Educate me on fork rake/trail

[ANAO]

I just got off the phone with Billy and, amongst top tube slope, bb drop and a slew of other geometry design points, we discussed fork rake and trail.

We're building a crit-centric bike and he mentioned that he likes to use 45 rake forks so that the bike feels nice and stable diving into a corner with 70mm of BB drop.

I gave him a Columbus Grammy that uses 43.

He said it was no problem; that he noticed and just adjusted the headtube angle to account for the difference and I wouldn't notice any more toe overlap than I would if I were on a 45. After hanging up, I'm now wondering: why is 45 better than 43, or vice versa? It seems like a very small difference, but what difference is there?

[cadence90]

http://www.spectrum-cycles.com/geometry.php

From Tom Kellogg, Spectrum:
Trail and Its Effects
Fork rake:The distance that the front axle leads the imaginary line drawn through the center of the head tube (or steering axis).

Head angle:The angle described between the ground and the steering axis.

Trail:The distance between the front wheel contact point with the road and the imaginary point where the steering axis meets the road.


As a general rule when dealing with 700-C wheels, a trail of about 56mm will give a frame set "neutral" handling. My use of the term "neutral" here refers to two things. First, neutral handling means that a frame set will respond to steering input in the same manner no matter what speed the bicycle is traveling. Second, while cornering, a neutral handling bike will have neither a tendency to climb out of a turn nor have a tendency to dive into the turn, it will simply hold the line that the rider sets up unless further rider input is applied.

Decreasing trail below the neutral range has a couple of effects as you might expect. The first thing a rider will notice about a low trial bike is that it appears to resist attitude changes (lean angle changes). It requires more physical effort to get the bike to lean into a corner and more effort to get it to straighten up. The second thing that you will notice is that while cornering at higher speeds, the bike will have a tendency to climb out of the turn on its own. Finally, you will find that the way the bike responds to rider input is affected by the speed of the bike. As you might have guessed by now, at lower speeds, a low trail bike will have a tendency to want to go straight and do so pretty much on its own. What you will find at higher speeds (like over 30mph) is that a low trail bike will become quite vague in the front end. The front wheel will feel as though it is wandering a bit and the contact patch feel will simply go away.

Increasing trial above the neutral range will cause opposite effects for the most part. At lower speeds, handling response will be light and consequently, attitude changes will be much easier. During cornering, the bike will have a tendency to drop into a tighter arc than the rider might have intended. Finally, speed's effect on handling is reversed. While low speeds give a light feel during handling maneuvers, high speed sets up a very solid front end feel.

Although high trail frame sets give safer (more inherently stable) handling than low trail frame sets do, high trail frame sets are still inconsistent in the way they respond to rider input. Interpreting from the basics above you can see why we usually aim for neutral trail. It does not require the rider to consciously hold a bike down during hard cornering, nor does it require different rider input depending on changing speeds.

For some frame designers though, it is not always that simple. For example, look at the way Eddy Merckx designs most of his frames. He usually uses less trail than the "ideal" as he did much of his racing on the pave and likes the way a low trail frame tracks under really horrible conditions. Granted, they do not act as consistently under a variety of speeds on good roads, but they really work on northern Europe's country tracks.

When we are asked to build frames for either loaded touring or Randonneur riding, we need to use lower trail numbers to keep handling consistent even when extra weight is loaded onto the bike in front of the steering axis. The degree to which we change the trail numbers depends on both the additional weight being carried and the handling requirements of the rider.

Track frames are a whole different thing though. If you would like me to, I can get into that. Let me know.

I did these drawings for a builder some years ago. They may help.

45mm rake versus 43mm rake = pretty minimal.
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[Peter P.]

A 2mm difference in fork rake will not be noticeable on handling or toe overlap.

But since "Billy" (whoever he is) said he would adjust the head angle to accommodate the fork rake, it sounds like he has a specific trail figure in mind.

Typically, as the frame gets larger (longer top tube), builders and manufacturers try to keep the wheelbase near-constant across the size range so they steepen the head angle. If the fork rake were left unchanged as head angle was steepened, there would be less trail. The result is a bike with less self-steering stability.

So builders and manufacturers use less fork rake on bikes with steeper head angles to keep trail relatively constant. Many manufacturers just use a 43mm fork across the entire size range because it's "close enough" to work with a variety of head angles. Custom builders, building steel forks, have more flexibility.

I would caution you against building a "crit-centric" bike. While ultimately rideable in all conditions, it can be less enjoyable for day-to-day riding, long term. An all around racing bicycle is perfectly capable as a criterium bike, as evidenced by many of the bikes currently seen at criteriums across the nation.

[ANAO]

Billy is from COARSE. He and I decided to build a crit bike because, firstly, he used to build for spooky and is very familiar and comfortable with the geometry of the skeleton (which he tweaked a bit to make a steel ride react how he wants, such as a lower bb and tighter rear end) and, secondly, I only race crits. When I'm not racing, I'm on my commuter.

Maybe one day, if my wife let's me disappear for days or even 12 hours at a time, I'll drive somewhere foreign and try a road race. But for now, this is what the metro-nyc offers and this is what I like to do.

Thanks for everything else in your post - really educational. [emoji481]

[Mark McM]

I agree with the others - a 2mm difference in fork offset is relatively small, and would be barely noticeable. If the builder wants to modify the head angle to achieve the same trail, they would only have to steep the angle by about 1/4 degree for the shorter offset fork.

But I have to comment on the Tom Kellogg article on the affects of trail - and my comment is that I find very little there to agree with. Trail is just one variable in bike handling, and the article appears to draw overly generalized conclusions about the affects different amounts of trail.

I suspect that Mr. Kellogg has been building a narrow category of bikes for a long time, and has not really looked at a wider range of different types of bikes and riding environments. For example, he states that low trail bikes require more rider effort to turn at low speeds, and that high trail bikes are easy to maneuver at low speeds. This goes counter to the experiences in mountain bikes, where low trail bikes are more nimble in low speed handling, while high trail bikes require more rider input to make rapid maneuvers.