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Old 11-29-12, 06:06 AM   #1
ftwelder
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steering geo, trail and fork offset.

The topic came up in C&V so copied it here also. I hope that is OK. Feel free to point out errors. It's still early.

There are three dimensions on road bikes that when changed can make significant changes in the way a bike will steer. They are 1) head angle 2) fork offset and the result of these numbers is the third, trail.

Quickly, trail is the amount the wheel follows the bike (in your brain) and a a big number means the bike has a strong tendency to go straight. a small trail number and you change lanes when you turn your head to spit.

Trail is a measurement taken on the ground. If you project a line through the center of the head tube all the way to the ground and make a mark. Then using a framing square or a plumb bob, you locate a point on the ground directly below the axle. The difference when viewed from the side of the bike is "trail".

Randy mentioned something in the recent PX thread (C&V) about different forks having the same steering geometry. The top row is a demonstration of that. Only the radius of the bend in the fork was changed but it has a large visual effect. No change in steering geometry.

The second row is what changes when head angle and fork rake (offset) are changed.

We can look at this a bit and discuss it more later if you like. There are other small factors, wheel diameter/fork length that effect the steering geometry. This is a small part of overall handling which weight distribution and leverage make big effects on handling. The image on the bottom right is a bike with a 74 head tube and a bent fork with a short offset number. It's counter intuitive that it would have a greater tendency to go straight. That is what I was talking about at the beginning, "the amount the wheel follows you" can really be felt in this example when ridden. You may have to click on the image to see it clearly. My monitor looks like you are peering through a screen door so it's difficult to see anything without a blur!



steeringgeo by frankthewelder, on Flickr
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Old 11-29-12, 11:27 AM   #2
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Very true Frank.

To add to this, the easy formula for calculating trail is (units are millimeters and degrees);

Trail = (Wheel Radius * Tangent(90 - Headtube angle)) - (1 / Cosine(90 - Headtube angle) * Rake)

Looking at the above, it's apparent that entirely different fork rakes can produce the same trail with an adjustment in head angle.
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Old 11-29-12, 11:35 AM   #3
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Originally Posted by ftwelder View Post
....It's counter intuitive that it(larger trail #) would have a greater tendency to go straight. That is what I was talking about at the beginning, "the amount the wheel follows you" can really be felt in this example when ridden.



steeringgeo by frankthewelder, on Flickr
Yes it is! Regarding the use of offset crowns(typically offered with 7degree offset) and straight blades, the blade length will determine the the trail, the longer the blades the longer the trail and therefore the more "stable" handling(given other frame variables are equal), correct, or do I have this reversed? Sort of hard for me to visualize this without drawing it. I realize as the blades lengthen the shallower the ht angle becomes(again, all else =), so blade length must be built exactly as designed to maintain desired characteristics.

I have heard trail described as under or oversteer(commonly used to describe car handling characteristics); with oversteer the large trail number requires more effort to turn while understeer requires less effort("you change lanes when you turn your head to spit"). I think I have this correct!

The often discussed "low trail touring bikes" need the "quicker" handling to better control the loaded bike, is that right?

Thanks for posting the graphic FTW, explains trail very simply and well.

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Old 11-29-12, 12:11 PM   #4
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....Looking at the above, it's apparent that entirely different fork rakes can produce the same trail with an adjustment in head angle.
OK, what's the difference between two bikes with the same trail, one with a 70hta and one with a 74hta, the 70hta bike will "absorb" more vibrations/shocks? Or is there no difference(all else being=)? And thanks for the formula, nice to have, also makes me realize how much I've forgotten about trig.

Brian






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Old 11-29-12, 12:50 PM   #5
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OK, what's the difference between two bikes with the same trail, one with a 70hta and one with a 74hta, the 70hta bike will "absorb" more vibrations/shocks? Or is there no difference(all else being=)? And thanks for the formula, nice to have, also makes me realize how much I've forgotten about trig.

Brian
Pretty much as you say.

For say 58mm of trail with 700C wheel;

A 74 Ht needs 38mm of rake
A 70 Ht needs 61mm of rake. So, yes a bit more cantilever.
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Old 11-29-12, 01:30 PM   #6
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Not to point out errors, just to add. Dave M has some interesting articles in general frame design considerations. I suspect everyone has seen these already, but just in case, here is a link to his article on handling. http://davesbikeblog.squarespace.com...?currentPage=2 and page 4: http://davesbikeblog.squarespace.com...?currentPage=4

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Old 11-29-12, 05:50 PM   #7
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Best explanation of fork trail that I have read is from Tom Kellog;
http://www.spectrum-cycles.com/geometry.php

Too often, I think new builders try to tune a desired fast or slow steering feel with trail instead of with HTA.


For the two examples bikes having same trail with 74 and 70 HTA, keep in mind that the combine effect of shallower HTA+increased rake also will both contribute to making the front-center measurement (Front wheel hub to bottom bracket) longer, so increase the wheelbase of the bike and change the weight distribution (more rearward). These later two side-effects probably have at least as much enfluence on the overall handeling of the bike (if not more), than the direct effect of HTA.

Low trail touring bikes need the low trail because every time you turn the bars you are also lifting all the dead weight of the gear being carried on the front end, increases the effort it takes to turn bars. The low-trail minimizes the amount of lift produced during turning so get the steering effort back to an easier, more familiar level. A high trail bike with heavy front-end load would turn like a freight train.
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Old 11-29-12, 05:59 PM   #8
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Maybe I'm a little slow today but having just read the two blogs posted above(admittedly quickly) I don't see any conflicts. SJX426 (or anyone else), please point me in the right direction(with proper trail of course). Great thread, thanks, Brian
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Old 11-30-12, 04:43 AM   #9
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I have to use some type of physical representation. Either a CAD drawing or welding a bunch of scrap and door hinges together.


The "lifting of the weight" is something I never noticed separately. I notice that head angle alone makes plenty of difference when when building a bike for specific terrain angle and is easiest to find an improvement.

I am working on a linkage that changes a rotary action to a tilting action at specific location. The guy before me gave up when he had four sets of levers/links and still had nothing. It took me a couple of days staring at it making mock-ups before solving the problem with one spherical rod end and one axis (with two bearings) It was a very simple solution but so hard to find. Props to you guys with math skills.



**update! trail calc

http://yojimg.net/bike/web_tools/trailcalc.php

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Old 11-30-12, 10:43 AM   #10
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Back in the day when i was starting to build I tried to absorb all i could WRT steering geometry. I went through dozens of frame spec charts from manufactures (now I know that these were not fully what the actual bikes used...) calculating the trails. Discovered Castor Angle (Eisentraut and Bill Boston), measured as many frames for lift or drop during steering angle changes. I remember the English publication that had a column on mechanical things (Mullet's Mechanics, or something like that) that had math formulas to derive the head angle, wheel size, rake combo that produced the "right" amount of lift/drop needed for a handling goal. 81* castor angles, Wheel Flop Factors, over/under steer all are just different ways of looking at the same thing, choose your language right and others will follow you like a Pied Piper.

After a few years I kind of gave up trying to be a student of steering spec. I found that the range of dimensions were narrow when other frame design needs were taken into acount (like front center/toe overlap or available tire/wheel sizes). I also found that how you loaded the bike (I was into loaded touring at the time) had far more import to handling then a CM of trail difference. Lastly i decided that i liked a certain feel to the handling and for my bikes centered my steering geometrys there.

It seems that so much of this discussion is cyclic (intended pun) with a new generation of builders that weren't around BITD. Only now there's the interweb to be able to draw many others into your thinkings. The best example of this in Jan Heine's Bicycle Quartery, and his attempts to understand bike handling WRT trail and tire size.

After so many thousands of builders/designers that came before us and have explored the varying dimensions and specs that can be produced and ridden that the vast majority of bikes have so little difference should say something. I have no problem reading one more discussion on steering geometry, even to add my views, but I don't expect to really have any "ah ha" moments.

Having said all that i tend to build with around 6cm of trail and choose the head angle with reference to the front center as much as the handling response. I don't carry much weight on the front of the bike (when loaded for touring there's a lot more rear weight the front). I feel that there's FAR more tire profile/air pressure controled road surface damping going on then fork blade flex, certainly for the high frequency buzz. Andy.
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Old 11-30-12, 02:08 PM   #11
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I found the old magazine articles I mentioned. Too many to post here but I've uploaded them onto my Flicker site. The copies I have are not easy to read what with the third or fourth generation copies that they are. The images can be enlargened. A few are about components but most are about frame design, as it was known back in the 1970s. Andy
http://www.flickr.com/photos/7319558...7632140831010/
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Old 11-30-12, 05:45 PM   #12
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Mike Burrows , UK Bike design Iconoclast , build a Long wheel base recumbent, head tube was vertical
, trail was The Offset..
so like a low trail , you didn't have to lift the front of the bike going off center..
but the trail was larger..

I think Powells carried his Book on the subject.


But I realize the Bike Biz is conservative. front wheel is in the same line as the rear.
Mike was, It seems, OK with them being on parallel lines.

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Old 12-02-12, 06:13 PM   #13
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I don't really have strong opinions on any of it. I haven't done any real experimentation with different fork offsets beyond a couple of examples. I have a lot of bikes with a lot of different geometrys. I suspect everyone has their own magical combo.

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Old 12-02-12, 08:38 PM   #14
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For me, more important than trail or head angle is front center. If you mind the front center, a lot of other issues don't surface.
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Old 12-03-12, 11:08 AM   #15
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For me, more important than trail or head angle is front center. If you mind the front center, a lot of other issues don't surface.
Craig, can you comment on Front-Center a bit more? As a tall rider all my bikes have a longer f-c, I wonder if this should be compensated for with a shorter trail?
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Old 12-03-12, 11:42 AM   #16
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For me, more important than trail or head angle is front center. If you mind the front center, a lot of other issues don't surface.
The first frame I built (for myself) with a purposefully short front center was a nice surprise.
58 CM front center with longish 42.5 cm chainstays. 58 mm of trail.

I think the slight forward weight bias solidifies the handling.
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Old 12-04-12, 03:44 PM   #17
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Craig, can you comment on Front-Center a bit more? As a tall rider all my bikes have a longer f-c, I wonder if this should be compensated for with a shorter trail?
Shorter fork trail will actually make the F-C measurement longer, not shorter. Keeping the F-C short on a big frame can best be accomplished by using a combination of steeper HTA and less fork rake to keep the trail neutral.
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Old 12-04-12, 05:31 PM   #18
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I think you should use rake/offset, of the fork.. since trail is a distance laid out on the ground plane.

true .. conventional bikes the longer the forward bend , fork rake , the shorter the trail, as measured on the ground.

such is used on front loaded Porteur Bikes which carried Bundles of Newspapers to the sales Kiosks of Paris
in the 40's ..
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Old 12-04-12, 06:50 PM   #19
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Originally Posted by ftwelder View Post
I am working on a linkage that changes a rotary action to a tilting action at specific location. The guy before me gave up when he had four sets of levers/links and still had nothing. It took me a couple of days staring at it making mock-ups before solving the problem with one spherical rod end and one axis (with two bearings) It was a very simple solution but so hard to find.
Glad you found your solution.

A fascinating source of thousands of such solutions is the four volume set of Franklin Jones's "Ingenious Mechanisms for Designers and Engineers". If you ever find a copy for under $100, jump on it.
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Old 12-04-12, 08:52 PM   #20
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Craig, can you comment on Front-Center a bit more? As a tall rider all my bikes have a longer f-c, I wonder if this should be compensated for with a shorter trail?
I don't think it's hard to keep a large frame balanced between FC and trail. GrayJay pointed this out well. It gets hard when you're building small frames, or frames for people who have limited reach. Once you get below a FC of about 585 you suddenly have toe overlap becoming an issue. If you slacken the head tube and add rake there is more of a tendency for wheel flop, and added rake might lower the trail more than you'd like. Small bikes hit all the edges of normal. In the frames I've built for myself to ride, I've found a FC of 590 to be ideal for my size. With adequate FC the range of trail is less important. I don't know, just seems to work for me, but it's a holistic thing. People talk about how great a short wheelbase/FC frame handles, but to me good handling is confidence at speed, and I've never had trouble getting around corners. Probably the most important factor in all of this is the rider and his/her ability to be fit for the bike.
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Old 12-15-12, 03:41 PM   #21
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I have to use some type of physical representation. Either a CAD drawing or welding a bunch of scrap and door hinges together.


The "lifting of the weight" is something I never noticed separately. I notice that head angle alone makes plenty of difference when when building a bike for specific terrain angle and is easiest to find an improvement.

I am working on a linkage that changes a rotary action to a tilting action at specific location. The guy before me gave up when he had four sets of levers/links and still had nothing. It took me a couple of days staring at it making mock-ups before solving the problem with one spherical rod end and one axis (with two bearings) It was a very simple solution but so hard to find. Props to you guys with math skills.



**update! trail calc

http://yojimg.net/bike/web_tools/trailcalc.php
That weblink has a calculating tool that uses the same basic math that was given earlier in this thread But Jim G also added a few niceties, like dealing with the issue, "I know I have a 700x28c tire, but what's the dang diameter?"
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