Tuesday, November 15, 2011

Fork and headset options for old French frames.

I mangled the fork on my MB, as a result of an epic curb smash. Of course there was a vehicle involved. My helmet was crushed, my noggin was hardly scratched, and my was bent. I’m lucky it didn't fold the tiny split top tubes. Even the rim, technically worth more than the bike, survived. I guess the fork gave itself to save it’s comrades. The Panaracer T-serv 32c Messenger tire survived as well, it should be noted.

Changing the headset out on an old French bike isn’t much of a hassle. You may or may not have to ream. If you do, take a little at a time. You want a good, tight fit on the bearing races. It should take a moderate amount of force to seat the races.

I used a block of 2x4 construction wood the protect the race and distribute the impact evenly, and a firm rubber mallet to do the driving.  The materials are important. . . you can only put so much power through them. If it’s not enough, take a another tiny shave. Don’t beat it repeatedly. It’s not easy, but I’m sure you can destroy not only the replacement headset but the steer tube itself, if you really try NOT to use your brain. It would be hard to with the wood and rubber set up. I’m sure there’s a special tool, but I learned this from an old pro.

I managed to fit a threaded Shimano standard-thread headset with just the teeniest bit of reaming.  Which opened up my options tremendously to 2 decades of more standardized threaded forks, though closed the door to ever putting another original MB Mirage fork on. So that was a tough decision, forced by the fact that this bike is my car, and there was no suitable length fork to be found in any of the local good-old-bike graveyards.  

Most carbon fiber forks are thread-less. A fellow bike hacker has also installed a thread-less headset in a similar vintage Peugeot frame. The real limitation, as far as trying to use a modern carbon fiber fork, is the one inch diameter. Carbon forks came in just as steer tubes were making a transition to larger sizes. There are a few 1“ carbon forks available, but not exactly a selection.  Don’t just jump out and grab the first one you see. You’ll want to understand how to measure the fork rake, and how fork rake affects handling.

As a general rule, pushing the front tire further ahead of you will give you greater straight line stability, at the a cost to turn in speed. Pulling the axle in, closer to the crank and the front-rear center of mass, will give you sharper turn in, at the cost of straight line stability. Small changes of a millimeter or two can have surprisingly big changes. If it’s dialed in somewhere you’re comfortable, we call this stable or quick. All things considered, unless you know what you intend and have a decent sense of the results, I would recommend erring on the side of stable.  Especially before spending out on a carbon fork.  If you’ve got a proper hacker’s stash of old parts, you might try a few dustbin forks of various rakes, after getting the new (to you) races installed.  This is how I did it, with my own fork too mangled to get an accurate read on it. The first on installed was too quick, so I picked another one with more rake. On the third fork, she felt much like she had before, if perhaps a bit more stable or sluggish, depending on taste. She rides like a Mercedes now. Definitely get the desired rake desired in before ordering a fork.

In my opinion, though, a classic curved rake fork gives the right smooth ride for this type of touring frame, and I would feel that a carbon fork would both look odd and feel strangely light on the front of such a (comparatively) heavy frame. But you may have more exotic steel than I, and different aesthetic tastes. And while carbon is lighter than steel and more compliant than aluminum, I don’t think it would feel enough different to justify the expense. I tend to want to replace aluminum forks with carbon. But I tend to want to sell aluminum bikes and get something either more or less advanced, as it were. They were a great improvement, especially in cost vs. weight vs. stiffness. But a terrible time for elbow joints. Good thing I was pretty young during the reign of aluminum.

If you need more information on fork rake and understanding how it affects handling, you might read the wikipedia article on motorcycle and bicycle handling here, just as a primer.

Livestrong offers a decently simple method to measure the fork rake here. Yours to figure out how to translate ‘lower the kickstand’ into something meaningful, since this is an odd bit of advice for anyone who rides with any seriousness, on any serious hardware. A kickstand? Really? (I’d find a hard time measuring the rake accurately with the bike at a kickstand angle and the wheel consequently trying to flop to one side.) But if the fork’s already off, and you have some to pick through, you can hold them side by side and see if, with the steer tubes lined up, the ends of the fork are ahead or behind the original fork. Provided the original fork isn’t as mangled as mine was.

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