# Talk:Bicycle and motorcycle geometry

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## Merger candidates

I've tried to incorporate all the details from

into one comprehensive article. Any reasons left not to redirect them all to here now? -AndrewDressel 16:21, 12 December 2006 (UTC)

Done. -AndrewDressel 22:26, 13 December 2006 (UTC)

## Steering Geometry

This article should really be called bicycle and motorcycle steering geometry. The "geometry" of a bicycle also refers to the length of its various tubes, which is of more concern for ergonomic fit than for its effect on handling characteristics. -- pde 23:30, 13 December 2006 (UTC)

Those other parameters could certainly be added. They don't exist anywhere yet, as far as I know. That would also keep the title from becoming unwieldy. -AndrewDressel 23:37, 13 December 2006 (UTC)

This could look something like this:

• Ergonomics
Other geometric parameters influence the rider's experience. These include:
• Bicycles
-Seat tube angle influences the position of the seat relative to the pedals. This can be countered somewhat by sliding the seat forward or back on its rails.
-Hanglebar height
-Top tube length and stem length control the cockpit length. Along with handlebar height and seat position, this influences how far the rider stretches to reach the handlebars.
-Chain stay length, along with seat tube angle and seat position, influences how close the rider is from directly over the rear wheel which impacts ride comfort.
• Motorcycles
-Seat height
-Handle bar position
-AndrewDressel 17:12, 14 December 2006 (UTC)

Hmmm. The bicycle frame article already has a good discussion of these other aspects of frame geometry. I've cross linked the articles. Is there an equivalent for motorcycles? I can't find it. -AndrewDressel

## How geometry affects handling

It would be a valuable addition to this article to add information about how each characteristic influences the handling ("behavior") of bicycles and motorcycles and which geometries and characteristics are best suited to different types of riding.

For example, I believe that a bicycle with more trail is more stable at higher speeds while a bike with less trail is easier to handle at lower speeds and/or when there is a load on the front wheel.

Steeper head angles are used on time-trial bikes and less steep on touring bikes.

Etc.

There is already a good start in the Bicycle frame article. Perhaps it should be moved here and expanded, as necessary. -AndrewDressel 14:01, 26 June 2007 (UTC)

Hi, I've been reading the articles concerning fork rake, and there's a fascinating section concerning it on Pages 33-34 of Mike Burrows book "Bicycle design" (Published 2004 Pedal Press, ISBN 0-9520603-2-9), which references a work called "Motorcycle Chassis Design" by Foale and Willoughby (apparently now out of print so I can't reference it) which debunks the handling aspects of fork rake - experiments using a BMW 650 concluded that fork rake had less to do with the handling dynamics than ergonomics - it's easier to package the seat and steering with rake. Their experiments with a 0 degree fork showed that the bike was markedly more stable in this form, could be ridden hands off at 100mph, but suffered a degree of brake judder as the fork was not designed for this geometry. Burrows also points out that with modern mountain bikes having 200+mm travel on their forks, the head angles can change +/- 10 degrees, but no-one notices any handling changes. My e-mail is wylko@blueyonder.co.uk —Preceding unsigned comment added by 80.192.74.89 (talk) 16:36, 9 January 2009 (UTC)

## Errors?

I might be misunderstanding, but in the picture of the chopper, doesn't it have an unusually long trail? The rake doesn't seem that different. —Preceding unsigned comment added by 144.212.105.66 (talk) 20:19, 22 April 2008 (UTC)

I cannot tell what the trail might be, the fork is turned and it is difficult to guess where the steering axis might intersect the ground, relative to the front wheel contact point. However, the angle of the steering axis, which in motorcycles, is called the rake and is measured counter-clock-wise from the vertical when viewed from the right side, appears to be quite a bit more than usual. It might be as much as 45° instead of the 23.7° (with 3.8 in of trail) listed for a 2007 Honda CBR® 600RR, or the 29° (with 4.5 in of trail) listed for a 2008 Harley FXD Dyna® Super Glide®. -AndrewDressel (talk) 21:25, 22 April 2008 (UTC)
I was going to raise the very same point. Head angle, trail and obviously fork length are unusual but rake is small as normal. —Preceding unsigned comment added by Killerisation (talkcontribs) 00:11, 31 December 2009 (UTC)
Probably should be made clear that 'rake' and 'head angle' are synonymous. - 68.109.89.36 (talk) 04:53, 1 October 2012 (UTC)musant
The article already states "in motorcycles, the steering axis angle is called the rake," and "in bicycles, fork offset is also called fork rake." Is that not sufficient? -AndrewDressel (talk) 13:15, 1 October 2012 (UTC)

## Wheel flop

I'd like to discuss this in the article, but I can't find a good source for it:

• Wilson's Bicycle Science doesn't mention it by name, and doesn't tell how to calculate it, but does mention a reduction in potential energy when the front end is turned.
• Foale's Motorcycle Handling and Chassis Design doesn't mention it by name, and doesn't tell how to calculate it, but does mention a reduction in potential energy when the front end is turned.
• Cossalter's Motorcycle Dynamics doesn't mention it by name, and doesn't tell how to calculate it.
• Sheldon Brown doesn't appear to mention it at all.
• Bicycle Quarterly lists it in their glossary, but doesn't tell exactly how to calculate it. Instead, they say "The amount of wheel flop is determined by the factor "f", the weight distribution of the bike, and an eventual front load attached to the bicycle's forks," and "Wheel flop factor "f" = b sin ∂ cos ∂ (b = geometric trail, ∂ = head angle). The amount of wheel flop (how much the front of the bike lowers when the handlebars are turned) is directly proportional to f."
• yojimg.net calculates a value, but doesn't reveal how. Are they just calculating the b sin ∂ cos ∂ that Bicycle Quarterly mentions?
• kogswell same here.

Any ideas? -AndrewDressel (talk) 20:12, 6 April 2009 (UTC)

I could model it and develop equations, which I could publish in Cycloculture, but I'm not sure that Cycloculture would be viewed as a reliable source. Thoughts? Ebikeguy (talk) 22:35, 1 June 2010 (UTC)
You are right about Cycloculture. I guess it just isn't that big a deal if we can't find it written up in any reliable source. If you had an equation with more detail than Bicycle Quarterly's, you could just post it here for now, but it begs the question: where did you get it? If you derived it, where did you find the definition of wheel flop detailed enough for you to base an equation on it? -AndrewDressel (talk) 01:25, 2 June 2010 (UTC)
The basic equation should not be that difficult. You've got a defined geometry and one degree of freedom. I think I could figure it out without too much trouble, although I've done a lot more acceleration modeling in the past than stuff like this (See "The Other Side of the Mountain" in BQ a couple years ago). That said, I have put the question to a couple of people who might already have the equation(s). Let's see what they say before I go trying to derive anything. Ebikeguy (talk) 04:33, 2 June 2010 (UTC)
A KOG member defines wheel flop, in mm, as "WF = Trail x sin(Head Angle) x cos(Head Angle)." I am not sure what that means, though. What is the significance of the calculated distance? Ebikeguy (talk) 15:10, 2 June 2010 (UTC)
Oops. I just noticed where you listed that equation from the BQ glossary already. Sorry I missed it at first. Ebikeguy (talk) 17:45, 2 June 2010 (UTC)
I posted what I think is an adequate starter section on this topic. The references are imperfect, but they seem good enough for now. If anyone wants to improve on the section, please do! Perhaps reading Foale's book or the BQ editions Jan references in the glossary would help, but I just don't have time to do that much research right now. I was working off some graphs from Foale's book and the BQ glossary. Cheers, Ebikeguy (talk) 05:20, 4 June 2010 (UTC)
The formula may or may not be correct, but the symbol for head angle is incorrect: ${\displaystyle \partial }$ is an operator (the partial differential operator), not a quantity. Assuming the customary symbol for head angle (if there is one) is ${\displaystyle \vartheta }$, I have edited accordingly. Lovibond (talk) 19:52, 23 March 2012 (UTC)

## Need to flip BMW Image ?

The BMW image shows the left side of the bike. Electricmic (talk) 10:33, 2 May 2009 (UTC)

## Track bike bottom bracket height

I've copied this here from the Track bike article in hopes of finding an answer. -AndrewDressel (talk) 13:41, 2 May 2009 (UTC)

"higher bottom bracket for additional cornering clearance." I've often heard this, but have never heard how much higher.

Technical specifications
Except where stated to the contrary, the following technical specifications shall apply to bicycles used in road, track and cyclo-cross racing.
1.3.015 The distance between the bottom bracket spindle and the ground shall be between 24 cm minimum and maximum 30 cm.
• Others, such as Bianchi and Surly, just say "track legal". What does that mean exactly?

I get the general "heigher for pedal clearance with a fixed gear", but it would seem also to depend on crank arm length. Does anyone know what the real requirement is? How would a particular bicycle at a track event be found to be legal or illegal?-AndrewDressel 00:03, 20 October 2007 (UTC)

Crank arm length affects the leverage on the gear. So if you have a long crank arm, it has to be mounted high enough to clear corners. I belive there is a mathematical formula but I do not know it. I personally use 65 size crank arms and a bottom bracket that messures 11 and 3/4" from the ground to center of allen bolt attaching crank arms. —Preceding unsigned comment added by Mrmike23 (talkcontribs) 18:12, 9 December 2007 (UTC)
We sure would like to see that formula. Anybody? -AndrewDressel (talk) 13:29, 2 May 2009 (UTC)
I'm confused by the reasoning here. Surely a track bike will lean less relative to the surface on which it is riding; the whole point in a banked track is to stay more perpendicular to a leaning bicycle than a flat surface? 86.0.203.120 (talk) 15:27, 24 August 2008 (UTC)
That a track bike leans less on a banked track is only true when the bike is following a line concentric (I suspect that is not the correct word, but I don't think parallel is correct either, and I can't find a better alternative) to the track. During turns to avoid another cyclist, or when the bike leans from side to side during a sprint, the bank of the track is not a factor, and it is important that the pedals, which are of course forced to rotate by the forward motion of the bike, not strike the track surface. The questions remains: "What exactly does 'track legal' mean?" -AndrewDressel (talk) 17:32, 25 August 2008 (UTC)
Track bike bottom bracket clearance plus crank arm length is a factor (for whether one's pedal will strike the track surface) when turning hard/sharply either up or down the track bank when on the track corners (where the banking is steepest). Such maneuvers are common in track riding. (yes, I have track racing experience; no, I've not measured the bottom bracket clearance of my track bike. FWIW I use 170 mm cranks on it) =JeffH (talk) 23:21, 2 March 2011 (UTC)

I'd like to add more information on head angle, and include information on seat angle. The seat angle info would have little to do with motorcycles, but is important to bicycle design. Any objections? Ebikeguy (talk) 01:53, 23 March 2010 (UTC)

Isn't this already covered in Bicycle_frame#Frame_geometry? -AndrewDressel (talk) 02:43, 23 March 2010 (UTC)
It is touched upon, but since this is the main article for bicycle geometry, I think it should be covered here in detail. Note that the section to which you wikilinked links back to this article for more info. Ebikeguy (talk) 14:45, 23 March 2010 (UTC)
Or perhaps a previous poster was correct in saying that this article should be titled Bicycle and motorcycle steering geometry, since it deals primarily with the front end geometry. Yesterday, I saw a request to create a Bicycle geometry page, so I just created a redirect to this page, but maybe we should create a separate bicycle geometry page which we would then link to this page for as the primary article on steering geometry. The bicycle geometry page would flesh out the subjects touched upon in Bicycle_frame#Frame_geometry and cover new subjects as well. If we do create the new article, we should probably rename this page "Bicycle and motorcycle steering geometry." Thoughts? Ebikeguy (talk) 15:10, 23 March 2010 (UTC)
I guess you'll just have to add what you want before anyone else can decide how well it fits. It looks like the page only gets about 150 hits per day, and fewer than 5 people watch it, so I doubt many readers will be upset. -AndrewDressel (talk) 23:13, 23 March 2010 (UTC)

## roll of caster angle

do u know the roll of caster angle provided in bicycle? -117.241.64.222 19:51, 8 June 2010

This article is merely about explaining what the parameters are and giving some sample values. For a discussion of their roll, see the separate article on bicycle and motorcycle dynamics. -AndrewDressel (talk) 13:09, 8 June 2010 (UTC)

## BB height/drop in the image

The image that is currently at the top of the article shows a bicycle with no bottom bracket drop. I'd like to edit the image to have some BB drop. If you have anything to say about that please leave a note on the image's talk page (as the image is used on other wikipedia pages too) --Keithonearth (talk) 23:45, 7 September 2011 (UTC)

That would be great, and you might also want to define "BB drop" in the article, if you can find a suitable reference for the definition. -AndrewDressel (talk) 00:14, 8 September 2011 (UTC)

## Image of front end geometry

I've recently uploaded a new, but nearly identical, of the image displaying trail. The original image was great, but was a jpeg, a file format not ideal for diagrams, and difficult to edit. I uploaded a svg, that eliminates jpeg artifacts and is easy to edit.

I duplicated the original image very closely, however I think it could be improved. The image is of 4 forks/wheels, but is only represents 3 setups, but this doesn't seem as clear as it could be. The two on the right hand side are identical, both showing a moderate rake and moderate head angle. I'd like to change this diagram to replicate the fork/wheel 3 times and have the 3 setups arranged horizontally, with "moderate rake and head angle" in the centre and "shallow head angle" on one end and "less rake" on the other, both resulting in more trail. How does this sound to others? --Keithonearth (talk) 19:28, 13 October 2013 (UTC)

Nice work. I guess there is no need for the 4th image. If it is easy enough, can you post your proposal here for a look over? -AndrewDressel (talk) 21:47, 13 October 2013 (UTC)
Hi Andrew. I uploaded it to the same file name, (ie updated the live image) as the image is only used on this page, and it's easy enough to change again if need be. My concerns about the previous layout are:
• it is not clear that the two images on the right side are identical, in fact at first it looks like one should have "steep head angle" and the other "more fork offset" relative to each other.
• it is not clear that the two labels at the bottom (more trail, less trail) refer to both images above the labels.
• While the images on the left are labeled "shallow head angle" and "less fork offset" in relation to all the others in the image, the ones on the right are labeled only in relation to the one to their left.
I should point out that when I say "not clear" I'm really just meaning not immediately clear -- and that it certainly can be discerned by a reader, it shouldn't be as much work as it was with the old format. I believe my new version clarifies most of these issues, by making it more clear that we have one setup in the middle, with variations away from it on the left and right. I'm open to other ways of formatting the image, but I do think that this version is a upgrade from the original format. --Keithonearth (talk) 21:52, 14 October 2013 (UTC)
I like your new image. It makes the points more clearly. I have only a couple of text copy edits to suggest:
• It appears that one "Increased trail" has a capital I and the other does not.
• Use "Decreased" for both head angle and force offset.
Thanks again. -AndrewDressel (talk) 00:01, 15 October 2013 (UTC)
Good catches, both those things have been fixed, and the new image has been uploaded. I've been thinking of making some other tweeks, maybe incorporating colour for the alignment lines, and changing the boxes at the bottom to be more indicative of measuring distance. Let me know if you have any other ideas or concerns. --Keithonearth (talk) 20:01, 15 October 2013 (UTC)
ok, one more tweak is up, nothing major, but I think that it's an improvement. --Keithonearth (talk) 00:51, 17 October 2013 (UTC)

## Comments on differences in definitions between bicycle and motorcycles

The terminology used throughout the Bicycle and motorcycle geometry page is confusing. This seems to be in large part due to the different way that "rake" is used on bicycles and motorcycles. I suspect that multiple authors were at work and they failed to seamlessly merge their contributions. What follows is mainly an attempt to straighten this out. I don't have a Wikipedia account and am new enough to editing Wikipedia pages that I thought it best to run this by you, who I gather is the biggest contributor to the page. There are two images below that I couldn't insert, apparently because of my not having an account, so I've uploaded them to Tinypic and included links in the text.

Steering axis angle:

1. "In motorcycles, the steering axis angle is called the rake and is measured from the vertical." would be clearer if it read "In motorcycles, the steering axis angle is called the rake angle or rake and is measured from the vertical."

Done.

2. As it's a motorcycle, the caption on the BMW would better be stated "Telescopic forks on a BMW motorcycle reveal the steering axis or rake angle".

Done.

3. "A 0° rake would be vertical." could be improved to "A 0° rake would be vertical. Note that as rake angle increases, head angle decreases. Head angle = 90° - Rake angle."

Not done.

Fork offset:

1. "Required rake angle arose from early times when lightweight bicycles suffered fork failures from road shock." Bicycles are being mentioned here. Use "head angle" instead of "rake angle".

Removed entire long-unsourced paragraph.

2. "Before most roads were paved, fork rake had a lower angle so the fork would be loaded axially on rougher surfaces." I suggest "fork rake had a lower angle" be "lower fork angles were used". Furthermore shouldn't it read radially rather than axially? The author of this statement, which I hope isn't you, is careless.

No, I didn't write it, but I believe 'axial' is correct.

3. "In motorcycles with telescopic fork tubes, fork offset can be implemented by either an offset in the triple tree, adding a rake angle (usually measured in degrees from 0) to the fork tubes as they mount into the triple tree, or a combination of the two." "Rake angle" is confusing and inappropriate here as the term has already been defined. I think "triple tree rake" is the appropriate term. See it being used at http://www.rbracing-rsr.com/rakeandtrail.html.

Done.

Trail:

1. "Trail is a function of head angle, fork offset or rake, and wheel size." tends to lead to confusion because of the use of "rake". I suggest that the word either be removed or that the sentence be changed to "On bicycles trail is a function of head angle, fork offset or rake, and wheel size."

Done.

2. "Motorcyclists tend to speak of trail in relation to rake angle. The larger the rake angle the larger the trail. Note that, on a bicycle, as rake angle increases, head angle decreases." In view of the suggested changes in the steering axis section, I suggest changing it to "Motorcyclists tend to speak of trail in relation to rake angle, http://i44.tinypic.com/t0j2pd.png. In terms of rake angle, the formula for trail is: http://i39.tinypic.com/293x091.png"

Possibly done.

Mechanical trail:

1. "Mechanical trail is the perpendicular distance between the steering axis and the point of contact between the front wheel and the ground." This doesn't make sense.

It makes sense to me, but go on.

The steering axis is just a line of infinite length.

Yes.

Where on that line does the perpendicular measurement to ground begin?

At the point from which a perpendicular line would intersect the point of contact between the front wheel and the ground.

If it begins at the steering head (I'm guessing), where on the length of the head does the measurement begin.

Nope. On most bikes, that would be pretty from from correct.

I.e. a typical motorcycle steering head is around 6" in length. Alternately, might mechanical trail be the perpendicular distance from the steering axis to the front axle? -Bill Vyn 184.187.191.197 (talk) 21:39, 12 December 2013 (UTC)

Nope. A picture would probably be worth a thousand words in this case. Let me see what I can find. -AndrewDressel (talk) 15:59, 14 December 2013 (UTC)
On second thought, and after wasting an hour trying, I find the tools available that support the preferred SVG file format unusable. Someone else is welcome to play with it. A decent image can be found online at: http://www.peterverdone.com/wiki/index.php?title=Mechanical_Trail -AndrewDressel (talk) 17:01, 14 December 2013 (UTC)
Good work Bill and Andrew. Thanks for fixing that.

On the KTM 390 Duke I noticed they were using steering head angle instead of rake [23][24], and it seems BMW does the same [25] now. Triumph still uses rake. A similar thing happened a few years ago, where without any kind of announcement, all the brands shifted from adverting dry weight to curb/wet weight. It would be nice to verify when BMW and KTM made this change, or if it was recent. --Dennis Bratland (talk) 20:18, 14 December 2013 (UTC)

Hmm. So long as they don't start measuring from the vertical and calling it rake angle, we might be okay. It would be nice to find a reliable, third-party source to explain things, though. -AndrewDressel (talk) 16:38, 15 December 2013 (UTC)

## New version of the Trail diagram

I've just updated the image for the Trail section, with the addition of a 3rd Variable (wheel size). I've also redone the layout to accommodate the 4 variations (baseline + 3 variables). Please have a look at the new image and let me know of any issues that could be addressed. Thanks! --Keithonearth (talk) 20:05, 10 August 2014 (UTC)

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## unclear trail section

I've just read the sections on trail and mechanical trail, and I think that they are saying that less trail makes a bike more twitchy/agile and more trail makes a bike track better and less agile. That seems to be the central importance of trail, so I'd like to see it stated more clearly and more prominently. --Keithonearth (talk) 02:50, 22 February 2016 (UTC)