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 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!

I copied this to frame builders, I hope that is OK!


steeringgeo by frankthewelder, on Flickr

I was just thinking of this las night while laboring up a 14% grade on my 3 speed Phillips Roadster. Slack slack head angle, big diameter (28 x 1 1/2") wheels and plenty of fork rake. I did not do the calc's but talk about wheel flop!

That and it is amazing how without careful study a steepish head angle (74° as per your example) and little fork rake has more trail than less. What "looks" fast and twitchy may not be. Late Paramounts and Waterford's often have this look.

In looking at my own bikes that I as time permits are throwing onto a CAD program, Front center (beeline between BB and front axle) often means just as much to me as how a bike feels. The one surprise to me was a bike long gone, a British built "criterium" machine. 74 HA, 73 SA, 22.375" front center (can you say OVERLAP) 15.25" chainstays, 11" high BB with 250 Clement tires. Great Criterium weapon, I could pedal through corners with 165's that only one guy could dare pedal too and he had a Schroder with an even higher BB. Other guys tried to pedal through to keep up and I would hear the pedal strikes and epithets that followed. This was very useful in the final turn before the sprint.

Another comment related to this is when inputting the take-off frame dimensions, I have had to go back and double check from time to time as what one expects might be a "clean" dimension of the top tube is actually messy, like 558mm, not 56 cm. The CAD program won't let you fudge.

Thanks for sharing your wisdom! Growing up on Gazelle 3 speeds, my first time on a road bike was revelation in terms of steering and handling. I love to ride many kinds of road bikes and comparing the handling with each other, slowly trying to find out what I like best. When I find that goldilocks geo, I probably move on to MTB's and start the process anew .

By the way, sorry if this is a dumb question, but why don't they just build bikes with a shallow headtube and a non-raked fork? that seems to be easier to manufacture (hence cheaper). Thus far, I've only seen one (modern CX) bike like that, why is this style not more common? tradition? Aestethics?

chopper bikes have those.. so does my pedicab

Actually, that chopper fork in the picture HAS rake because of the offset dropouts and offset fork legs (in relation to the steerer tube).

When say "non raked", I think you are talking about "rake at the crown". If that is true, it's like the top row of drawings, for steering geometry it makes no difference. I am not sure of the outcome of what you suggest. Someone did a comprehensive write-up on this many years ago. I think it was in one of the 70's bicycle books.

Steering is a funny thing to think about on bicycles. If you've ever had a bike with a steep head tube and a long-rake fork you know how bad it can be. The headtube can "drop" as steering angle is increased. This is not fun.

Stability and trail are one factor, but steering "quickness" is another. Steep head tube and low-rake forks are stable but can feel "quick" due to short wheelbases and short pivot (term??) when the bars are turned. It's almost trivial to think about turning the bars, though, as you barely turn the front wheel in normal riding.

I've got a bike with a 74 or 75deg. headtube and 35mm rake fork that will ride straight all day long. I've riding a more relaxed looking Raleigh DL1 that was treacherous.

Now let's add weight and loading to this hehehe. Racks and bags?

This is a bit of a peeve of mine: I know we bicyclists use the word "rake" to refer to offset, but I wish we wouldn't. Rake, if you look it up in a dictionary, really refers only to the inclination from vertical or horizontal.

One mustn't discount the effects of wheel size. Since most bikes have the same wheel size (anything in the 26 to 28 inch range is effectively the same) we can do this, but when you use smaller wheel sizes, the amount of offset has to be reduced accordingly. When you get down to a 16" wheel, the balance of these factors is so critical that changing the tire size of a bike can make a radical difference to its handling. I used to ride a folding bike that handled best with a 2" tire in front and a 1.25" tire in back; with tires of the same size, handling was barely tolerable.

Relative weight loading is also pretty important. I have a couple bikes (relaxed angles, lots of offset) that shimmy badly unless I lean forward.

So I've always gotten cornfused when trying to remember HT angle/fork rake/trail. But I know the front-center measurement I like for my given frame size. Is this an adquate measurement for determining handling (steering) characteristics?

edit: Within diamond framed road bikes and excepting any specific applications (Time trial, cargo hauler, etc), I'm hoping the answer is yes.

Good discussion, Frank. I've been asked questions about this before. The key item people seem not to get, though it is pretty simple when you look at the geometry, is that "rake" reduces trail. That is its purpose. Steep-angled bikes have less rake because the trail is lower already. That correlation makes people erroneously conclude that less rake makes the steering faster. But if you were to swap forks to one with less rake it would steer more slowly.

One just has to accept the long used terminology. Talk to a motorcycle designer and he will also pull his hair out regarding all of the bicycle nomenclature.

Great thread! I am working on a Kogswell P/R that has a "low trail" design with a 73degree head angle and a 55mm offset fork.

I am also currently using a front basket on my Raleigh Sports 3 speed and the "low trail" design works well for this.

Can someone please explain why a low trail design handling improves with weight at the front? I'm trying to wrap my head around this. Many thanks

I'm not sure this is the complete answer but here is a point not mentioned yet.

Even if rake is "correct" such that trail is the same between one frame and another, the shallower head angle will induce more camber change for the same amount of steering input. In other words, the wheel leans into the turn. This lowers the wheel center.

With a "normal" bike, the non-zero trail means that the contact patch rises when the steering is turned, which really means that the wheel center falls, which means that gravity wants to pull the wheel sideways (i.,e. flop over). But now imagine two bikes, one with a vertical head angle and no rake, and one with a "normal" head angle but enough rake to give it zero trail. The flop-over contribution from trail is zero in both cases, but the camber induced by the non-zero head angle still means the wheel center will fall and thus "want" to flop over.

One final question is where the "extra weight at the front" is located. If it's in a handlebar bag it is in front of the steering axis. If you're talking about rider weight, it is obviously further back. So you'd need to be more specific.

See, that is the thing.. OK, wildwoods question. I came across this when building my tandem frame. Tandems are funny and what everyone seems to be talking about relates to the amount of weight on the front wheel and speed. Tandems and tourers have a great deal of outside forces trying to effect the balance. I don't know the answer.

If you take the average British roadster and try to ride the sidewalk at a snails pace, it's difficult. Take that same roadster down a choppy dirt road at speed and it's pretty darn nice. Also, with tandem it's better with steering geometry that is less effected by subtle lean angles.

The head angle seems to create a leverage ratio. If you had a 90 degree head angle, the bike would ride a circle radius that matched the angle of the handlebars. As the head angle gets slacker the resulting turn radius becomes larger. Lift begins to take place and the contact patch increases. It seems a balance of "wanting to go straight" and the ability to initiate lean lean in a short amount of time is pretty ideal. Weight distribution seems to be what changes day to night with a certain set of numbers.

I think trail provides lean "effort" with more leverage to upset the balance. This allows any outside influence to effect the balance. Though that does not really sound correct. I think it's being done with a steeper head angle and the resulting lost trail is how it's described.

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

Nice!

Great discussion! It has made me rethink my decision to put slightly beefier tire on the back wheel. I ride on rough and occasionally rubble-y roads, and my thought was that I should have a 32mm wide on the back and a 28mm on the front, so that the stiffer and more heavily loaded rear triangle would have more tolerance before I experience a "pinch flat" on the rear if I fail to "post". The trail discussion changes my thoughts - a slightly larger front tire would mean a slightly larger trail and therefore slightly more stable steering - though I think that for large size wheels, the difference will only be slight.

A year or two ago there was a chorus of voices suggesting people (everyone?) switch their 27" wheels to 700c. I raised the point at the time that it would reduce the trail by about 1.16mm, and asked whether this would affect handling. The chorused answer at that time was no, it wouldn't make much difference. Since both A and B were true, where A = enough people say it, and B = I read it on the Internet, then it must be true, right?

So carrying this thought forward, a switch from 32mm to 28mm tires means a cross-sectional radius difference of only 2mm. (Does this really mean a change in ride height by only 2mm, or something different?) That's less than the supposedly insignificant 630 to 622 wheel switch. A smaller front tire means the bike will point slightly downhill but much less than 1deg. This will further reduce trail, but is a tiny amount by itself.

The point being that you probably wouldn't notice much difference. But that's only because the Internet and enough people said so.

Take a bike, run 21-23 mm tires one it and then run 27-29 mm tires on it. Take some descents. It will not feel like the same bike.

Great discussion! Thanks all.
I'm gonna try this.
Edit: Post #1000. A milestone.

+1.
wheels (esp "aero" front wheels) and tires influence handling tremendously, so do chainstays. But this was originally a thread about steering geometry.

I think it is but, even for a given rim size, the tire cross section difference especially in front has to at least be acknowledged.

I discount some of the extreme views on this with the 650b enthusiasts but the dynamics of the bike are effected.

I take your point (but I think I will not do the experiment ). However that tire switch makes many things different, such as tire deformation itself, the total wheel&tire weight, the amount of cushioning from the tires, etc. I wonder how much of the change in feel is due to each.

Oh well, it's fun to speculate.