So you’ve got an old mountain bike sitting around your house that you used to think was the bomb, but now it’s just a bomb. Sure the frame is great, but the suspension fork has gone the way of my belly. It’s old and saggy and really nobody wants to look at it anymore including you. So it remains hidden from site in the garage or under the basement stairs or maybe on the back porch (your bike, not my belly).
What’s a bike lover to do? You don’t feel like shelling out $800 or more for a new fork that’s just going to lose its appeal in a couple of years and leave you feeling guilty again.
Enter the 26” rigid fork with a 29”wheel. What? You can’t do that without messing up the geometry, can you?
Chances are, you can. If you do it right.
What we want to end up with is a higher axle and a shorter fork that combine to equal the same distance from the ground to the headset that the bike was originally intended for. Thus we haven't tilted back the frame, changed the trail, or altered the effective frame angles.
Plus you can shave 1200 grams off the front end of your bike in the process! It's a pretty good rule of thumb that you pay at least a buck for every gram you lose so think if this as a 1200 dollar upgrade for three or four hundred bucks. If your old bike already has disc brakes it’s super easy because you don’t have to worry about the brake pads needing to line up with a larger rim that will be at a different height. If you use V brakes or cantis it’s more effort and expense, but occasionally do-able if you have a little skill with a torch and file, but it's easier to just go with a cable actuated disc setup in the front and leave the rim brakes in the rear.
Here’s the deal. Say your fugly old fork has an axle to crown measurement of 440mm (more on how to measure that later). That’s where we're going to start in order to be sure that the geometry of the bike remains in tact. If you end up with something too long the frame tilts backwards on the axis of the rear axle and the end result is more trail, slacker effective frame angles, and a bottom bracket that's super high. If the fork is too short, the opposite happens and you might even end up whacking your pedals on the pavement.
Either way the bike won’t steer like it used to. (For the purposes of this blurb we’re going to assume you like the way it handled before. If not, come back later for another yack about changing the geometry without altering the frame.)
A 29” wheel is about 64mm larger than a 26” wheel (if you do the conversion to inches you’ll probably think that sounds wrong but it isn’t because a 29” wheel isn’t really 29 inches in diameter and a 26” wheel isn’t 26 inches in diameter, but try not to freak out about that right now). Take that 64mm measurement and divide it in half. That makes 32mm. We’re dividing it in half because that’s how much higher the center of the front axle will be when you put the larger wheel on. Note that you need to use close to the same width 29” tire as you used on the old 26” wheel or there will be some slight differences in steering again because wider tires are also taller.
Ultimately, we end up with the approximate axle to crown height (408MM) we want for the new rigid fork. The math is like this:
440 axle to crown
-32 from the increase in axle height
= 408 (mm)
Now all we need to do is find a fork that's close to that length, (there are quite a few 410mms out there), pop the fork on the bike, put on the new wheel, install the disc brakes and whoopee! You’ve got the bump absorbing, obstacle clearing power of the larger 29” wheel paired with the joy of the far lighter rigid fork and no worries about maintenance or dialing in fork dampening.
Yes, the 29" wheel/tire combo weighs a skosh more than an equivalent 26" combo, but not a lot and certainly not as much as that silly 5 pound suspension fork in anodized Gatorade green. In my own conversion, the new setup is 1,226 grams lighter according to my Alpine scale.
But wait, what about the rim brake crowd? Yes, that’s me most of the time. In fact, I’m so old, many of my mountain bikes run cantilever brakes and I kinda like it that way.
There were a couple of issues to deal with since I wanted to stick with rim brakes. The big one is that the brake mounts were not at the right height. I had to saw the brake mounts off and file the fork smooth. Then I brazed on new mounts in the correct location. If you want to do this kind of conversion yourself, avoid the trouble and get the Kona 2 disc fork, a disc compatible 29" front wheel and a cable actuated linear pull disc brake. Assuming you were already running V brakes, your current levers should work just fine. If you were running cantis, you need a new lever because canti’s require different levers than V brakes or mechanical discs. You could either replace the rear canti with a V brake (why bother), or do what I do and ride with a mismatched pair of levers. It won’t kill you and it looks cool. It says “I work on this friggin' thing myself and I don’t need no stinkin' matchy matchy parts because I’m a manly/womanly/transgendered cyclist of doom!” Or something like that.
You may be able to get what you need for the conversion from your local bike shop, but if your LBS is mostly BS and doesn't want to talk to you about axle to crown height and doesn't get what you're doing (or why you'd do it instead of buying the latest blingey $2,000 crap from them) you can always talk to my friends (who I don’t know personally but they’re still my friends) at Bikeman.com. I like them. They’re really the folks at Bath Cycle and Ski in Woolwich, Maine. They’ve had what I need for many a year now and they have darned good service too. There was a time when V brakes had gotten popular and cyclocross hadn’t yet, so nobody was making or using cantilever brakes. You couldn’t find a front cable hanger for love nor money in those days and I was exiled in South Texas desperately needing my bike back in working order and not wanting to buy new brakes and new levers (which were annoyingly attached to my shifters which I'd have had to buy new as well). Bikeman came to the rescue and I’ve pretty much been a major fan ever since.
Project 2 forks from Kona are a really good buy and come in a few different lengths. The 410 worked well for me, but this is the absolute shortest fork you'd want to try to stuff a 29" wheel into. It has the minimum height clearance for a 2 inch wide 29er tire (the WTB Vulpine in this case which has minimal knobs). Forks from Vicious Cycles come in several different lengths with disc mounts. They’re more costly than the Konas, but they’re super nice (I sometimes put them on customer's bikes but usually don't spend the money for myself) and you can get them in bare steel (directly from viciouscycles.com) if you feel like painting the fork yourself. There are also several carbon mountain forks out there and some even have adjustable canti mounts. But frankly I’m reluctant to recommend them. I’m a hefty fella and I’ve been known to ding a few tubes in my time. Carbon doesn’t ding. It cracks, it snaps, it shatters all to heck. I stay clear of the stuff myself.
Don't worry too terribly much about making sure the fork is exactly the perfect height. A difference of 10mm in either direction should be just fine. If you really feel like worrying about it, think of it this way: If you like twitchier steering go with the shorter fork and if you like more stable steering go with the longer one. If you plan to ride this bike at 35mph or more, go with the longer fork for more trail which is a good thing at high speeds.
Oh yeah, measuring that fork height! I promised earlier to come back to that. If you already have a good grip on how to do that, cool. If not, the easiest thing to do is take the wheel off and pull out the quick release. Then re-install the quick release without the wheel. Slip that little chromed clip at the end of your measuring tape center of the QR rod and measure up to where your headset begins. That isn't a strictly perfect measurement, but it's more than good enough for our purposes.
Good night, and good luck!
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