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 All Forums
 Motorcycle Safety
 Physics and the theoretical
 Rake and Response
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Mikeydude
Male Advanced Member
749 Posts
[Mentor]


Ft. Worth, Texas
USA

Harley-Davidson

03 FXD Super Glide

Posted - 06/21/2006 :  2:53 PM                       Like
I was wondering if you folks could help me better understand the relationship between front fork rake and response and handling of the bike?

I think that the longer the rake, the poorer the handling at slow speed maneuvers (u-turns, etc...), and the shorter the rake the more responsive the bike in slow speed maneuvers. And the longer the rake the smoother the handling at higher speeds... Is this accurate?

How does an offset riser affect handling? (Raising the bars and moving them closer to the rider).

The forks are set forward of the steering stem (?) in the triple tree, and that has some affect on steering, right? So wouldn't moving the bars further back (closer to the rider) without moving the steering stem have an affect too? What would that be?

(I'm not explaining this very well...)

Thanks

MD

scottrnelson
Advanced Member
6950 Posts
[Mentor]


Meridian, ID
USA

Honda

XR650L, 790 Adv R

Posted - 06/21/2006 :  2:59 PM
I was going to put together a bit of an explanation, but in looking for a picture to go with it, I realized that tip 163 already tells you most of what you need to know about it:

http://www.msgroup.org/forums/mtt/t...?TOPIC_ID=76

Ultimately less rake and trail means quicker steering but less stable. A shorter wheelbase also makes for quicker steering and less stability. For slower steering and a more stable ride you want more rake and trail and a longer wheelbase.

Edited by - scottrnelson on 06/21/2006 3:00 PM
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Mikeydude
Male Advanced Member
749 Posts
[Mentor]


Ft. Worth, Texas
USA

Harley-Davidson

03 FXD Super Glide

Posted - 06/21/2006 :  3:08 PM
Thanks Scott... I've read that article too... and got the general concept of it.

How about the offset riser stuff... any thoughts?
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James R. Davis
Male Administrator
17380 Posts
[Mentor]


Houston, TX
USA

Honda

GoldWing 1500

Posted - 06/21/2006 :  3:22 PM Follow poster on Twitter  Join poster on Facebook as Friend  
This is actually not well understood by most riders so I will attempt a bit of an explanation.

If your steering stem pointed straight down (in other words, your rake angle was zero) then if you turned the handlebars by 10 degrees to the left or right, the front wheel would point either 10 degrees to the right or left. That's as 'fast' as your steering can be - direct.

But if you imagined that your front wheel was sticking out directly in front of you with the steering stem pointing horizontally to the front, then if you turned the handlebars 10 degrees your front wheel would still point straight ahead (it would lean 10 degrees, but it would still point forward.) In other words, you would then have the 'slowest' possible (zero) steering.

If your steering stem was pointing at a 45 degree angle toward the front (a 45 degree rake angle), then turning the handlebars 10 degrees would result in a 5 degree change of direction of the front wheel as well as a lean of that wheel. That would be 'extremely slow' steering. Instead, your steering stem points forward at something close to 30 degrees. It should be clear to you that a 10 degree turn of the handlebars will result in something less than a 10 degree change of direction, but it will feel 'normal' to an experienced rider.

Thus, your rake angle determines how 'fast' (faithful) your steering is.

The offset of the steering stem and your forks (via the triple tree) is designed to negate a part of your 'trail'. (Which is determined by where the steering stem points to the ground vs. the center of your contact patch.) Where your forks tie to your front wheel hub, along with the diameter of the wheel and the rake angle, determine exactly how long your trail is.

Trail is what creates the 'righting force' which tries to keep the front wheel pointing in the same direction the bike is noving. (When you are in a turn the front wheel is always out-tracking, thus pointing slightly outside the turn and the righting force tries to correct that misalignment.) The longer the trail, and the greater the weight that is being carried by the front tire, the stronger is that righting force.

This explains why it is almost impossible to 'steer' while in a panic stop - the righting force is enormous when virtually all the weight of the bike is on the front tire.

If you use pull-backs or straight risers to adjust where your hands grip the handlebars you have not affected the steering dynamics of the motorcycle in any way.
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Mikeydude
Male Advanced Member
749 Posts
[Mentor]


Ft. Worth, Texas
USA

Harley-Davidson

03 FXD Super Glide

Posted - 06/21/2006 :  3:48 PM
James... I follow you very well on the angle points you've made. I have a small model motorcycle that I can use as a visualization tool that helps me see that...

I'm not arguing your point about risers not changing the steering dynamics at the tire end, but it does change the way the rider moves his arms to initiate a turn, doesn't it? The forks are set forward of the pivot point in the triple trees, so this means that the forks actually swing around that pivot point. If the handlebars are positioned directly over the pivot they will is esscence turn with a "screwing" motion. If the bars are mounted at any other location they would actually "swing" around the pivot. So it seems that adding an offset of 2 more inches would increase that swing... And that would mean the rider has to change the way he moves... Instead of "screwing" the handlebars, he now has to "swing" the handlebars. And the more the offset, the more he has to swing to turn the bars...

Does that make any sense?
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James R. Davis
Male Administrator
17380 Posts
[Mentor]


Houston, TX
USA

Honda

GoldWing 1500

Posted - 06/21/2006 :  4:00 PM Follow poster on Twitter  Join poster on Facebook as Friend  
I understand what you are trying to say, I'm just puzzled by what you are asking about.

Look, your steering is using the steering stem as the pivot point. Steering involves no more than revolving the steering stem. Though your handlbars LOOK like you are simply pushing either the left or right grips forward to turn, in fact what you are doing is pushing them forward and UP because of rake angle.

But extending the handlebars back or making them higher (if wider) only changes the amount of leverage you have on the steering stem, in no way does it change the front-end geometry. Longer handlebars require less pressure to torque the steering stem - i.e., it will feel easier, but it will not be 'faster' or 'slower' steering.
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Mikeydude
Male Advanced Member
749 Posts
[Mentor]


Ft. Worth, Texas
USA

Harley-Davidson

03 FXD Super Glide

Posted - 06/21/2006 :  5:08 PM
LOL... I told ya I wasn't explaining this very well. I can picture it in my head, but I can't even figure out a way to draw a picture of it. I guess I threw things off when I titled this rake and response... My bad.

My question is actually about where the bars are mounted on the trees and how an offset riser comes into play. Not the height of the riser, but the length of the offset. I understand while turning I am actually pushing a certain grip forward as well as up because of the rake. But this assumes that the bars are mounted directly over the steering stem.

Let's use a left turn (and left grip) for discussion purposes here... If the bars are mounted closer to me, rather than directly over the steering stem, I am not only pushing forward and up on the left grip, but also out to the left... And if the bars are mounted farther away from me, instead of directly over the steering stem, I am I actually pushing forward, up, and in to the right. Is this correct?

And the greater the distance from the stem the bars are mounted, the more I have to push laterally, because the radius is greater. Is that right?

Are these differences negligable, or do they have any dramatic effect?

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James R. Davis
Male Administrator
17380 Posts
[Mentor]


Houston, TX
USA

Honda

GoldWing 1500

Posted - 06/21/2006 :  5:20 PM Follow poster on Twitter  Join poster on Facebook as Friend  
quote:
I understand while turning I am actually pushing a certain grip forward as well as up because of the rake. But this assumes that the bars are mounted directly over the steering stem.



Nope. It is always true, regardless of where the grips actually are. (Because you can only turn the stem on its axis.)
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scottrnelson
Advanced Member
6950 Posts
[Mentor]


Meridian, ID
USA

Honda

XR650L, 790 Adv R

Posted - 06/21/2006 :  6:56 PM
quote:
Originally posted by Mikeydude

My question is actually about where the bars are mounted on the trees and how an offset riser comes into play.


The only thing that matters is how far away the grip is from your steering stem, it doesn't matter if the bar and triple clamp takes some circuitous path to get there. Unless there is some flex in the system, of course.

Moving bars further back or making them wider gives a little bit more leverage. Moving them too far back would make steering more like working a tiller than regular handlebars, though.

Oh, and I learned something new (or understood it better) from James' explanation of steering at 0 degrees and 90 degrees. Thanks for that description, James.

Edited by - scottrnelson on 06/21/2006 6:57 PM
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Mikeydude
Male Advanced Member
749 Posts
[Mentor]


Ft. Worth, Texas
USA

Harley-Davidson

03 FXD Super Glide

Posted - 06/21/2006 :  8:47 PM
quote:
Originally posted by scottrnelson


Moving them too far back would make steering more like working a tiller than regular handlebars, though.



Yes... this is what I was talking about. I am guessing from the responses though, that the effect of a couple of inches would have negligable effect? Once again, I'm not refering to how the bike moves, but what the rider does to execute the move.
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subvetSSN606
Senior Member
418 Posts
[Mentor]


Ellettsville, IN
USA

Suzuki

800 Intruder

Posted - 06/22/2006 :  6:03 AM
I think I get what you're asking Mikeydude, and I think its correct that in the usual range of configurations the effect you're asking about is negligible, especially if you are not fighting yourself.

Others are exactly right that as far as the bike is concerned, it makes no difference as the steering head can only rotate around it's pivot point.

You are raising the point about what your hands are doing. If the handle bars are a straight line over that pivot point, then the bars are a diameter of the circle centered over the pivot point... in that case if you push forward with your left hand, it follows an arc forward and to the right. Simultaneously then, the right hand follows an arc backwards and to the left.

However with pull-backs, both your hands are behind that line that is a diameter of the circle of rotation around the pivot point, you're not on that diameter... So now when you push forward with the left hand, rather than following an arc forward and to the right, your hand follows an arc forward and to the left. Simultaneously, your right hand follows an arc backwards and to the left. So in other words... in this case BOTH hands are moving left!

Visualize a car steering wheel. If your hands are at 3 and 9 O'Clock, your hand motion is as I described in the first case. If your hands are at 5 and 7 O'Clock, your hand motion is as I described in the second case... To turn the wheel right, both hands move left.

Having said all that to try and help others understand what you're asking...
You're over thinking it. ;)
Whatever bars you have, you'll figure out what you need to do to turn them, and how the bike reacts is unrelated to the geometry of how you had to move your hands to turn the front wheel.

Tom

Edited by - subvetSSN606 on 06/22/2006 6:15 AM
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Mikeydude
Male Advanced Member
749 Posts
[Mentor]


Ft. Worth, Texas
USA

Harley-Davidson

03 FXD Super Glide

Posted - 06/22/2006 :  11:36 AM
Thanks Tom... thats a great explaination... It's exactly what I was trying to say... lol. And I'm sure you're right; I am over thinking it. I've been told that before.

Thanks again!

Edited by - Mikeydude on 06/22/2006 11:36 AM
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