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Fork rake on fork swap - can I swap these forks?

5K views 15 replies 7 participants last post by  BentChainring 
#1 ·
I have a 2005 Bianchi FG Lite with the curved fork and what I believe is a 43mm rake. I really want to switch it to a straight blade fork and I can pick up a straight blade Bianchi fork that they use on the Ducati bikes they make. However, that fork has a 45mm rake and I know absolutely nothing else about the forks used on the Ducati bikes made by Bianchi. Can I use this fork on my FG Lite, or should I stay away from it?

What will the increased rake do to handling? I am assuming that it would make the bike handle quicker, which would be fine by me because I plan on using it as a crit bike.
 
#2 ·
If you run the numbers the "fork flop" will increase. What does this mean? It means that as you lean the bike, the force your hands feel is greater. So you will have a bit more "road feel".

Will it matter? In my opinion, no. It may feel slightly different the first couple times. You will adapt quickly.

Have fun.

nK
 
#3 ·
fork flop??

SLOEIT said:
If you run the numbers the "fork flop" will increase. What does this mean? It means that as you lean the bike, the force your hands feel is greater. So you will have a bit more "road feel".

Will it matter? In my opinion, no. It may feel slightly different the first couple times. You will adapt quickly.

Have fun.

nK
If by "fork flop" you mean steering trail, more rake reduces trail and makes the steering a bit quicker, assuming that both forks are also the same length. If there happens to be a length difference, it could increase or decrease the effect of the additional rake.

More fork rake (and less trail) requires less rider input to effect a change in direction, not more.

The formula for steering trail is (R/tanH) - (rake/sinH), where R is the tire radius and H is the head tube angle. You'll notice that any amount of rake reduces trail.
 
#4 ·
The bike was designed around a 43 MM rake. I would assume that you bought the bike at least partially because you liked the way the bike handled. Why would you change that?

Len
 
#5 ·
All the forks on my Colnagos are straight blade and I love the way they handle. They also have a 43mm rake like the curved fork currently on the Bianchi. I'd love to get the new Bianchi FF16 straight blade fork, but I cannot find it anywhere and the price is right on this Bianchi Ducati fork (i.e., $150). Plus, this frame is all aluminum and it is going to be my main crit bike. So, a little faster steering would be a little nice with 100+ corners a race.

The reason I bought the frame is because I have always wanted a Bianchi and I got it brand new on ebay for $750. Plus, the geometry was pretty close to my Colnago Cristallo, so I figured it would fit and handle about the same.

I think I am going to get the Bianchi Ducati fork and see if I can actually notice any difference in handling.
 
#6 ·
illusion

fabsroman said:
All the forks on my Colnagos are straight blade and I love the way they handle.
Straight vs. curved fork has no effect on handling. A curved fork of otherwise identical construction MIGHT flex a tiny bit more longitudinally than a straight one, but most forks don't flex much at all anyway. The handling you like on the Colnagos is a function of geometry, including head angle and fork rake. Whether the fork is straight or curved is mostly a matter of fashion, including a miniscule weight saving. Where the fork goes between the steerer junction and the dropouts is irrelevant, if it puts the dropouts in the same place.
 
#8 ·
Dave_Stohler said:
"Fork flop"? Now there's a technical term they don't teach you in engineering vehicle dynamics courses.......
Thats exactly where I learned it. Single Track Vehicle Dynamics...rather "bicycle design"

Straight vs. Curved forks do make a difference in handling, if your not changing the Head Tube Angle. The stiffness change in carbon forks....Ill have to look into this, a few friends spent a year bending and breaking forks for a senior project. I dont recall the conclusions at this moment.

C40, yeah your right, youll reduce trail. Decrease fork flop... So your "feel" of your leaning will go down, making handling "quicker". Its not that your actually handling quicker, but the dynamic forces resist you less.

nK
 
#9 ·
I have read that there is a difference between the handling of curved forks and straight forks. The curved forks tend to push back on the forward momentum of the bike when encountering bumps whereas the straight forks tend to push upwards when encountering bumps. As far as the fork rake and related trail, I understand there is no difference in trail if the rake and length to dropout of a curved and straight fork are the same.

I've decided I'm going to get the straight blade fork and see how I like it. For a little over $150, it won't be too expensive to chuck it if I hate it.
 
#10 ·
Confused :(

fabsroman said:
I have read that there is a difference between the handling of curved forks and straight forks. The curved forks tend to push back on the forward momentum of the bike when encountering bumps whereas the straight forks tend to push upwards when encountering bumps.
This conception can only be described as propagation of error. There is no basis for such a statement, either in the fundamentals of mechanical design or in the practical experience of riding bikes. Any differences in the behavior of straight and curved blade forks are due to differences in the stiffness of the fork. Where ever you read this, you should doubt any other claims they might have made, as they are obviously talking through their hat.
 
#11 ·
Sad thing is while I have a good memory, I don't have a great one, and thus cannot remember exactly where I read that. Not being an engineer, I wouldn't have come up with it on my own though. I'm still going to buy the fork for the looks and to see if a 45 degree rake is any different than a 42 for crits.
 
#13 ·
I hit 40+ on almost every ride I do, even in training. Bianchi uses this fork on its Ducati model with the same headtube angle as my FG Lite (i.e., 72 degrees). I would seriously doubt that Bianchi would be using a fork that is unstable on the Ducati model at 40+ mph.

How can I figure out the trail of the bike to see if it is at least 55? I know I saw formulas somewhere, but I don't think I know the length of the fork, which will probably be a problem in the calculation. However, I can always ask the seller for the length.
 
#14 ·
Dave_Stohler said:
It will be. You'll be less stable at speeds with a 45mm rake, but more agile at low speed maneuvering. If you plan to do ANY high speeds at all, you need at least a 55 of trail. Anything less will cause serious instability over 40 mph.
I don't understand the statement, that you need at least 55 (mm i assume) of trail to be stable at speeds over 40mph. I have seen recumbents (which are inherently less stable) with less trail reach over 70mph.

The stability of bicycles typically improves with speed above a certain point, that point is generally around 3-5mph.

As with bicycles in general. As long as you are riding someting WITH some trail (and even sometimes without) you will get used to the steering response fairly quickly. It will feel different, but you should be able to compensate in a relatively short period of time.

If you can find it "Lord of the Chainrings" by Bill Patterson describes the dynamic equations I am basing my statements on.

nK
 
#15 ·
Recumbents are, almost without exception, closer to the ground than a conventional bicycle is. As a result, the length of the 'lever arm' from the road up to the center of gravity is much shorter, therefore amplitudes of cyclical motions will be less. Also, recumbents tend to have greater (or much greater) wheelbases.

Having no knowledge of just how much trail a recumbent SHOULD have, I would guess that it could be as stable with roughly half the trail of a common bicycle. Am I correct?
 
#16 ·
Dave_Stohler said:
Recumbents are, almost without exception, closer to the ground than a conventional bicycle is. As a result, the length of the 'lever arm' from the road up to the center of gravity is much shorter, therefore amplitudes of cyclical motions will be less. Also, recumbents tend to have greater (or much greater) wheelbases.

Having no knowledge of just how much trail a recumbent SHOULD have, I would guess that it could be as stable with roughly half the trail of a common bicycle. Am I correct?

You actually have it backwards. This is a very common misconception.

Stability of bicycles increases:
increasing CG height
decreasing wheelbase

A bicycle is an almost stable vehicle. If you push one at a high rate of speed straight, it will stay up for a while, then inevitably turn into a spiral and fall. Your job as rider is to correct for those little bumps that caust it to turn. You do that by rolling and yawing the bicycle.

The shorter the wheelbase the more "yaw authority" you have, meaning, for X amount of roll you obtain more yaw, thus correcting the fall.

The higher your CG, the slower the oscillations in roll. IE you have more time to correct for a fall. THink of it as an inverted pendulum. A short pendulum has a fast period, while a long one has a long period of oscillation. It even surprised the hell outta me to see people ride bikes with the seat 6 ft off the ground. They say they are quite easy to ride (just dont use the brakes too much.

The specific amount of trail necessary is generally more on a recumbent. But the handling is different. You have to get used to the fact that you lose yaw authority, so you have to roll the bike more to correct and turn.

A big part of this is handlebar width. So a poor handling bike can be made a bit better by adding wider handlebars. And in the ened, it doesnt matter how badly a bicycles stability is, someone can always learn to ride it, maybe not well...but decently.

nK
 
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