Forking Instability
 

My Carbon Fibre Trek (mid 90s) came with an Aluminum fork which I slightly bet recently in an accident. More than one bike shop said I was going to die if I didn't replace it, but it is a 1" with a long steerer tube. I found one after buying 3 on ebay that fit. It is a 'composite' Kestral. It fits fine but I had to put a 3mm spacer on the upper part of the bearing. It turns perfectly when not riding, but when riding it feels completely unstable. With the bent AL original fork I could ride without my hands on the handlebar, but with this new fork, I feel like the bike with go out of control even if I look behind me and not let go of the handlebars. It is the worst stability on a bike I have felt, but the shop says everything is good. What could be going on ? The new fork is a little lower (3mm) but could that f*ck up the whole geometry?
Thanks

How much rake (offset) was built into the old fork versus the new? If the new fork has more rake (offset) than the old, you'll have less trail and thus a less stable bike.

Did the shop test ride the bike? Or did they just verify the adjustment of the headset?

...just so you know, you will die eventually whether you replace the fork or ride with your old one.

There are several issues involved in replacing a fork on a bike that has been damaged in an accident.

Rake and trail are the most common, but if you crashed badly enough to damage your fork, the
entire frame is suspect from where i am sitting, and I have no way to see it or to verify that
whatever shop you decided to finally go with is capable of doing the inspection.

A front wheel issue can also affect your steering dramatically, as can a loose headset.

Find a local source that you are certain has the expertise to analyze and diagnose your problem
while looking at your bicycle....whatever that costs is cheaper than another crash.

Dave Barry would say that Forking Instability is a great name for a rock band.

And good luck with the fork fix.

No mention of the frame's condition after the accident. Andy.

Since it's described as a "carbon fiber Trek", I would bet that any frame damage would be readily apparent. On the other hand, if it's one of the carbon-tubed, aluminum-lugged Treks, a loose lug could cause a wobble, but that's iffy.

The OP says it's stable with the old fork, but not with the new Kestrel fork. I'm betting on a change in geometry causing this.

Jeff- you are probably correct. I passed by the carbon fiber Trek reference. It is interesting how sensitive some are to handling and geometry, or changes to such. I test ride hundreds of bikes, after servicing them, each year and do notice differences in the handling between otherwise similar types of bikes. But rare is the bike that i would label as unstable.

Perhaps the ones with their forks splayed to one side are the least stable as they want to turn to one side always. But then what is the definition of "unstable"? A bike that always handles in a certain manor could be called very stable, perhaps just not what most would call acceptable. I recall reading about birds VS airplanes WRT this topic. Planes are so stable they are unmaneuverable compared to the highly nimble birds, which are very unstable but expertly controlled. Andy.

Thanks for the replies. The shop didn't test ride it, I just rode it to them and had them verify I had put it together right. The frame is good, the accident was minor, in fact it is hard to tell the AL fork is bent. After the accident, the headset was fine. I changed the fork out because I was told to. The frame has been checked out and deemed solid. I have another cinelli fiber fork I can try, but it is threadless and I need to cut it and thread it, but I am not sure I want to do that and end up with a bike I can't ride. If I put the AL fork back on I can ride it as new. It is really weird how it rides with the Kestrel fork, as if the bike is a plane that doesn't want to change direction. I know I have other metal fork options, but I don't want an asian replacement. I also don't want a $500 Colnago fork. I have 2 places locally that make forks, but those are also $500 options. I wonder if someone can look at the orig fork and tell if it is ok to use.... thanks again and yes I plan to die, but hopefully not from a failed fork.

1. Don't cut and thread a threadless fork. A threaded fork is the correct inside diameter for your quill stem, which is not always the case with threadless steerer tubes.
2. "Doesn't want to change direction"?? That sounds like the new fork has less rake (offset) than the old one, which creates more trail and a bike more resistant to changing direction.

Curiouser and curiouser.

It would seem to me that one of the outfits that could make the OP a new fork, that are near by, could also check the original fork's alignment. As well as give the fork a close inspection for obvious external issues. While this exam might not speak to the forks life span remaining it would be one more data point from which to make judgments from. They could likely also measure the replacement fork (although that's not a hard job to do with simple tools and a little thought) and then the OP could push the numbers to compare the two geometries. Andy.

Are you sure it isn't just a stiff headset adjustment?

If it is the fork and you want to try the threadless one, simply get a threadless headset and stem. You'll most likely need to use a 9/8" one with a shim. Bonus: then you can use 31.8 bars, a worthwhile upgrade.

i have several bikes and they all ride differently when riding "hands free" (HF). some are difficult but not due to headset adjustment or damage. i'm not talking about wobble here, just the amount of sensitivity to weight induced steering input. worst is a '79 Trek 710 touring frame. very sensitive. best is a Gary Fisher '95 Tassahara. completely stable. i imagine it has something to do with rake and trail. tire width IME has an influence too. also found i benefited from a bit of refining in my HF riding technique.

in one case, things improved greatly when the tires "squared off" after a few hundred miles.

Thanks for the replies. The way the bike acts now shifts my attention to not crashing from what is ahead. So it needs to go. I have tried it with all sorts of headset tightness and each renders this scary ride quality. I will take my original fork to the fork dudes and pay them to examine it. It still give me perfect ride quality and will keep me from guessing if what I bought on ebay will work.

Besides the frame geometry there's rotating weight, tire width with air pressure and the resulting contact patch shape that influence the front wheel's easy of rotating off center (steering). I also have seen many riders with too high a seat height that have no handed control issues. Andy.

I took my orig AL fork to a guy who makes forks and frames and he put it on this huge 'fork tuning' machine and was able to negotiate the one fork back in perfect alignment and tested it for rigidity and said it was solid. Put it back on my bike and it rides like new. The Kestrel fork had a 38mm rake and the OE trek fork is 44mm, so that must be where the ride handling issue lies. Alles ist jetzt in ordnung. Thanks for the tips.

Ach! Alles ist gut.

Yep, As I said: less rake equals more trail means more stable and more resistant to changing direction.

I wonder what kind of machine tests forks for rigidity.

...I bet it uses beer, somehow.

More like schnapps.... ;)