Fork vibrating when braking: what is wrong?
Karl Olav Lillevold TFS
I have a problem with my mountain bike. Whenever I use my front brake
the front fork starts vibrating violently back and forth. This means I
have to apply a hard pressure on the rear brake, and only a tiny
amount of pressure on the front brake, even though the front brake is
more efficient for stopping on hard surfaces with good tire traction.
My headset is tight, but I am beginning to wonder if my front fork is
too soft? It is a normal Tange MTB steel fork (a couple of years old)
I am looking for some advice. Should I buy a new stiffer fork, or
perhaps even a suspension fork?
Karl Lillevold
--
Karl Olav Lillevold | Image Sequence Compression
Karl.Li...@tf.tele.no | Norwegian Telecom Research
Mr. S.J. Wooding
: I have a problem with my mountain bike. Whenever I use my front brake
: the front fork starts vibrating violently back and forth. This means I
: have to apply a hard pressure on the rear brake, and only a tiny
: amount of pressure on the front brake, even though the front brake is
: more efficient for stopping on hard surfaces with good tire traction.
: My headset is tight, but I am beginning to wonder if my front fork is
: too soft? It is a normal Tange MTB steel fork (a couple of years old)
: I am looking for some advice. Should I buy a new stiffer fork, or
: perhaps even a suspension fork?
Try toe-ing your brake blocks in. The front of the block should contact the rim
about 2mm before the back. This is because the rotational force of the rim pulls
the block into the rim. No toe-in means that only the back edge of the block
is stopping the wheel, and this can have the vibrational effect you speak of.
The rear edge of the block 'catches' the rim and skids back repeatedly. I
cured this on my MtB this way. Hope it cures your problem too ;o)
Steve JW (MadBritishMountainBikerAndMaterialsScientist)
!faster and faster until the thrill of speed overcomes the fear of death!
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
S.J. Wooding
Radiation Damage Group
Dept. of Materials Science and Engineering
University of Liverpool
PO Box 149
L69 3BX fax: (051) 794-4675 tel: (051) 794-5384 raddamsw @liv.ac.uk
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
Michael J. Edelman
>
>I have a problem with my mountain bike. Whenever I use my front brake
...I am beginning to wonder if my front fork is
>too soft?...I am looking for some advice. Should I buy a new stiffer fork, or
>perhaps even a suspension fork?
The problem is not with your fork; steel forks do not soften with age!
I suggest you look at your brake shoes, to see if perhaps the pads have
hardened or have foreign matter in them, and also check your wheels to see if they
are in true. You may also have loose brakes; this could cause chattering as well.
Probably wouldn't be a bad idea to have a good bicycle mechanic look into this,
as I suspect you may be a bit of a novice at bike mechanicals.
--mike
Davin Lim
is one possibility (as someone has already mentioned.) Another
possibility is that one of the cantilever brake bosses is loose or
cracked. This very thing happened to my commuter bike just last week.
The front brakes started to vibrate quite violently when applied, and
I noticed that the brake was not firmly attached to the fork. When I
applied the slightest pressure when tightening with an Allen wrench,
the brake boss snapped off - from the rust that was visible, it was
obvious that it had been cracked for a while. I'm just glad it
didn't break off while I was actually riding.
-Davin (li...@arraytech.com)
Mike Fike, Manchester, Maine
In article <KOL.93Ju...@axil.nta.no>, k...@nta.no (Karl Olav Lillevold TFS) writes...
Check your pads and rims first. Clean thoroughly - you may have some
stickiness grabbing your pads. Sand the pads to a fresh surface. Check to see
that your pads have a slight (1mm) toe-in. If this fails check to see if you've
bent your fork backwards a bit- if so , this might be a nice time to upgrade to
a suspension fork!
Mike
********************************************************************************
Mike Fike * No comment until I see a lawyer....
17 Myrtle St. *
Manchester, Maine, 04351 *
********************************************************************************
T...@slacvm.slac.stanford.edu
bearings omit this test, but if you have ball bearings; lift the bike so that
front tire clears the ground. Move the rim side to side, using your brake pads
as a guide, there should be no play in the wheel. The other check is the
headset; put on the front brake and rock the bike back and forth, again there
should be no play.
good luck,
-tom
Fred Grosby
MJE> The problem is not with your fork; steel forks do not soften with
MJE> age!
Not true. Steel frames/forks lose 25-33% of their original strength to metal
fatigue. Aluminum, on the other hand, has no fatigue limit; it just keeps
getting weaker and weaker and weaker and ...
Karl Olav Lillevold TFS
Thanks for all your suggestions!
The answer to my problem was quite simple. I had to adjust the
toe-in and also sand the braking surfaces a bit. Since the
front brake pads don't wear as much as the rear ones I had not
adjusted toe-in for a while. Also, I thought adjusting toe-in
had more to do with preventing screeching braking noises than
this vibration. To sum up: in my case the vibration was caused by
(i) no toe-in and
(ii) sticky braking surfaces.
I had just built myself a new wheel-set using non-anodized rims
instead of anodized ones, and the braking pads stick much more to
the non-anodized ones.
Karl.
Peter Epstein
|> The answer to my problem was quite simple. I had to adjust the
|> toe-in and also sand the braking surfaces a bit. Since the
|> front brake pads don't wear as much as the rear ones I had not
|> adjusted toe-in for a while. Also, I thought adjusting toe-in
|> had more to do with preventing screeching braking noises than
|> this vibration. To sum up: in my case the vibration was caused by
|> (i) no toe-in and
|> (ii) sticky braking surfaces.
|> I had just built myself a new wheel-set using non-anodized rims
|> instead of anodized ones, and the braking pads stick much more to
|> the non-anodized ones.
Why do your front brakes wear more slowly than your real? The front brakes
should take 70% of the braking load (even more under extreme braking),
so they should wear faster.
If you want that nice braking you get with non-anodized rims but you're
stuck with anodized rims, place a piece of sandpaper between the brake
pad and rim (facing the rim) and turn the wheel while braking. This
will sand off the anodization leaving an ugly looking multi-color braking
surface that is almost as good as a non-anodized surface. You found yet
another reason to go with non-anodized rims :)
--
,__o ,__o
_-\_<, Peter Epstein pe...@objectime.on.ca _-\_<,
(*)/'(*) (*)/'(*)
Dave Hayes
writes:
> MJE> The problem is not with your fork; steel forks do not soften with
> MJE> age!
>
>Not true. Steel frames/forks lose 25-33% of their original strength to metal
>fatigue. Aluminum, on the other hand, has no fatigue limit; it just keeps
>getting weaker and weaker and weaker and ...
I don't have my Materials text close by, so I can't confidently refute these
statements. However, I do know that Fred has at least confused strength with
stiffness (aka "modulus of elasticity" or Young's modulus).
When people talk about a material "softening," they usually mean that the
structure is less resistant to flexing or will flex farther given the same
force. The FACT of the matter is that, unless stressed beyond the yield point
(where the structure is "bent" and does not return to its original shape by
itself), the modulus of steel DOES NOT CHANGE, no matter how many times it is
flexed. This is also true of aluminum.
The significance to us cyclists is that steel and Al frames and forks DO NOT
"soften" with age. (This may not be true of a composite frame.)
You can break a piece of metal by repeatedly bending it back and forth--but
you are stressing the material well beyond the yield point to do so.
I'm sort of foggy now (It's been 4 years--use it or lose it!), but I think
Fred is referring to the behavior of steels and Al alloys in response to
repeated below-yield stresses ("cycling"). The theory has been that you can
cycle a steel forever and (after, say 10^6 cycles) it reaches a minimum
STRENGTH, whereas Al will continually weaken until it eventually cracks. Note,
however, that the modulus of either material stays the same throughout. My
materials prof suggested that steel may well continue to weaken, but that no
one had had the patience to cycle it long enough to find out, as the strength
vs. # of cycles curve gets pretty flat after 10^6 cycles.
Perhaps one of the material scientists on the Net can give a better
explanation.
Dave Hayes
Geoffrey Hall
Maybe some metallurgist out there can help out, but as best as I can
explain it, metal fatigue is a term for an undesirable work HARDENING
of the metal to the extent that it gives rise to fractures, like the
way a paper clip bent back and forth a number of times stiffens before
it fails. (A properly built frame/fork would never get to that point
under normal use.)
Geoff Hall (ge...@zeus.sps.mot.com)
Fred Grosby
DH> In article <74089015...@Clone.his.com> Fred Grosby,
DH> fred....@his.com
DH> writes:
DH> > MJE> The problem is not with your fork; steel forks do not soften with
DH> > MJE> age!
DH> >
DH> >Not true. Steel frames/forks lose 25-33% of their original strength to
DH> >metalfatigue. Aluminum, on the other hand, has no fatigue limit; it
just
DH> >keepsgetting weaker and weaker and weaker and ...
DH> I don't have my Materials text close by, so I can't confidently refute
DH> these statements. However, I do know that Fred has at least confused
DH> strength with stiffness (aka "modulus of elasticity" or Young's
modulus).
My statement is based on what I was told by professor of
metallurgy/framebuilder Julian Edwins. The references to fatigue and fatigue
limit are his. Given his qualifications, I choose to accept what he todl me
as being authoritative.
Anders Jacob Truelsen
[snip-snip]
>If you want that nice braking you get with non-anodized rims but you're
>stuck with anodized rims, place a piece of sandpaper between the brake
>pad and rim (facing the rim) and turn the wheel while braking. This
>will sand off the anodization leaving an ugly looking multi-color braking
>surface that is almost as good as a non-anodized surface. You found yet
>another reason to go with non-anodized rims :)
I've done it without sandpaper ! One trip through the woods on a rainy day, and
my rims had that lovely raw look.....
Cheers
Anders
--
_~o Anders Truelsen email : leg...@daimi.aau.dk
_-\_<, CS. Dept University of Aarhus phone : +45 86202711-5208
(*)/'(*) 8000 Aarhus C fax : +45 86135725
DENMARK Priv. : +45 86188915
Frank Krygowski
In a previous article, ge...@zeus.sps.mot.com (Geoffrey Hall) says:
>In article <74089015...@Clone.his.com> fred....@his.com (Fred Grosby) writes:
>Maybe some metallurgist out there can help out, but as best as I can
>explain it, metal fatigue is a term for an undesirable work HARDENING
>of the metal to the extent that it gives rise to fractures, like the
>way a paper clip bent back and forth a number of times stiffens before
>it fails. (A properly built frame/fork would never get to that point
>under normal use.)
>
I'm not a metallurgist, but I'm a mechanical engineer.
All the evidence I've seen says : steel forks do not soften with age.
Or, more precisely, their stiffness (resistance to the deflection caused
by an applied load) should not change measurably.
In mechanical design, it's standard practice to design with stresses
below the "endurance limit" of steel. If this is done, there should be
no fatigue. But in any case, stresses above the endurance limit don't
gradually turn the metal to jelly. As I understand it, _eventually_
such stresses will result in sub-microscopic cracks, which will
eventually coalesce into microscopic, then visible cracks and failure.
Typically this failure happens _without warning_ ... which is what
makes it so fearsome. If the metal's stiffness went all to hell
ahead of time (that is, more than a few cycles before total fracture)
we'd worry less about fatigue failures. They'd be easier to prevent
because we could tell they were in progress.
Now, vibration (not fatigue) is used to remove residual stresses
sometimes ... like in large weldments. But as far as I know, the
effect is on the microscopic, not macroscopic level. Again, I'd
doubt any measurable effect on a bike's fork.
I'm curious: what _do_ the metallurgists have to say?
--
Frank Krygowski ae...@yfn.ysu.edu
Harry Phinney
: MJE> The problem is not with your fork; steel forks do not soften with
: MJE> age!
: Not true. Steel frames/forks lose 25-33% of their original strength to metal
: fatigue. Aluminum, on the other hand, has no fatigue limit; it just keeps
: getting weaker and weaker and weaker and ...
As has been pointed out, the stiffness of the forks is unrelated to the
strength of the material, but is instead related to the stiffness of the
material (i.e. the modulus of elasticity). I do not believe that steel
(or any other metal, for that matter) loses 25% - 33% of its original
yield strength, or ultimate tensile strength through low-strain cyclical
loading. Its modulus of elasticity most certainly does not change. A
cold worked material, such as cold-drawn tubing, can lose significant
strength and hardness under cyclical loading, but I believe only at
cyclical stress levels above the yield stress of the material (1). In
any case, there are very few times when one cares about the yield
strength, let alone the ultimate tensile strength of a bike frame. Bike
frames do not plastically deform under riding stresses, so the only time
the yield strength matters is in crashes and when trying to straighten
or bend the frame in the workshop.
Also, while it's true that steels exhibit a true endurance limit
(generally of around 50% of the steel's ultimate tensile strength), it
is unlikely that most steel bike frames operate at stresses under this
limit at all times. If they did, then steel frames would never fail,
but they occasionally do so. In any case, if a particular type of frame
never fails, then builders will construct lighter frames until they
begin to eventually fail. An aluminum frame can be designed and built
to last multiple human life times, but I suspect they are instead
generally designed to provide a life span similar to that of an
equivalent quality steel frame. In no case does a metal frame get
"softer" (i.e. more flexible) with age.
Harry Phinney ha...@cv.hp.com
1. Polakowski, N.H. and Ripling, E.J. "Strength and Structure of
Engineering Materials" Prentice-Hall; pages 493-500.
Fred Grosby
GH> In article <74089015...@Clone.his.com> fred....@his.com
GH> (Fred Grosby) writes:
GH> > m...@pookie.pass.wayne.edu (Michael J. Edelman) writes:
GH> >
GH> > MJE> The problem is not with your fork; steel forks do not soften with
GH> > MJE> age!
GH> >
GH> >Not true. Steel frames/forks lose 25-33% of their original strength to
GH> >metalfatigue. Aluminum, on the other hand, has no fatigue limit; it
just
GH> >keepsgetting weaker and weaker and weaker and ...
>
GH> >
GH> Maybe some metallurgist out there can help out, ...
As I have mentioned, what I know of this I learned from a metallurgist. Maybe
he explained it incorrectly.
scott dexter
>
>In article <KOL.93Ju...@axil.nta.no>, k...@nta.no (Karl Olav Lillevold TFS) writes...
>>
>>I have a problem with my mountain bike. Whenever I use my front brake
>>the front fork starts vibrating violently back and forth. This means I
>>have to apply a hard pressure on the rear brake, and only a tiny
>>amount of pressure on the front brake, even though the front brake is
>>more efficient for stopping on hard surfaces with good tire traction.
>>
>>My headset is tight, but I am beginning to wonder if my front fork is
>>too soft? It is a normal Tange MTB steel fork (a couple of years old)
>>
>>I am looking for some advice. Should I buy a new stiffer fork, or
>>perhaps even a suspension fork?
>>
>>Karl Lillevold
I think there are a lot of people misunderstanding Karl's problem (please feel
free to correct me Karl :-) ) because I have the same "problem" with my `92
Spec RockHopper (DirectDrive CroMo fork). If I brake REAL hard on pavement,
the fork (NOT the brakes) flexes in a forward/backward motion (the same as
above). The harder I brake, the more my front tire wants to lock up (I adjust
my weight so I don't go over the top). I tend to believe that the fork is
okay, I just don't brake so hard on concrete. (At least) Mtn bike forks are
designed to flex - soaks up bumps a little better that way (and the bike lives
longer).
Karl, I don't think there's anything wrong with your fork, unless you have
treated it like sh!t and ran into a couple trees or something....
My 2 cents......
Scott
-----------------------
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Computer Science Undergraduate,
University of California, Riverside
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