Re-raking of fork

Jayves

Speeding Up
Nov 20, 2009
115
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Yokohama
jayves-rando.blogspot.jp
#1
I want to increase the stability (aka, less twitchiness) of my bike a little bit by increasing the wheel base and decreasing the trail and one option is to replace the fork with more rake/offset. However, I cannot find a 700c steel fork or even a carbon fork with greater than 45mm rake (Some fork also doesn't publish the rake online). I probably won't go this route if I can find a fork with more rake.... Also, I'm not sure that will be the 'new' rake but is willing to experiment/test.

My question to he group is, does anybody has experience re-raking a fork and tools to use or knows a shop that do this kind of work?
 

GSAstuto

Maximum Pace
Oct 11, 2009
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www.roadfixie.com
#2
Actually keeping everything the same - if you decrease rake you will increase trail and make the bike more stable. We adjust trail to make the bike handling as neutral as possible providing a consistent touch regardless of the speed travelled. I have several different geometry forks you are welcome to try out. Patrick has used a few and experienced the differences especially between ascending and descending. Only a few mm makes quite a difference. To adjust your steel fork, I'd go to Tanabe san. In addition to adjusting the fork, he can check overall straightness of the frame which can also affect certain handling aspects.
 

Jayves

Speeding Up
Nov 20, 2009
115
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Yokohama
jayves-rando.blogspot.jp
#3
Searching for Tanabe-san in this site resulted in Kalavinka Shop in NakaMeguro. I looked at their website and this is the shop I need to visit! It seems they provide different sort of steel frame modifications.

I visited Cherubim once on replacement steel fork but they only make 1".
 

GSAstuto

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Oct 11, 2009
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#4
Yes - that's it! If you have time and can visit with me or my staff then maybe it's a little faster since we do some work together with Tanabe san :) He is a real master and in my opinion the best real builder in Japan. He can 'read' your bike and make the appropriate adjustment perfectly.
 
Jun 9, 2011
241
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tokyo
#6
there's more to front end handling and stability than just wheel base and fork rake. head tube angle makes a huge difference as does the overall rigidity of the fork blades. stem length, body position and weight distribution can also affect stability.

under what conditions are you experiencing instability or twitchy handling? what type of riding do you normally do and what are the road conditions like? do you know your bike's current geometry (HT/ST angle, offset, axle to crown length, BB drop)?

messing with the offset on a fork can also change your stack height which will change your HT angle and BB drop. the ST angle will also change but not so significantly you can't compensate with a saddle adjustment. and change in the HT angle, however, can make a big difference in handling.

I've got forks that range from around a 32mm offset to 45mm and i've found that as long as the head tube angle is between 73° and 74° degrees there's not a huge difference. I've had problems with instability at high cadence (>150rpm) with HT angles under 73°.

For speeds under 30kph i find that an offset of about 32-35mm is more stable and allows for tighter handling. over 35kph an offset over 43mm tends to track better on straights and feels more stable in corners.

The fork I'm riding now is full carbon and has 45mm of offset and is 170mm axle to crown. it gives me a HT angle a little steeper than 73° but probably not quite 73.5°. I've tested another very similar carbon with with the same geometry but more compliant blades and I found the stiffer fork more stable at low speeds and comparable at higher speeds. I don't think I'd want to go further out front than a 45mm offset.

The best thing to do is try forks at the extremes and see how your bike's handling changes. I've got a short steel fork and long carbon fork you're welcome to try. the short fork has around 32mm of offset and 165mm axle to crown. the long fork has 43mm of offset and is 175mm axle to crown. these two will result in different stack heights and alter your HT angle which should help determine if you want to ultimately move your HT angle one way or the other.
 

Jayves

Speeding Up
Nov 20, 2009
115
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Yokohama
jayves-rando.blogspot.jp
#7
Patrick, the bike I'm trying to tweak is 2007 Lemond Sarthe (see attached image). I like this steel bike and love the ride. I think it was designed as a road racing bike. However, I have been using it more for brevet rides and light touring and ride characteristic that I'm looking for is a little bit different. When I ride a brevet, I have Carradice Pendle saddle bag and Montbell handlebar bag attached. Depending on the distance of the brevet, I could load a few kilos on both bags.

At first I was using the default factory carbon fork. I can manage the handling OK for the first 10-12 hours but after 16 hours and beyond on the saddle, any small twitchines (or call it tight handling), become a balance struggle. This is especially on a very slow climb or when I'm very tire/half-sleep on the bike. I feel I always need to 'correct' the 'squirrelly' steering.

Then I change the fork to a reynolds steel fork (45mm offset) with longer blade (can accept 28mm tire with fender). I guess this might have lower the HT angle and the ride become more stable than the factory carbon fork. I wanted to experiment more (after my brevet series) and compare the stability of a fork with different rake especially when my front saddle bag is 'loaded'.

I would like to try you long carbon fork. I guess you are using a long reach caliper?

(Note that I don't know and cannot find the geometry of my bike size (53) as Lemond Bikes website removed all the references of all their bikes.)
 

kiwisimon

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Dec 14, 2006
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#8
I think you need a less race styled frame. Something with longer chainstays and the stability will be much better than fluffing around with the front end. Horses for courses and it looks like you are asking a thoroughbred to do a pack horses job. Save the Lemond/Trek for day jaunts and find another frame. The money you pay on customisation can easily pay for a frame.
 

Jayves

Speeding Up
Nov 20, 2009
115
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Yokohama
jayves-rando.blogspot.jp
#10
@kiwisimon, ah! thanks for the archive reference....

I'm still hoping that bending a steel tube is *a lot* cheaper than a new frame and I still have the old carbon fork for these day jaunts. Probably need to buy a similar fork crown race so I can easily swap forks rather than hammering it in/out :)

Also, I don't need to worry about the '# of bikes rule' ... for now. :)
 

GSAstuto

Maximum Pace
Oct 11, 2009
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#11
Start here:

http://socrates.berkeley.edu/~fajans/pub/pdffiles/SteerBikeAJP.PDF

To increase low speed stability you will typically increase HT angle or decrease fork rake. Both will increase trail which directly affects the castering component of steering.

Too much trail is not good as well, the bike will feel heavy and at low speeds actually feel like it will flop over once you get past the highest pivot point. And at higher speeds it will be hard to initiate turns.

Increasing wheelbase also has an affect - mainly on the capsize speed of the bike. Increase the wheelbase and the capsize speed will reduce.

Lastly - there is pneumatic trail. This is the trail that is induced by the tire compliance itself. Choosing slightly fuller profiles and /or lower pressure will result in higher pneumatic trail.

So - you can see that many things combined together will affect the overall stability and rideability of the bike. Some of them are somewhat fixed, but others may be easily changed by simply changing a few parts (like wheels, tires, fork).

One thing I'm waiting for (patiently at the house) is the arrival of a new set of 23x23 wheels. I find when I'm riding my typical training wheel (Conti Sprinter Gatorskin) which has a fuller profile, compared to a higher performance tire (Evo Corsa CX) , the Conti is more stable feeling under most riding conditions, making it very comfortable on longer rides especially when I get tired and start to lose my core strength and stability. My current rims are about 19-20mm and with the 23mm I'm looking for a slightly broader footprint with the same profile (23C) tire. This should provide reduced rolling resistance at lower speeds AND slightly reduced pneumatic trail. Thus, I can ride at little lower pressure - increasing compliance , without sacrificing overall performance. Interesting, huh?

Pete will attest to the subjective advantages of such a system as he followed me down a reasonably fast descent during which, even under some fairly tricky corners, my bike handled quite well both in terms of stability through a corner and actual corner initiation. Now, then, on my older 'Pro Tour' carbon bike, descending was a 'wing and a prayer' by comparison. Mainly because trail was less, slightly shorter wheelbase and the fork had a strong tendency to 'wind up' on deceleration and forward loading which caused it to literally bump steer. My new(er) bike has about 1cm longer overall wheelbase, about 5mm more trail and much stiffer fork. The difference is amazing. Combined with a little more compliant tires and I can negotiate any corners at much higher speed and confidence than my previous bike.
 

kiwisimon

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Dec 14, 2006
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#12
Good post Tim, also center of gravity is an important factor. Looking at the loaded bike this might also be part of the problem, front bags aren't reknowned for improving stability.
 

Yamabushi

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Jun 1, 2010
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#13
Pete will attest to the subjective advantages of such a system as he followed me down a reasonably fast descent during which, even under some fairly tricky corners, my bike handled quite well both in terms of stability through a corner and actual corner initiation.
I do so attest! Tim was on fire! I've ridden with him countless miles, and know firsthand that he is an all around competent and accomplished cyclist, but I've never seen him descend like this! I'm looking forward to seeing more of it!!
 

Jayves

Speeding Up
Nov 20, 2009
115
3
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Yokohama
jayves-rando.blogspot.jp
#14
The Lemond is probably a wrong bike for the specific brevet riding that I do. Seems also there are lots of debates out there on High Trail or Low Trail. One thing I just found is this article from Jan Heine, editor of Bicycle Quarterly.

http://janheine.wordpress.com/2011/02/10/a-journey-of-discovery-part-4-front-end-geometry/

On the comment part, there is a reader who ask the question how to try a bike with a low trail in which Heine answer:

"The second option is to take an existing bike and either have the fork re-raked, or have a new one built. As long as your bike has a head angle of 72-74 degrees, this is an easy option. It also increases your front-center, which can be a good thing if your bike has toe overlap. At the same time, you may be able to incorporate a good rack, so your handlebar bag has good support…"

I already tried the two mid-trail geometry with the current setup and the factory fork. And will experiment with the re-raked fork. Would also like to experiment with Patrick short steel fork (32mm rake). If I'm not happy with the result...... [will cross the bridge when I get there]...
 

GSAstuto

Maximum Pace
Oct 11, 2009
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#15
If you increase rake, you decrease trail which makes the bike a little lighter, especially at lower speeds. Actually, if you mount front pannier, surprisingly a little less trail 'feels' better as the bike tends to be more neutral (to a point) at low speeds. But you do have to apply more steering control due the reduced caster effect. For the last 100yrs or so, the bike has evolved to the point that the sweet spot is hardly changed. 71-74 degree head tube with 50 - 65mm of trail. Outside that range and the bike tends to feel unacceptable in terms of combined ride performance and stability.
 

Jayves

Speeding Up
Nov 20, 2009
115
3
38
Yokohama
jayves-rando.blogspot.jp
#17
The best thing to do is try forks at the extremes and see how your bike's handling changes. I've got a short steel fork and long carbon fork you're welcome to try. the short fork has around 32mm of offset and 165mm axle to crown. the long fork has 43mm of offset and is 175mm axle to crown. these two will result in different stack heights and alter your HT angle which should help determine if you want to ultimately move your HT angle one way or the other.
Just a quick update...

Thanks to Patrick, and after my series of brevet, I'm testing his steel fork for a couple of weeks now to helps me understand better how bikes handles with different front geometries. More update to follow.

IMG_0471.jpg