Newb Question: Trail, what is too far?

[jamaris]

Okay, this might be already covered in some capacity and may get some groans, but the question (which also probably has too many variables) is...

If things are generally normal (nothing too crazy in terms of chain stays or head tube angles or lots of other things I am sure I am missing), can you make trail as "low" as you want it/handle it?

Or maybe a better question is what is the lowest trail value you have ever built a road bike around (let's say 700x23 tires for ease of use even if we know 25< is better)?

[WadePatton]

I'm not answering your question directly but using your post (do you feel dirty?) to share some trail observations as yesterday i spent a while studying upon it.

First, as a newb, I simply used established figures for front end angle/rake to get trail right in the middle of the range for RR bikes. It works, and I was re-using a store-bought fork.

Now that I've built a fork (to match the one I designed around above), I know that the angle and rake can be completely manipulated by my work. And the long story short of what I was studying yesterday is that Angle and Rake are inversely related...i even measured with a laser pointer.

wait i'm getting too deep.

The range i just dug up is 50-65mm. I used to consult Paterek's book, but i misplaced that in 2009. I'd stay away from the short end unless putting extra weight up front or wanting a nervous nelly.

[GrayJay]

Probably the most informative description of the effects of fork trail I have found is on Tom's spectrum site;
Spectrum Cycles | Geometry

Ive gained my best firsthand experience with trail by riding bikes with extreme trail to magnify the effects, make it easier to feel. Try taking an old junker fork and re-raking it to alternately setup a bike with 100mm trail and 40mm trail. Ride them both and feel the extreme of the range (then toss the fork).

Interesting to note that Merckx had a preference for low fork trail number on rough roads, seems rather counter-intuitve as most all modern bikes use a relativly high trail for dealing with rough/offroad conditions (for example, production CX and MTB bikes that are most typically in the 65-80mm trail range).

I noticed a cyclocross magazine pro bike profile recently (maybe it was Johnathan Page's bike??) where he listed that he hauled around a couple of forks with different rake to the races, apparently in order to be able to fine tune the bikes trail to suit different CX course conditions. All I can figure is that on very soft conditions (like sand), the wheel contact point shifts forward as the tire sinks, effectivly making the trail even higher. Is this reason enough to start with a lower trail setting, so that when it sinks a more normal trail is reached? What other CX trail tuning considerations might there be to warrant switching out a fork?

[jamaris]

Probably the most informative description of the effects of fork trail I have found is on Tom's spectrum site;
Spectrum Cycles | Geometry

Ive gained my best firsthand experience with trail by riding bikes with extreme trail to magnify the effects, make it easier to feel. Try taking an old junker fork and re-raking it to alternately setup a bike with 100mm trail and 40mm trail. Ride them both and feel the extreme of the range (then toss the fork).

Interesting to note that Merckx had a preference for low fork trail number on rough roads, seems rather counter-intuitve as most all modern bikes use a relativly high trail for dealing with rough/offroad conditions (for example, production CX and MTB bikes that are most typically in the 65-80mm trail range).

I noticed a cyclocross magazine pro bike profile recently (maybe it was Johnathan Page's bike??) where he listed that he hauled around a couple of forks with different rake to the races, apparently in order to be able to fine tune the bikes trail to suit different CX course conditions. All I can figure is that on very soft conditions (like sand), the wheel contact point shifts forward as the tire sinks, effectivly making the trail even higher. Is this reason enough to start with a lower trail setting, so that when it sinks a more normal trail is reached? What other CX trail tuning considerations might there be to warrant switching out a fork?

Maybe a separate thread for this type of trail? Since as I understand it, it varies significantly from road bikes.

[bellman]

You might look for info on the older 80's trek frame/fork combos. It seems to me like they were running 43-49mm of trail (not rake in case someone thinks i'm confused) and they handled quite nicely as they still enjoy an enthusiastic following.

[Andrew R Stewart]

There's a lot of variation in what people believe a "proper' trail should be. The Jan Heine dimension (not picking on Jan, just using his vast writings as a category) might be 38mm. This is considered to be pretty low trail. The claims, if i understand them, is that on rough roads and or with a front bag the handling will be "right". No over or under steering. The last couple of decades has seen the production racer crowd use 60-65mm trail often. This might be considered long trail. This seems to produce understeering/stability at speed and/or with a rear placed load. This is a very simple condencing of what i've read and experienced but I think it represents the range and handling natures. I am open for correction or a better understanding.

My riding experience and preferences go towards the longer trail dimension. I'm a short guy with the usual light upper body of a rider. I do carry some weight over the rear wheel many times, my touring loads are 65/35 rear biased. I don't have issues with low speed control/stability and look for mid to higher speed stability. I have had a lot of bikes with "speed man's shimmy". I like a more flexible frame.

So over the years of building my frames I have tended towards 61-63mm of trail and 71-72 degree head angles. My last couple of frames have been the most shimmy free and nice handling that I have made. The couple of frames from 10+ years ago with 55-58mm trail have not been as nice feeling.

But i will say that trail taken as a goal independent of other factors is not the fate complete that some seem to make it. The balance of frt to rr weight, the bb height, the stem length, the seat set back, the frame's stiffness torsionally all contribute to the road results. As another said- experiment and ride a lot of bikes then make something. Just don't expect the world to open up with the first bike you make. Andy.

[Jamie Medeiros]

A little light reading from Physics Today circa 1970
David Jones' classic article on bicycle stability.
http://www.phys.lsu.edu/faculty/gonz...9no9p51_56.pdf

[Dorman]

I like trail in the 5.9 t0 6.1 cm range. It just works right for me at 5' 8" and 160 lbs. General experience is the more weight on the front end the less trail you need/want. People are top heavy, so as the human gets larger more weight is going on the front end. There are so many variables that it is almost impossible to get a quantifyible answer. Preference is a whole 'nother question.

[jon_norstog]

Trail:

When you are building around a suspension fork or other ready-made fork, you are stuck with varying the headtube/steering axis angle. I never bothered to calculate trail since the fork manufacturers seem to have agreed on standards for "offset" (rake). My own approach is to go for sharper handling than the mass-produced mountain bikes, and to dial in the BMX bikes by racing them and going for a bike that will hold a line rather than diving in or flying over the top on a bermed turn. I also dial in the extreme-ride/hucking bikes and now that there is a 4X track in Boise I plan to work the kinks out of the 4X bike, finally!

On mountain bikes, the industry standard angles for XC bikes were 73/71 with a 16.75" chainstay. One size fits all. Now the majors have caught on to extreme riding and are building hucking bikes with very tight rear ends - 15, 15.5" chainstay length, relaxed seat tubes and lazy front ends. 70/68 angles. This gives you a bike that will straighten out if you blow it on a landing, even with the fork at max compression. But they have to build the front center over-long to make those slow front ends work riding on the level.

I think that as framebuilders we should let experience guide us. The majors are all building to the lowest common denominator customer, while we are building for individual clients, many of them advanced riders.

Topic drift: the question was what is the right amount of trail. The answer is, of course, "it depends." It's up to us to learn **how** it depends and make something of it.

jn

[GrayJay]

I noticed a cyclocross magazine pro bike profile recently (maybe it was Johnathan Page's bike??) where he listed that he hauled around a couple of forks with different rake to the races, apparently in order to be able to fine tune the bikes trail to suit different CX course conditions. All I can figure is that on very soft conditions (like sand), the wheel contact point shifts forward as the tire sinks, effectivly making the trail even higher. Is this reason enough to start with a lower trail setting, so that when it sinks a more normal trail is reached? What other CX trail tuning considerations might there be to warrant switching out a fork?

I got this backwards when I wrote it - sinking tire & forward moving contact patch result in decreased trail, not increased.

[Curt Goodrich]

I got this backwards when I wrote it - sinking tire & forward moving contact patch result in decreased trail, not increased.

I think you're over thinking this. In the case of Jonathon Page and the different forks, I'll bet a dozen donuts the decision process about which fork offset to use is based on just what feels right at the time. The trail is never discussed and doubtful that it's even considered. Most racers and for that matter most riders like what they like and that can change from day to day.

[jamaris]

I like trail in the 5.9 t0 6.1 cm range. It just works right for me at 5' 8" and 160 lbs. General experience is the more weight on the front end the less trail you need/want. People are top heavy, so as the human gets larger more weight is going on the front end. There are so many variables that it is almost impossible to get a quantifyible answer. Preference is a whole 'nother question.

I guess that is the tricky part. I generally like lower trail numbers for my personal road racers, in the 53-55 range whereas I don't think that is the norm. So preference can be a tricky thing.

[nick.graham]

Related question on trail: if a person had an ideal trail target or range (e.g. 50 - 65 mm from the Paterek manual, as referenced above), would this range change based on wheel size? I’m planning to build my daughter a bike with 24” wheels, and trying to sort out the fork offset. Most of the references I’ve seen to “neutral” trail figures seem to make reference to 700C wheels. Has anyone thought through how these trail figures might change based on wheel size. My assumption is they would be scaled down (presumably as a function of wheel radius), but thought I’d check in here to see if anyone has worked through this.

[Andrew R Stewart]

Nick- 8 years... not a record thread age but good job at plumbing the archives before just posting. I'm probably one of the more experienced hobby builders here when dealing with 24" (actually 520 ISO) front wheels.

Of the 4 frame/forks I have built with smaller ft wheels one used 600A(540ISO) and the other 3 used 520ISO, all had tires from 25 to 28mm real widths. The trails ranged from 45 to 50mm. Each bike was described as fairly quick steering. The shorter trail bikes were more loaded touring focused and the longer train ones had more "club" riding intentions. All bikes were for the same 5'2" female rider who wasn't fast but rode 4K+ a year (she had eye issues that made driving a challenge).

At the time she was alive and with me my thoughts were that trail should be relative to tire diameter. I initially sought advise from a friend with vastly more experience with smaller wheels, Georgena Terry. She followed Bill Boston's design guide line of about an 81* castor angle. What's that? See the image for explanation. The red line is the castor angle.

I've spent a LOT of time running my bikes and published specs of many others through a castor angle calculation and find most bikes range from about 79* to 81*. Very few are greater then 81*. The formula I use for castor angle is: Tangent of Castor Angle = tire radius / trail

Here's a cool trail calculator http://yojimg.net/bike/web_tools/trailcalc.php There are other interesting dimensional calculators available there. Andy

Castor Angle Chart 10-22-20.jpg[ATTACH=CONFIG]116882

[nick.graham]

Nick- 8 years... not a record thread age but good job at plumbing the archives before just posting. I'm probably one of the more experienced hobby builders here when dealing with 24" (actually 520 ISO) front wheels.

Of the 4 frame/forks I have built with smaller ft wheels one used 600A(540ISO) and the other 3 used 520ISO, all had tires from 25 to 28mm real widths. The trails ranged from 45 to 50mm. Each bike was described as fairly quick steering. The shorter trail bikes were more loaded touring focused and the longer train ones had more "club" riding intentions. All bikes were for the same 5'2" female rider who wasn't fast but rode 4K+ a year (she had eye issues that made driving a challenge).

At the time she was alive and with me my thoughts were that trail should be relative to tire diameter. I initially sought advise from a friend with vastly more experience with smaller wheels, Georgena Terry. She followed Bill Boston's design guide line of about an 81* castor angle. What's that? See the image for explanation. The red line is the castor angle.

I've spent a LOT of time running my bikes and published specs of many others through a castor angle calculation and find most bikes range from about 79* to 81*. Very few are greater then 81*. The formula I use for castor angle is: Tangent of Castor Angle = tire radius / trail

Here's a cool trail calculator http://yojimg.net/bike/web_tools/trailcalc.php There are other interesting dimensional calculators available there. Andy

Castor Angle Chart 10-22-20.jpg[ATTACH=CONFIG]116882

Thanks so much for the prompt and thorough response, Andy! Very helpful!

[guido]

My Elephant NFS calculates out to 35mm trail with BabyShoePass tires. The steering is quick and intuitive with a light front load (tools, tubes and a jacket) in an Acorn tall rando bag. My notion of perfection. Long trail bikes just feel sluggish in comparison. I ride several of them but they never feel right.

[jon_norstog]

I did a post on low-trail forks and have since got a fair bit of experience with them on touring bikes. You may find it here:

https://www.velocipedesalon.com/foru...rks-44962.html

I may add a little to it soon.

jn