Preferred fork rake
Joe Gross
IRD Mosaic 57 fork. I've previously waxed poetic about this fork due
to it being carbon, has room for a 57mm standard reach caliper brake,
and has eyelets and room for a front fender. It's served me well for
many brevets as well as PBP last year. The fork has a 43mm rake.
One thing I'm not trilled about is the stability of the bike's front
end. I cannot ride without any hands for even a moment for fear of the
front end starting to shimmy badly, certainly leading to a crash.
I also own a Specialized Roubaix with a 49mm rake that is *very*
stable, although less appropriate for randonneuring than the Airborne.
I'm considering replacing the fork with a similar fork but going for a
49mm rake. It looks like my options for the same type of fork with
larger rake are a semi-custom modem from Seven or Wound Up that will
run me around $500.
Before I spend the money and time to try something that may or may not
help I'd like to gather opinions on preferred rake and whether
increasing rake will help stabilize the bike.
Thanks,
Joe
Dave Pyle
For a given bike, a larger rake means less trail. But the head tube
angle is also part of the equation so the 49mm fork on your
Specialized may not produce the same results on the Carpe Diem. Once
of the best short explanations I have seen of trail and its effects is
from Tom Kellogg <http://www.spectrum-cycles.com/612.htm>. In this
article he does not consider weight on the front end, so if you use a
handlebar bag that will introduce another factor.
Dave
littlecircles :: mikeb
is very stable, even with the dual Schmidt lights mounted. The head
tube is 72 degrees and the fork offset is 50 - according to my
paperwork the 'trail' is 58.2mm.
I cannot ride the bike no-handed with an Ortlieb handlebar bag - the
load is high and the front end shimmies. I've inquired about a
different fork from IF to make a porteur style rack and bag work -
steel forks start at about $450 and can be set up for whatever brakes
you choose (I'm using the Shimano long reach).
-Mike
AlexY
distribution of your Airborne vs. Roubaix.
You can do this by comparing the ratio of the chainstay length to the
front-center on each bike.
I believe more weight on the front wheel makes it easier to ride with
no hands, even with the same
steering geometry. At least that's my observation.
- Alex
NickBull
series of articles on fork rake, frame geometry, tires, etc. In
particular, Vol. 5, No. 3 (Spring 2007) had an article called How to
Design a Well-Handling Bicycle that would serve as a pretty good
summary article of what they've learned over the years. Well worth
buying the back issue, if you're considering spending $500 on a
fork.
For that matter, the spring 2008 issue that is supposed to arrive in a
couple of weeks includes several more potentially relevant articles:
26 Fork Blades Optimized for Comfort and Speed
43 Top Tubes, Planing and Shimmy
44 Curing Shimmy on a Bike
Of all the publications I spend money on, Bicycle Quarterly is without
doubt the best bang for the buck for randonneuring interest.
Nick
On Apr 11, 4:09 pm, "littlecircles :: mikeb" <mike.bega...@gmail.com>
wrote:
Mike Dayton
Lathe, a local builder and randonneur. When we planned it out, he
asked a lot of questions about what kind of front bag I'd be using and
how often I intended to ride with it. We also discussed my preferred
tire size. He adjusted the fork rake accordingly. It was nice to work
with a builder knowledgeable and experienced on those issues. And one
you see out there on the course.
I have a Silk Hope frame, built in 1971 or so, that suffers from the
wheel flop that Jan describes. The bike wants to steer itself,
especially at slow speeds. But it's still fun to ride because it's a
bicycle.
Mike / Raleigh, NC
On Apr 12, 10:43 am, Jan Heine <hein...@earthlink.net> wrote:
> >From: Joe Gross
> >One thing I'm not trilled about is the stability of the bike's front
> >end. I cannot ride without any hands for even a moment for fear of the
> >front end starting to shimmy badly, certainly leading to a crash.
>
> >I also own a Specialized Roubaix with a 49mm rake that is *very*
> >stable, although less appropriate for randonneuring than the Airborne.
>
> oscillation of the steering, back and forth. It occurs on some bikes,
> not on others. Shimmy with the hands on the handlebars occurs with
> some riders, while other riders on the same bike have no problems.
> (No-hands shimmy appears to be less rider-related.)
>
> Nobody seems to know why shimmy occurs, but there may be ways to cure
> it. In the latest issue of Bicycle Quarterly (Spring 2008, Vol. 6,
> No. 3), we report how we got rid of shimmy on a bike with a simple
> headset swap, replacing a Chris King with a needle-bearing
> Stronglight. Others have reported similar success, but of course, I
> can't guarantee it.
>
> Regarding stability, a bike with too little fork rake for its
> intended purpose will have too much wheel flop: It will tend to veer
> off course when you try to ride no-hands, or when you ride slowly up
> a hill. Increasing fork rake can stabilize the bike, but it's not as
> simple as that. For narrow tires (25 mm or narrower), this may not
> work well, because with a low-trail design, you need the stabilizing
> effect of wide tires. Also, we found that "intermediate" trail seems
> to work less well, especially with a handlebar bag, than either
> "high" or "low" trail - see an article on that subject in Bicycle
> Quarterly Vol. 4, No. 3.
>
> Before you order a new fork, I suggest you read our article on
> front-end geometry that goes into lots of details, including sample
> geometries (Bicycle Quarterly Vol. 5, No. 3).
>
> Based on that information, a number of readers have had new forks
> made for their existing bikes, and found that stability, especially
> with a front rack, has been much improved, with no detriment to the
> high-speed handling or cornering. As long as your bike's head angle
> is around 73 degrees, there is no reason why you could not get the
> geometry of a true randonneur bike. If you get a steel fork, you even
> can get a custom rack with it, including light mounts, fender
> braze-ons, and everything else you might want. Carbon forks limit
> your options a bit more, but it appears that some forks offer the
> amount of rake you'd need.
>
> Jan Heine
> Editor
> Bicycle Quarterly
> 140 Lakeside Ave #C
> Seattle WA 98122www.bikequarterly.com
russell...@yahoo.com
Mr. Brandt on shimmy. My three road bikes all shimmy when riding
above 25 mph no hands coasting down a hill when I am using a Mavic
Open Pro front wheel. 32 spoke, 14/15 3 cross, brass, Record or
Chorus hub, 23 or 25 mm Continental tire wire or kevlar. Same bikes
with a much heavier Vento front wheel, no shimmy when riding no hands
above 25 mph coasting down hills. Vento wheel is 38 mm deep aluminum,
14 spokes, heavy. The bike frames/forks range from 1995 Waterford
1200 with steel fork, 2004 Cannondale CAAD7 with carbon fork, 2005
Litespeed Tuscany with carbon fork. All have 40mm rake. All have
73-74 head tube angles. All are 58-59 cm c-t frames. Simply putting
a knee against the top tube when coasting no hands prevents any shimmy
from occurring. No shimmy occurs when riding at any speed with both
hands on the bars.
Before spending money on a new fork, you may want to experiment with
different wheels if you have not done so already. Heavier, lighter
wheels. Heavier, lighter, wider, narrower tires. See if you can
achieve the same desired outcome with a different front wheel for a
lot less money and trouble than a new fork.
Robert Magyar
After experience shimmy a few times (4 times on 3 different bikes) I did a bit of research on it.
http://draco.nac.uci.edu/rbfaq/FAQ/8h.5.html
Speed Induced Shimmy:
Brandt's article paints a clear picture of what is happening, and switching headsets may help dampen the effect,
but take great care when trying out a new set of stiff wheels with hard tires, as these tend to make the effect even worst.
(I read an account on the ultra list, where a guy didn't know about laying one leg against the top tube, and ended up crashing after loosing control due to a shimmy.)
Cross Wind Induced Shimmy:
But I did not experience Speed Induced Shimmy (I reached past 45mph on numorous occasions with no problems), but rather Cross Wind Induced Shimmy, same problem and same solution, but different cause. So if you have had issues with speed induced shimmy, you should also watch out for crosswind induced shimmy, when you are descending. On a good decent, one normally feels heavy on the bars and with good weight on the front wheel, but when I am having a problem with a cross wind, I notice that I am suddenly feeling light on the bars, and that is when I might start experiencing a shimmy, now if I start feeling light on a decent, I lay my knee against the top tube and all is well. (I believe the cross wind somehow is creating lift, that is un-weighting the front wheel, thus setting up conditions for a oscillation to start.)
Bob
> Date: Sat, 12 Apr 2008 07:43:27 -0700
> To: ran...@googlegroups.com
> From: hei...@earthlink.net
> Subject: [Randon] Preferred fork rake
>
>
> >From: Joe Gross
> >
> >One thing I'm not trilled about is the stability of the bike's front
> >end. I cannot ride without any hands for even a moment for fear of the
> >front end starting to shimmy badly, certainly leading to a crash.
> >
> >I also own a Specialized Roubaix with a 49mm rake that is *very*
> >stable, although less appropriate for randonneuring than the Airborne.
>
jimg
As an experiment, before spending the big $$$ on a custom carbon fork,
you could get a steel low-trail fork from Kogswell and try that on
your bike -- I think they cost around $80-100.
Today I hope to finish the assembly of my Kogswell 700C P/R frameset.
I chose the 40mm trail fork option, but I have an extra 30mm trail
fork to test on it also. Once I get this thing built and dialed in,
if you're interested, we could meet up sometime and you could take a
test ride on it.
photo at http://www.flickr.com/photos/jimg/2402802291/
Cheers,
-Jim G
Harry Spatz
stem is on one side. I know that that is a problem with the boutique wheels
that require ultra long (heavy) valve stems. The wheel will settle heavy
side down. Tape some weight opposite the heavy spot until the wheel is in
balance, then do the same high speed test.
Harry Spatz
Mike Biswell
>From: Harry S
>Sent: Tuesday, April 15, 2008 7:49 AM
>Subject: [Randon] Re: Preferred fork rake
>Perhaps the shimmy comes from wheel imbalance. After all, the heavy valve
>stem is on one side. I know that that is a problem with the boutique
>wheels that require ultra long (heavy) valve stems.
????
Ya learn "something" new every day...unbelievable. Go rando!
????
Regards!
Mike
NickBull
(http://www.rivbike.com/products/list/odds_and_ends#product=31-371)
180 degrees opposite from the valve for this reason. It doesn't
perfectly offset the weight of the valve, but it's close.
Mike Biswell
>From: NickBull
>Sent: Tuesday, April 15, 2008 9:25 AM
>Subject: [Randon] Re: Preferred fork rake
>I put my Rivendell spoke reflector
>(http://www.rivbike.com/products/list/odds_and_ends#product=31-371)
>180 degrees opposite from the valve for this reason. It doesn't
>perfectly offset the weight of the valve, but it's close.
Whatever the effect, it's not the overall cause of 'shimmy,' nor need it be
transmuted into a luddite swipe at bikes.
Shimmy did indeed exist prior to the 'boutiquing' of bike wheels.
Regards!
Mike
russell...@yahoo.com
On Apr 15, 9:48 am, "Harry Spatz" <hsp...@baystatescale.com> wrote:
> Perhaps the shimmy comes from wheel imbalance. After all, the heavy valve
> stem is on one side. I know that that is a problem with the boutique wheels
> that require ultra long (heavy) valve stems. The wheel will settle heavy
> side down. Tape some weight opposite the heavy spot until the wheel is in
> balance, then do the same high speed test.
the wheel that does not shimmy. The hand built 32 spoke Open Pro
wheel with short valve stem is the one that does shimmy. Exact
opposite of what you say.
The shimmy is due to my weight, my weight distribution on the bike,
and my frame size. The lighter wheel has slightly less weight on the
front. The heavier wheel has slightly more weight on the front.
About a half pound per wheel more for the Vento over the Open Pro.
This weight distribution changes the dynamics of the spring (frame).
>
> Harry Spatz
>
>
>
> -----Original Message-----
> From: ran...@googlegroups.com [mailto:ran...@googlegroups.com]On Behalf Of
>
>
> - Show quoted text -
Harry Spatz
one. The heavier wheel would offer more rotational inertia to the shimmy.
I read on another forum of someone with a shake problem with wheels
requiring 60 mm valve stems. He fixed his problem by using a short valve
stem with a plastic extender that was lighter, so I know that at least in
some cases, imbalance leads to problems. Another possibility is an out of
round tire or rim. Perhaps, you know what causes your shimmy, (or perhaps
not), but there might be other causes and I do not think you can dismiss
them a priori. The tire-balancing people claim that an automobile wheel and
tire (many times heavier than a bicycle wheel) that is out of balance 1/2
oz. can exhibit shimmy and they use very expensive dynamic balancing
equipment to eliminate this. We don't need such sophistication due to our
lower speeds and un-sprung suspensions, but that does not mean that balance
is unimportant.
Bill Gobie
involving springs (frame, fork, rider) and masses (front wheel,
rider). It is not surprising that swapping a heavy wheel for a light
one alters the resonant frequency (speed at which shimmy occurs).
As for wheel balancing, an out of balance wheel produces a radial
force that rotates with the wheel. If the wheel is true enough to be
rideable, the force is virtually coplanar with the steerer axis.
Therefore the force produces no torque about the steerer axis, so it
cannot turn the wheel. Try it: spin a wheel in your hands, and move
it up and down. It will not turn. You will probably notice the
wheel is unbalanced. It's safe to say all bicycle wheels are
unbalanced, with no ill effect.
I don't know what to make of the valve stem story except to say
something else was probably going on.
Car wheels are wide enough to be out of balance side to side,
resulting in the out-of-balance force being significantly offset from
the steerer axis. In this case, through the mysteries of rotational
dynamics, shimmy can result. Also, as Brandt points out in the link
below, car tires are softer (more compliant), so they bounce if
unbalanced.
http://yarchive.net/bike/wheel_balancing.html
Bill Gobie
Robert Magyar
> From: bi...@billandlorene.com
>
> Jobst Brandt's article on shimmy makes clear shimmy is a resonance
> involving springs (frame, fork, rider) and masses (front wheel,
> rider). It is not surprising that swapping a heavy wheel for a light
> one alters the resonant frequency (speed at which shimmy occurs).
And if it was just a wheel balance issue, then one would feel the problem at all speeds, it would just be more noticeable as speed increased.
But with speed induced shimmy all is well until the magic speed is reached.
So if one changes out wheel sets, is trying different tires, a new bike, or has redistributed the load differently on your current bike, take care when you go ripping down your next high speed decent,
and if you start to shimmy, remember to lay your knee/leg against your top tube.
Bob
Harry Spatz
problem a cheap and easy experiment to try to fix their problem. I have
never had a shimmy problem, never tested shimmy problems, nor do I advocate
for or against any particular solution. I simply thought that it was
cheaper and easier to give the balancing thing a try before replacing forks,
head sets, wheels, etc. Here is where I got the idea that balancing might
help:
http://www.bacchettabikes.com/forum2/fb.asp?m=14913
This guy went to the trouble of contacting Zipp and getting them to give him
a solution for getting rid of his balance problem. He implemented their
solution and reports that the problem went away. Is it the placebo affect?
Perhaps, I don't know. He must have been quite convinced since the solution
required that he drill his expensive wheels!
I did look at Zipp's website to see if I could get a clue. I did. It is
here:
http://www.zipp.com/Support/Maintenance/FAQ/tabid/81/Default.aspx
Scroll down to the bottom and you will find Zipp's instructions on how to
balance their wheels. Are they trying to solve a problem that does not
exist? Perhaps, but why would they do that?
As to Jobst Brandt's analysis, he may be correct, or maybe not. I see no
evidence that he or anyone else has done a double blind experiment testing
an unbalanced wheel with shimmy against the same wheel balanced, in the same
situation that manifested the shimmy. Maybe that's something that Jan Heine
would like to tackle in his Quarterly. A simpler less precise way would be
for people with shimmy to balance their wheels and chime in as to whether it
helped or not.
Incidentally, I do understand and agree that resonance can play a factor
here. In fact one of the first things shown to us at RPI as first year
engineering students was this video:
http://www.youtube.com/watch?v=HxTZ446tbzE
This bridge was designed properly given what was known at the time.
Obviously, the effects of resonance were not known then, but this disaster
changed that and many famous bridges were modified subsequently.
Mike Biswell
Isn’t the common thread the fact that bikes typically only shimmy going downhill, and not that specific to speed?
Not presuming to know. Also, why don’t Eddy Merckx made bikes ever have a shimmy? “Low” bottom bracket on Eddy’s bikes? He seems to have figured this out…
Await disproof.
Regards!
Mike
Bill Bryant
YMMV,
Bill Bryant
Bill Gobie
On Apr 16, 2008, at 12:18 PM, Mike Biswell wrote:
> Isn’t the common thread the fact that bikes typically only shimmy
> going downhill,
Downhill is typically when one reaches the speeds where shimmy
occurs. In some cases pedaling seems to dampen shimmy, so even if a
bike rides fine when pedaling on the flat or a slight downhill, it
can shimmy when coasting downhill at the same speeds.
> and not that specific to speed?
>
I was going to write, "One of my bikes used to shimmy very
predictably at 23 mph." But in thinking it over, the shimmy only
happened on a particular stretch of somewhat rough pavement. Other
owners of this model bike either report no shimmy, or have
experiences like mine. Some in the latter group claim not to mind
(ohmigod!). So there are lots of factors at play: bike, rider, road
surface. I think someone mentioned crosswind, too.
Seeing the wheel dance was quite a sight. A new fork fixed the
problem and the bike is solid up to 40 mph, which is as fast as I
care to go.
Bill Gobie
Bill Gobie
On Apr 16, 2008, at 12:04 PM, Harry Spatz wrote:
> My idea about bringing up the balancing was to give people with a
> shimmy
> Here is where I got the idea that balancing might
> help:
> http://www.bacchettabikes.com/forum2/fb.asp?m=14913
Now I remember reading this thread. But the problem was not shimmy,
it was that the rider could feel the rear wheel thump due to the
weight of the long stem. On that bike (a Bacchetta Aero recumbent)
the seat stays transmit rear axle motion directly to the top of the
seat. So a bit of vibration that a DF rider's saddle and bum would
soak up might feel like chest compressions on this type of recumbent.
> This guy went to the trouble of contacting Zipp and getting them to
> give him
> a solution for getting rid of his balance problem. He implemented
> their
> solution and reports that the problem went away. Is it the placebo
> affect?
> Perhaps, I don't know. He must have been quite convinced since the
> solution
> required that he drill his expensive wheels!
>
I do believe that he solved the thumping problem.
>
> As to Jobst Brandt's analysis, he may be correct, or maybe not.
Brandt is a capable engineer and I generally trust his *technical*
opinions. Unfortunately he does not distinguish between objective
facts and subjective experience, so some of his writings are mere
rants. See for example his articles about leather saddles, helmet/
eyeglass mirrors, and recumbents.
> I see no
> evidence that he or anyone else has done a double blind experiment
> testing
> an unbalanced wheel with shimmy against the same wheel balanced, in
> the same
> situation that manifested the shimmy. Maybe that's something that
> Jan Heine
> would like to tackle in his Quarterly. A simpler less precise way
> would be
> for people with shimmy to balance their wheels and chime in as to
> whether it
> helped or not.
On further consideration it might be possible that imbalance could
exacerbate shimmy once some other factor initiated shimmy. Rather
than working out the math I agree it would be great to hear from
people who have tried balancing their wheels.
>
> Incidentally, I do understand and agree that resonance can play a
> factor
> here. In fact one of the first things shown to us at RPI as first
> year
> engineering students was this video:
> http://www.youtube.com/watch?v=HxTZ446tbzE
> This bridge was designed properly given what was known at the time.
> Obviously, the effects of resonance were not known then, but this
> disaster
> changed that and many famous bridges were modified subsequently.
"Galloping Gertie." I rode across her replacement on a brevet once
very late at night -- not the smartest thing I've ever done as I
almost toppled over the vestigial pedestrian barricade into the
traffic lanes. Anyway, I think it's more accurate to say it was not
thought resonance could occur, and so the design was not sufficiently
analyzed and tested. Cutting-edge designs are still a bit of shot in
the dark; the new Millennium Footbridge in London had to be shut down
for modifications after it make pedestrians sick.
Bill Gobie
Philip Kim
I used to ride a low trail bike, which was nice and stable with front load, but slightly twitchy unloaded, I believe it had a rake of 69.
I have since moved to a mid trail bike and it has more neutral handling, still can ride without hands loaded, no shimmy. Unloaded rides pretty nice. It's nice an stable with a front load, but I don't really load more than 7 or 8 pounds in my rando bag. Currently riding a 55 rake with 71 headtube angle. I think the trail is around 56.
Also here's a link to a trail calculator. http://yojimg.net/bike/web_tools/trailcalc.php.
Jason Marshall
Joe Gross
Just curious - why is this a pinned subject?
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