Forking Instability
#1
Junior Member
Thread Starter
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
Thanks
#2
Insane Bicycle Mechanic
Join Date: Mar 2008
Location: other Vancouver
Posts: 9,811
Mentioned: 34 Post(s)
Tagged: 0 Thread(s)
Quoted: 788 Post(s)
Liked 688 Times
in
367 Posts
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?
Did the shop test ride the bike? Or did they just verify the adjustment of the headset?
__________________
Jeff Wills
Comcast nuked my web page. It will return soon..
Jeff Wills
Comcast nuked my web page. It will return soon..
#3
Friendship is Magic
Join Date: Nov 2010
Location: Sacramento, CA
Posts: 22,932
Bikes: old ones
Mentioned: 304 Post(s)
Tagged: 0 Thread(s)
Quoted: 26251 Post(s)
Liked 10,232 Times
in
7,101 Posts
...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.
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.
__________________
#4
rebmeM roineS
Join Date: Jan 2006
Location: Metro Indy, IN
Posts: 16,217
Bikes: Bacchetta Giro A20, RANS V-Rex, RANS Screamer
Mentioned: 15 Post(s)
Tagged: 0 Thread(s)
Quoted: 653 Post(s)
Liked 345 Times
in
225 Posts
Dave Barry would say that Forking Instability is a great name for a rock band.
And good luck with the fork fix.
And good luck with the fork fix.
__________________
Bacchetta Giro A20, RANS V-Rex, RANS Screamer
Bacchetta Giro A20, RANS V-Rex, RANS Screamer
#5
Senior Member
Join Date: Feb 2012
Location: Rochester, NY
Posts: 18,003
Bikes: Stewart S&S coupled sport tourer, Stewart Sunday light, Stewart Commuting, Stewart Touring, Co Motion Tandem, Stewart 3-Spd, Stewart Track, Fuji Finest, Mongoose Tomac ATB, GT Bravado ATB, JCP Folder, Stewart 650B ATB
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 4172 Post(s)
Liked 3,793 Times
in
2,272 Posts
No mention of the frame's condition after the accident. Andy.
#6
Insane Bicycle Mechanic
Join Date: Mar 2008
Location: other Vancouver
Posts: 9,811
Mentioned: 34 Post(s)
Tagged: 0 Thread(s)
Quoted: 788 Post(s)
Liked 688 Times
in
367 Posts
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.
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 Wills
Comcast nuked my web page. It will return soon..
Jeff Wills
Comcast nuked my web page. It will return soon..
#7
Senior Member
Join Date: Feb 2012
Location: Rochester, NY
Posts: 18,003
Bikes: Stewart S&S coupled sport tourer, Stewart Sunday light, Stewart Commuting, Stewart Touring, Co Motion Tandem, Stewart 3-Spd, Stewart Track, Fuji Finest, Mongoose Tomac ATB, GT Bravado ATB, JCP Folder, Stewart 650B ATB
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 4172 Post(s)
Liked 3,793 Times
in
2,272 Posts
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.
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.
#8
Junior Member
Thread Starter
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.
#9
Insane Bicycle Mechanic
Join Date: Mar 2008
Location: other Vancouver
Posts: 9,811
Mentioned: 34 Post(s)
Tagged: 0 Thread(s)
Quoted: 788 Post(s)
Liked 688 Times
in
367 Posts
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.
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.
__________________
Jeff Wills
Comcast nuked my web page. It will return soon..
Jeff Wills
Comcast nuked my web page. It will return soon..
#10
Senior Member
Join Date: Feb 2012
Location: Rochester, NY
Posts: 18,003
Bikes: Stewart S&S coupled sport tourer, Stewart Sunday light, Stewart Commuting, Stewart Touring, Co Motion Tandem, Stewart 3-Spd, Stewart Track, Fuji Finest, Mongoose Tomac ATB, GT Bravado ATB, JCP Folder, Stewart 650B ATB
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 4172 Post(s)
Liked 3,793 Times
in
2,272 Posts
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.
#11
bike whisperer
Join Date: Dec 2009
Location: Melbourne, Oz
Posts: 9,533
Bikes: https://weightweenies.starbike.com/forum/viewtopic.php?f=10&t=152015&p=1404231
Mentioned: 15 Post(s)
Tagged: 0 Thread(s)
Quoted: 1520 Post(s)
Liked 716 Times
in
508 Posts
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.
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.
__________________
Sheldon Brown's bike info ~~~ Park Tools repair help
Half-step triple, using double gear ~~~ 6400 STI rebuild walkthrough ~~~ Want 8/9/10s @126mm OLD? OCR. ~~~ Shimano cassette body overhaul ~~~ Ergopower Escape wear repair ~~~ PSA: drivetrain wear
List of US/Canada bike co-ops ~~~ Global list
Sheldon Brown's bike info ~~~ Park Tools repair help
Half-step triple, using double gear ~~~ 6400 STI rebuild walkthrough ~~~ Want 8/9/10s @126mm OLD? OCR. ~~~ Shimano cassette body overhaul ~~~ Ergopower Escape wear repair ~~~ PSA: drivetrain wear
List of US/Canada bike co-ops ~~~ Global list
#12
Senior Member
Join Date: Dec 2010
Location: Above ground, Walnut Creek, Ca
Posts: 6,681
Bikes: 8 ss bikes, 1 5-speed touring bike
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 86 Post(s)
Likes: 0
Liked 4 Times
in
4 Posts
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.
in one case, things improved greatly when the tires "squared off" after a few hundred miles.
Last edited by hueyhoolihan; 11-29-13 at 11:19 PM.
#13
Junior Member
Thread Starter
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.
#14
Senior Member
Join Date: Feb 2012
Location: Rochester, NY
Posts: 18,003
Bikes: Stewart S&S coupled sport tourer, Stewart Sunday light, Stewart Commuting, Stewart Touring, Co Motion Tandem, Stewart 3-Spd, Stewart Track, Fuji Finest, Mongoose Tomac ATB, GT Bravado ATB, JCP Folder, Stewart 650B ATB
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 4172 Post(s)
Liked 3,793 Times
in
2,272 Posts
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 tooi 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.
in one case, things improved greatly when the tires "squared off" after a few hundred miles.
#15
Junior Member
Thread Starter
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.
#16
Insane Bicycle Mechanic
Join Date: Mar 2008
Location: other Vancouver
Posts: 9,811
Mentioned: 34 Post(s)
Tagged: 0 Thread(s)
Quoted: 788 Post(s)
Liked 688 Times
in
367 Posts
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.
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.
__________________
Jeff Wills
Comcast nuked my web page. It will return soon..
Jeff Wills
Comcast nuked my web page. It will return soon..
#17
Friendship is Magic
Join Date: Nov 2010
Location: Sacramento, CA
Posts: 22,932
Bikes: old ones
Mentioned: 304 Post(s)
Tagged: 0 Thread(s)
Quoted: 26251 Post(s)
Liked 10,232 Times
in
7,101 Posts
#18
Jack of all trades