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Changing Fork Rake from 45mm to 41mm

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Changing Fork Rake from 45mm to 41mm

Old 12-04-09, 10:05 PM
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cazzooo
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Changing Fork Rake from 45mm to 41mm

I just switched out my road bike fork from a 45mm rake to a 41mm rake and was wondering what the ride effects this will make. I road the 41mm rake fork today for the first time. The bike seems a bit "tighter" and quicker to respond. Does this make sense?
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Old 12-04-09, 11:10 PM
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Reducing the rake offset increases steering trail. Your bike should now have a greater tendency to continue in a straight line, especially at higher speeds. I don't think it should feel quicker to respond except possibly at very low speeds.

I replaced the 43 mm rake offset fork in my Trek with a 40 mm and I could definitely feel an immediate change but it was not a big change and it felt normal after a few rides. I could have gone back to the 43 but decided to stay with the 40.

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Old 12-04-09, 11:14 PM
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It also decreases the wheelbase.

Steeper rake increases the twitchiness feeling in bikes. Imagine how much of a pain in the ass it would be to steer with a 90d rake (fork goes straight to the ground from the head tube) .. that's because it would dive in too fast.

https://www.tonyfoale.com/Articles/RakeEx/RakeEx.htm

Last edited by tonski; 12-04-09 at 11:20 PM.
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Old 12-05-09, 08:35 AM
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Rake (in bicycle terms) is not an angle, so there is no such thing as a 90 degree rake. Rake is also called offset and that is really a more proper term, since it's the distance between two parallel lines - one is the center line of the steerer and the other is a line through the center of the axle.

Motorcycles use the term rake to describe what is called the head tube angle on bicycles.

Another dimension that is often overlooked is the fork length. A slightly different fork length could cancel out or double a change made to the offset.

Steering trail is defined as (R/tanH) - (offset/sinH), where R is the tire radius and H is the head tube angle.
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Old 12-05-09, 08:49 AM
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Originally Posted by tonski View Post
It also decreases the wheelbase.
That's insignificant. A change of 4 mm out of 1000 mm is negligible

Originally Posted by tonski View Post
Steeper rake increases the twitchiness feeling in bikes. Imagine how much of a pain in the ass it would be to steer with a 90d rake (fork goes straight to the ground from the head tube) .. that's because it would dive in too fast.
As DaveSSS pointed out, rake is a linear dimension, not an angle in bicycle terms. It is the offset distance of the fork tips from the centerline of the steerer tube and, along with the headtube angle and fork length (crown race seat to center of the front axle) establishes the "trail".

For a given frame and fork less rake=more trail. The OP's fork change should have increased trail and slowed the handling at normal riding speeds but, as noted, if the fork length changed too, anything could have happened
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Old 12-05-09, 12:07 PM
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Rake offset is affected by the rake which is effectively changed if the axle moves. Motorcycles have straight forks (no offset) and the angle is really from the bottom of the head tube to the axle, the offset is just measured by the head tube angle to how far out the axle is pushed - in this case 4mm "in". With all other things being equal decreasing the rake angle decreases the trail and makes steering feel less stable (or less desire to continue in a "straight line").

Just for clarity, with bicycles you use the head tube angle to determine the rake offset. The actual effective rake angle is the line from the headtube to the axle.

Last edited by tonski; 12-05-09 at 12:31 PM.
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Old 12-05-09, 01:20 PM
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On suspension MTB forks, the dimension called "offset" is the same dimension called "rake" on road bikes. The head tube angle and rake/offset along with wheel diameter determine the trail. AFAIK, it's the same for motorcycles.
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Old 12-05-09, 01:26 PM
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All right guys which one is it. More trail equalls quicker turning or more trail equals less quick turning?
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Old 12-05-09, 02:07 PM
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Here's how I've always understood it: Head angle and trail determine how quickly the front end handles. On a track bike you'll see a 75* head angle for quicker turning (for when they slow down to a walking speed to force the other rider to take the front) and short rake to increase the trail and slow down the front ends handling at high speed. If you look at old fashion motorpace bikes from the 40's, 50's and 60's, they have negative rake/offset to have lots of trail, slow handling at 60mph is a good thing. On a touring bike with 72* head angle for slow turning, the rake/offset is longer to increase the quickness by decreasing the trail.
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Old 12-05-09, 02:18 PM
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More trail = more stability/less quick turning.
Less "rake" should mean more trail. But "rake" is a term that gets used wrong a lot (possibly because it means something different for motorcycles). The proper measurement is "offset", how far forward the center of the dropouts sit from a line extending down the center of the steerer tube (measured perpendicular to that line).

A bike with a vertical ("90 degree") fork would be very unstable. But not because there is no rake, it would be unstable because there is no trail.
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Old 12-05-09, 02:19 PM
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Originally Posted by San Rensho View Post
All right guys which one is it. More trail equalls quicker turning or more trail equals less quick turning?
All else remaining the same (and often it doesn't):

more rake = less trail = quicker handling

less rake = more trail = slower handling.
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Old 12-05-09, 10:50 PM
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If you read what I wrote you could use the motorcycle research that's been done. Blaze your own path, I don't care. The information is there but you can ignore it, come to your own conclusion, and produce an even more complex explanation.
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Old 12-05-09, 11:18 PM
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Originally Posted by tonski View Post
If you read what I wrote you could use the motorcycle research that's been done. Blaze your own path, I don't care. The information is there but you can ignore it, come to your own conclusion, and produce an even more complex explanation.
If we were discussing motorcycles you would be correct, but we're not. With motorcycles the term rake is the rake angle. This equates to bicycle head tube angle.
In BICYCLE terminology rake is offset, the distance from the front axle center perpendicular to the steering axis. Decreasing rake offset increases trail. Increasing trail increases the bike's ability to continue in a straight line.

Rake offset and steering trail have been discussed extensively in other threads. All you need to do is search the archives. Also there are many references available on the net.
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Old 12-05-09, 11:22 PM
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Originally Posted by Al1943 View Post
If we were discussing motorcycles you would be correct, but we're not. With motorcycles the term rake is the rake angle. This equates to bicycle head tube angle.
In BICYCLE terminology rake is offset, the distance from the front axle center perpendicular to the steering axis. Decreasing rake offset increases trail. Increasing trail increases the bike's ability to continue in a straight line.

Rake offset and steering trail have been discussed extensively in other threads. All you need to do is search the archives. Also there are many references available on the net.
God [or spaghetti monster] forbid you accept that the same physics applies to bicycles AND motorcycles. Sorry, I'll just stick with real physics and research done by respected professionals and reliable sources. Quit dismissing research because it doesn't look EXACTLY like what you're used to and think about it. Everything I've posted is still completely relevant and the rebuttal is "This is bicycles not motorcycles", I've explained how they can be compared, and how it would explain why the OP's bicycle feels more responsive. It is effectively decreasing his trail.

Last edited by tonski; 12-05-09 at 11:28 PM.
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Old 12-05-09, 11:25 PM
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Originally Posted by tonski View Post
God forbid you accept that the same physics applies to bicycles AND motorcycles. Sorry, I'll just stick with real physics and research done by respected professionals and reliable sources.
Take a look at the online Wikipedia explanation for bicycle trail here: https://en.wikipedia.org/wiki/Bicycle...cycle_geometry
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Old 12-05-09, 11:40 PM
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Yea I'm aware of that, the reason bicycles measure it that way is because it the bend in the fork changes effective rake angle. Please start thinking, I'm not going to spoon feed you information anymore or respond to your idiotic responses because you refute physics. Look at the colorful picture and explain how the orange rake has more trail.

Obviously not to scale but any rational thinking person [and perhaps even you] can see a trend.
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Last edited by tonski; 12-05-09 at 11:59 PM.
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Old 12-06-09, 02:07 AM
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Originally Posted by DLM View Post
A bike with a vertical ("90 degree") fork would be very unstable. But not because there is no rake, it would be unstable because there is no trail.
Agreed, I guess I should have explicitly said 0 offset.

Atl, read about the mechanical trail that is source of dissonance and since I'm more of doer than teacher, maybe that will explain it more clearly. It also clearly explains why the OP would experience what he did. Everyone is using the formula posted above, saying there is more trail, and then applying that to known research and common knowledge of mechanical trail which are 2 different things. Mechanical trail being what my crude image shows above.
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Old 12-06-09, 08:15 AM
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Originally Posted by tonski View Post
... Look at the colorful picture and explain how the orange rake has more trail....
You have a line from the crown through the axle. If should be through the head tube/fork blades without any offset (see the wiki picture). You picture isn't right for bikes. Is this right for motorcycles?
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Old 12-06-09, 08:30 AM
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Originally Posted by tonski View Post
God [or spaghetti monster] forbid you accept that the same physics applies to bicycles AND motorcycles. Sorry, I'll just stick with real physics and research done by respected professionals and reliable sources. Quit dismissing research because it doesn't look EXACTLY like what you're used to and think about it. Everything I've posted is still completely relevant and the rebuttal is "This is bicycles not motorcycles", I've explained how they can be compared, and how it would explain why the OP's bicycle feels more responsive. It is effectively decreasing his trail.
It's not "research", it's geometry, and, yes, the same physics apply to all single track vehicles but the outcome isn't achieved the same way for bicycles and motorcycles.

So far all you've accomplished is to rant about how we don't seem to understand what we are talking about while completely missing the point that the terms applied to bicycle and motorcycle steering geometry aren't the same even if the results are similar. It's the semantics, not physics that you don't seem to grasp.
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Old 12-06-09, 08:55 AM
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Originally Posted by tonski View Post
Yea I'm aware of that, the reason bicycles measure it that way is because it the bend in the fork changes effective rake angle. Please start thinking, I'm not going to spoon feed you information anymore or respond to your idiotic responses because you refute physics. Look at the colorful picture and explain how the orange rake has more trail.

Obviously not to scale but any rational thinking person [and perhaps even you] can see a trend.
The illustration has one problem - the head tube angle didn't change when the OP changed the fork.

Therefore both forks (both "axle points", i.e. orange and red) need to use the original (red) head tube angle as the basis for measuring trail.

The red point is closer to the red axis than the orange point. The orange point has more trail.

Rake lets the axle vertical line to "catch up" to the headtube axis line. More rake moves the axle forward, reducing trail.

A 90 degree head tube angle (like a shopping cart) is unusable unless you get trail somehow. The way to do this is to have a "backwards rake" fork. This is how shopping carts work - 90 degree angle, negative rake, and boom, the shopping cart wants to go straight when you push it. Derny track bikes have the same thing, essentially vertical headtubes with negative rake forks.

Trail = stability = goes in a straight line.
Headtube angle = quicker turning

I forget the frame company but the ones that sponsored Spago's team in the early 90s, they had insane head tube angles (75?) with almost no rake. Although this seems counter-intuitive, the small rake made for more trail. If they increased the rake, there'd have been less trail.

Shorter height fork will decrease trail, increase headtube angle, and put slightly more weight on the front end (all extremely minor, granted). This could explain the quicker handling. On one of my overbuilt frame/forks I cut 5 mm off my lower head tube and perhaps 2 mm off my fork crown in an effort to quicken handling. It didn't make a huge difference.

cdr
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Old 12-06-09, 09:17 AM
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Originally Posted by HillRider View Post
All else remaining the same (and often it doesn't):

more rake = less trail = quicker handling

less rake = more trail = slower handling.
Exactly. The OP's impression of quicker handling may be the increased wheel flop which can be experienced at LOW speeds with increased trail.
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Old 12-06-09, 03:04 PM
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Originally Posted by CycleBiker View Post
Exactly. The OP's impression of quicker handling may be the increased wheel flop which can be experienced at LOW speeds with increased trail.
Or we could stop being idiots and apply those knowns to mechanical trail which the little formula above doesn't give you. Mechanical trail gets smaller with less offset (other variables remaining the same).
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Old 12-06-09, 03:12 PM
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Originally Posted by carpediemracing View Post
The illustration has one problem - the head tube angle didn't change when the OP changed the fork.

Therefore both forks (both "axle points", i.e. orange and red) need to use the original (red) head tube angle as the basis for measuring trail.

The red point is closer to the red axis than the orange point. The orange point has more trail.

Rake lets the axle vertical line to "catch up" to the headtube axis line. More rake moves the axle forward, reducing trail.

A 90 degree head tube angle (like a shopping cart) is unusable unless you get trail somehow. The way to do this is to have a "backwards rake" fork. This is how shopping carts work - 90 degree angle, negative rake, and boom, the shopping cart wants to go straight when you push it. Derny track bikes have the same thing, essentially vertical headtubes with negative rake forks.

Trail = stability = goes in a straight line.
Headtube angle = quicker turning

I forget the frame company but the ones that sponsored Spago's team in the early 90s, they had insane head tube angles (75?) with almost no rake. Although this seems counter-intuitive, the small rake made for more trail. If they increased the rake, there'd have been less trail.

Shorter height fork will decrease trail, increase headtube angle, and put slightly more weight on the front end (all extremely minor, granted). This could explain the quicker handling. On one of my overbuilt frame/forks I cut 5 mm off my lower head tube and perhaps 2 mm off my fork crown in an effort to quicken handling. It didn't make a huge difference.

cdr
Head tube angle to axle is mechanical trail so no, there is not a problem with the illustration.

The illustration shows mechanical trail no matter how you look at it. Moving the axle forward will increase trail. Are you all really under the impression that with a 72.5d head tube angle and, for the sake of this argument, 30 meters of offset will dive into corners more willingly than the 45mm offset fork? Wow these WSBK and MotoGP teams really should take a lesson from the Harleys, throw away all their research and evidence, increase their head tube angle and throw those front wheels way out front.

A ridiculous head tube angle will make it awkward to steer but the mechanical trail is still the only thing that changed here and it is less than it was before.

FWIW I've never used a shopping cart that went straight anywhere without me muscling it.

Last edited by tonski; 12-06-09 at 03:18 PM.
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Old 12-06-09, 03:20 PM
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If we bent the fork forward enough for the wheel to clear the downtube this thing would be a straight line machine. The negative offset here would make the trail huge, showing that the formula above doesn't correlate to the given "truths" of more trail and stability and that you need to use mechanical trail which I showed above how to do. What you're all arguing is so counter-intuitive it borders on insane faith.

Am I really the only person that can see you are applying that formula incorrectly? I'm not going to buy into the groupthink that is going on here and is most obviously wrong.
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Old 12-06-09, 03:57 PM
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tonski..

You need to cool your jets and do so more reading up on the subject. As a mechanical engineer who knows the geometry of both bikes and motorcyles, I noticed many errors in your postings, but refrained to comment.

When you're wrong, all the shouting and bullying in the world won't make you right. That drawing you made is not an example of a change to the fork offset. It shows a large change to the head tube angle and that is not what was asked by the OP. A proper drawing of that situation would have a fixed head tube angle, but the center of one axle would be horizontally forward of the other. There would be two lines drawn parallel to the steering axis, 4mm apart, as measured perpendicular to those lines.

Trail for a road bike has a very simple formula, that I already posted. The formula it (R/tanH) - (offset/sinH), where H is the head tube angle and R is the tire radius. The first half of this equation it the amount of trail, if the bike had a fork with no offset. The second half is the reduction in trail due to the offset. This formula is 100% correct and it's easy to derive all on your own if you know trigonometry (a high school subject).

Forks can have straight legs and still have offset. Colnago was one of the first companies to use straight blades, but they still had the same 43mm offset that had been in use for years before that time. The offset is produced by angling the straight legs right at the crown area.

Offset is the perpendicular distance between two parallel lines - one through the center of the steering tube and one through the center of the axle. It does not matter how that distance is produced. If a line is drawn through the center of the axle, parallel to the steering axis, the distance between those lines defines the offset (which can be negative).

IIRC, you mentioned something about motorcycles not having offset because the legs are straight. That is not true at all. Motorcycles have triple trees that move those straight legs out in front of the steering axis and produce offset. In addition, those triple trees can be angled, further complicating the forumla for steering trail.

Remember that atricles written about bikes will usually call offset by the term rake. Motorcycle articles will use the term rake to refer to head tube angle, but they at least use the term offset properly.

I suggest a google search on both subjects. There is lots of correct information out there.

https://www.performanceoiltechnology...._and_trail.htm
https://kogswell.com/trail.php
https://www.phred.org/~josh/bike/trail.html

Last edited by DaveSSS; 12-06-09 at 04:12 PM.
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