Bicycle Mechanics - Straightening A Bent Fork At Home?
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I have a bent fork :-(
It is not the common problem of fork blades bent rearward.
Rather, when viewed from the front, the fork blades are angled a little bit, so that the front dropouts are slightly displaced to the drive-side. As a result, if the top of the front wheel is centered in the fork crown, the wheel is at a slight angle relative to the rear wheel - it is leaned over a little bit toward the non-drive-side.
The bend is not large. I didn't see it when I bought the bike (22 year old road bike). You have to have everything lined up perfectly to see it.
I don't care about a bend that I can hardly see, except that this bike cannot be ridden hands-off. I discovered that when taking it out for a test ride tonight, after spending two weeks overhauling the damn thing. It veers to the right. I'm decent at riding hands-off, but can't do it on this bike at all.
The local bike coop charges $30 to straighten a fork. I'm wondering if I can do this one at home. I could clamp the steerer tube in my bench vise, slide a pipe over the fork blade, and carefully apply pressure.
Sound do-able? Have you done it? Or should I fork over (ha ha) $30?
I'm only wondering, because this bike has proven a PITA and I'm already in it for more than I'd hoped. 1980 Motobecane Grand Record, I have $252 in it so far and still need bar tape and $26 of decals to be done.
How much is "slightly?"
What material is the fork made of?
If it is less than 3/4 of an inch and it is a steel fork I would.....consider .....straightening it after much careful measuring and double checking of results and a very close inspection afterwards for any indications of buckling, cracking or failure around the crown to blade area.
Don't use a pipe. You will need to make a proper bender which can be done with a 2x4 and blocks.
It is steel - Vitus 172 I think - with conventional crown. The center of the front dropouts is displaced less than 1/2" to the drive side.
Looked on Sheldon Brown and it seems to say I can do this with the fork in the bike. I'm going to remove the fork and clamp it in the vise anyway. I think it will be easier to measure and verify.
First I'll verify and adjust the true and dish on the front wheel, since the position of the rim in the fork crown will be a visual guide.
By the way, can anyone think of any other reason, beyond a bent fork, for a bike to pull to the right when ridden hands off?
08-02-12, 08:57 AM
I've done it; put in the garage floor with the steerer tube suspended/viced, one foot on the away fork, grabbed the near fork and yanked or pushed, controlled fashion. Perfect, but I tend to like simple solutions. As long as you have the physical control to make small moves, like a 1/4 inch at a time, easy peasy.
I recall the steering feeling off, like it had a low side and a high side. Are you certain your cables are not interfering with a neutral steering position?
Cables are not interfering, I even tried re-routing them. I also tried loosening the headset, in case it was binding.
I took a hard look at the fork. Hmm. In addition to the slight lateral shift of the dropouts toward the drive side (DS), the DS blade has slightly (but visibly) more curvature than the non drive side (NDS) blade, which I think could be causing the DS dropout to be higher than the NDS drop out. If so, that would exacerbate the tilt of the front wheel.
When I stand over the bike, squint with one eye, and get the frame exactly vertical and centered, then the front wheel is tilted with its top toward the NDS. It is subtle, you don't notice it unless you are looking specifically for it (or, maybe, I'm just a Neanderthal).
I will remove fork, clamp in vise, make some careful measurements, carefully bend the blades to center the dropouts laterally, make myself a bender, and carefully bend to reduce the curvature. I think I can at least "do no harm", and can probably mostly correct the bends. Worst case, the bike just can't be ridden hands off. Really worst case, there is a stack of old French forks at the bike coop for $15-20 each, though the color won't match.
I rode the bike to work today to check how it goes. Answer: it goes very nicely, except for this issue. Surprisingly light (maybe not surprising, spec was 22 lb and this one has lighter wheels and saddle than stock), quick, comfortable, shifts beautifully (new cables, housing, adjusted), drivetrain silent (cleaned and oiled everything, repacked BB), brakes smoothly (new pads, cables, housing, adjustment), very nice French ride. I'm fixing it up for a friend who wants a vintage French bike, and wanted to keep the all-in cost for the bare bike under $250 for him, if I can DIY the fork then we'll almost make that target. I've decided he can choose whether to pretty it up w/ new decals.
08-02-12, 03:23 PM
It's difficult to know if the blades are only bent sideways - that is not always the case. If the co-op's $30 job is done with a Park fork guage or similar, as well as dropout alignment guages, so that when done the blades are centered on the column, in the same fore/aft position, and the dropouts are aligned it's money very well spent. It's difficult to get all three of those to spec on a home repair.
08-02-12, 08:37 PM
a known to be true and dished wheel is a reasonable guage for centerng the fork blades
put bike in stand, sit yourself in a chair in front of it
brace one foot against the bottom bracket/leftcrank head, grab a fork blade with both hands, pull hard
rotate the fork to face 90* left or right to bend whatever direction needed (you can only apply pulling force)
useful to have a friend grab the handlebar and keep things from rotating
go in small steps and put the wheel in place between adjustments to check centering
realign the fork ends afterwards
have had good luck with this
steel forks only
I did one recently at home.
I saw a posting somewhere where a guy built a jig out of hard wood that was really nice but I couldn't find it maybe you can.
Here is what I did (Mine was not bent backwards but out from side to side and drop outs misaligned, fork was made of hi-ten steel, not caused by a crash but by intentional bending by an idiot)
1. Removed fork from frame
2. Wrapped fork blades with old inner tube for protection
3. Inserted a scrap piece of tubing down the fork tube that extended to the drop outs (this was used on a s reference to measure if the blades were different distances from the centerline.
4. Placed fork tube in bench vice protected by wood blocks
5. Using pipe on for blades bend and measure (dial caliper) in small increments, repeat. brought the blades back equal distance from the center line with correct over lock distance.
6. Aligned drop outs with threaded rod, fender washers and nuts. as you do for the rear.
It was my opinion that my fork was still plenty safe to ride, primarily because it was really hard to bend with a big lever on it.
When I straightened some forks, I would take an all threaded rod about 36" long and place it in the drop outs where the wheel axle would go. This long straight rod will give you a very good reference to see how both sides of the fork are being aligned. I did not remove my fork from the bike while I made the adjustments. I would find ways to hold the side of the fork (like in a vise with wood blocks) and then I would use the frame of the bike as leverage to make small bends in the fork.
It can be done. Steel bends pretty darn easily.
I built a rough jig out of wood but there are many variables. Getting it right from side to side and centered is easy. Getting the bends right so they match left/right and the lengths from the crown to the DO is even harder. These are frighteningly hard to measure and compare. You can't really go by "looks" as the frame builder may have made them crooked to begin with and compensated a little when they were built up. They may "look" crooked but still be geometrically correct. Or, conversely, look right but then be crooked.
In the end it is a lot of trial and error -riding it to be sure you got it right with assembly/dissassembly each time. Make sure to align the dropouts as you go each time too. That will throw everything else off. You have to get everything lined up in multiple dimensions at the same time.
While it can be done and I've done it before, if I had a shop nearby that had the proper jig and could guarantee that it was right the first time I'd jump at paying just $30 to have it done for me on a real fork jig. It's just not worth the work and the time involved doing it the hard way. Most shops will not touch this work these days as the liability involved can be very high. The proper straightening jigs were huge, heavy and very expensive. Almost no modern shops have one these days or are willing to work on rebending forks. .
Well, it is not perfect, but it is much, much better. I can ride the bike hands off now, with a little bit of care. I'm going to call this good.
08-05-12, 12:45 AM
Had that same damned thing happen to me on my first build (still underway) Except th pull was to the drive side. What a pain in the &^% Finally took it to one of the LBS, they didn't have a jig, though bent it back(ish) for $15 its not perfect, and still has some pull though it is MUCH more rideable and the guy at the shop said he rode it hands off, I'll take his word for it cause i'm rubbish at that even with a straight fork. Bugs me that it still not perfect and i'm like $260 in in the bike. Happy yours turned out pretty good too.
The bike rode well with hands, even with the bent fork. But it pulled hard to the drive side as soon as you took your hands off. I can ride my bike hands off no problem, but couldnt manage this one for five feet. I wanted to fix that, as I worried about my friend inadvertently taking his hands off for a moment, maybe accidentally, and immediately crashing.
Now I can ride it hands off, not as easily as my own bikes, but not too tricky.
My friend won't likely try to ride hands off - I dont think he knows how to - but if he does lose his grip momentarily, he has a good chance of being okay.
As a bonus, the bike seems to ride a little better with hands - just a bit more responsive.
I used the wheel to judge the blades' lateral alignment, and used a flat surface to get the front-to-back position of the blades and dropouts.
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