
To this end, I revised my fork swap calculator to help play with the numbers. The goal of this is a swap that doesn’t disrupt the rest of the bicycle design and contains the changes to just the front end. It includes provisions for changing lower headsets as well as the usual parameters.
Interestingly, I found when doing this math that if I’m keeping the same front center and head tube location with a different wheel, then the trail and flop will be identical. That’s pretty neat and not something I had really considered until now but it’s obvious.
I may not attempt to replicate the exact trail and flop values though. The two systems will still behave very different if the frictional force of the two tires on the riding surface are different. Of course, they will be different. The 686-61 tire will be much longer and narrower than the 622-66 tire. Will this result in the same frictional force acting on the front wheel trail? More work will be done here but they may be close.
This is combined with the inertial differences in the two wheels. Here, the 686-61 wheel will be significantly greater. This will add weight to the steering and increase stability especially at speed. Do I reduce the offset of the 622mm wheeled fork to make it heavier than I want? Or am I increasing the offset of the 686mm wheeled fork to help it match the feel that I’m used to? Probably the ladder.
In BikeCAD, the two prints are as shown below. Values from the calculator are confirmed.
A quick view of the 32″ all-road front AR bike with the 29″ conversion front end. Actually doesn’t look too unfamiliar.
I need to do a bit of work to the fork models and bring them up to spec with the other tricks I’ve been doing lately but this seems to be a promising direction.
2025-05-18 – As the last few days have gone by, I’ve been thinking about all of this. It’s been nagging at me and that’s usually a sign that I need to belly up to the bar again and dig deeper. I was a bit lazy in developing that calculator. It needed more choices to match some of the more interesting choices that we might make.
For example. Let’s imagine that we have a 29er mountain bike with a 160mm fork. Everything is nice. Rides well and makes fun. Now, we want to swap that fancy squish fork for a rigid fork but with a new trend 32″ wheel. This is some strange stuff but I can easily see where it may be useful. More, lets say we want to make a slight change to the front wheel trail to make up for the inertia and tire contact. What makes this even harder is if we have an imaginary fork with the blades angled from the steerer by 5 degrees like some of the new gravel forks. Got it? Good.
I updated the calculator to help us with this. Notice that the numbers don’t exactly go where you would expect. I believe that this is the final version of the calculator that does all thing things that it should.
So this leads me to the conclusion about the fork sizing for the new bike.
I will design the bike around a 29er RockShox SID 100mm fork with a 44mm offset and 64mm tire and 30% sag and a 68.5 degree head. This means that a rigid fork for 29er and a 66mm tire will be 479.14mm long and 44.73mm offset. For a 32″ rigid fork and 61mm tire will be 454.02mm long and 54.63mm offset.
This gives me three fork options with theoretically identical geometry. This is a quality solution.
