Trail and frame sizes

[shand]

Hi

following on from the other thread about trail, thought I'd ask the question here on what peoples thoughts are on 'ideal' trail figures for bikes of various sizes.

As a builder, you obviously want to build for a trail figure you think is right for the job. Does that ideal trail figure vary for small frames and big frames? I've always tried to keep the trail figures on our bikes close across the size range and this means if I need to steepen or slacken a head angle to get the right fit, I'll alter the offset to suit, so I'm working with a trail figure that's fairly consistent across a size range.

However, many of the the bigger brands (and I assume some smaller builders too) that are speccing a production (probably carbon) fork with a fixed offset have a relatively wide range of trail figures for what is essentially supposed to be the same frame design (albeit in a different size) as the smaller frames will mostly have a steeper HA while retaining the fixed offset of the production fork.

I'm not really quite sure what my question is here, I just find it interesting that so much emphasis (at the bespoked/custom level) is placed on trail, yet on high end production bikes we don't see the same level of attention to this particular detail.

Actually, on re-reading this, perhaps I've just summed up why a custom frame is a custom frame!

[e-RICHIE]

I'm not really quite sure what my question is here, I just find it interesting that so much emphasis (at the bespoked/custom level) is placed on trail, <cut>

I don't know what the trail measurement is on my frames so can't add much. But having said that, I never designed a frame around trail or considered it.
For me, on every frame and order, front center in mm is a critical measurement and I will alter other elements if the f.c. doesn't resonate with me atmo.

[Clockwork]

I don't know what the trail measurement is on my frames so can't add much. But having said that, I never designed a frame around trail or considered it.
For me, on every frame and order, front center in mm is a critical measurement and I will alter other elements if the f.c. doesn't resonate with me atmo.

Richie, would you share your thoughts on front-center and what numbers you use for different bikes/riders? I've always focused on cockpit length and then trail, so I'm interested in the front-center focused method.

Thanks, Joel

[e-RICHIE]

Richie, would you share your thoughts on front-center and what numbers you use for different bikes/riders? I've always focused on cockpit length and then trail, so I'm interested in the front-center focused method.

Thanks, Joel

I focus on cockpit length too; that measurement is for the rider, the front center is for the bicycle. They work together. Let's say I am the example. My reach on a road unit is normally 51cm - 52cm from saddle nose to center of handlebars. There are a myriad (that's Middle Eastern for lots of...) of ways to achieve this AND they each will result in a different f.c. too. I prefer to have my Aspide saddle at about 71cm extension and with a 4.5cm (or so) setback.

I will make the frame 54cm c-t with a 14.5cm setback. Now, since I like my fronts to be in the 59.5cm range, I find a way to hit that number using a combination of top tube length, head angle (which I never measure), and fork offset range. Since I already know my forks are usually 4.7cm - 5.2cm in rake, that is a basis for altering the other two options to get my desired reach.

My Bike Machinery Hydra fixture also serves a giant template or design-o-meter. I can lock down 2-3 measurements and vary others around them to see what others will be down to the millimeter. If the seat tube is leaning at 14.5cm and the rake setting is at 5cm (as a starting point), I then proceed to decide what top tube length with what lean of the head tube (I change the lean back and forth, never looking at the head angle hash marks on the back plate...), and see what the tool says the resultant f.c. would be. Oh, I also know that I desire to use an 11cm stem, so that is also a control.

Whatever top tube length and head tube lean permit my cockpit measurement to work with the other key control elements I use - this is the length (and the lean) I will use for my frame. Then, I cut pipes to fit the fixture, as it has now become my finished draft.

I began thinking in linear measurement terms (and going away from using angles completely) in the late 1970s. It's long since been an intuitive decision, the one I make to select and feel confident about which front center and other linear measurements control the design. The Hydra makes it easy too because it's like a Ouija board in many respects. In most cases, the mental energy devoted to arriving at a solution to each frame order borders on a minute or three. There is rarely any deliberation or second guessing. No formulas. No cad. It's repetition, repeated over again, refined, and repeated - frame after frame. Sooner or later it makes sense, even though the process may seem confounding or even contrary in this app-it-all era we are in.

Now, to maybe answer your question going forward, each time I start an order, I look at the rider's contact points, how his bicycle is laid out, ask a few questions about heartaches and groin pulls, and use all of this information to then spend some 120 seconds to decide how the Hydra will be configured for that client's frame. It's jazz, really. No over-thinking or analysis need apply atmo.

[shand]

front center in mm is a critical measurement....

do you consider this critical for fit, handling or both?

EDIT: Looks like we posted at the same time and you answered my question!

[e-RICHIE]

do you consider this critical for fit, handling or both?

Both - fit and handling are inseparable atmo. I should also add there it's very easy for me to be as cavalier about this as I am. Over many years I have used many bicycles and made many, many more for others. Sooner or later something makes sense, and the natural tendency to want to fit stuff into neat little compartments passes. What's left (or next) is the time in which everything just gets done without thinking about it, or asking why.

There really is no formula or magic bullet to this frame design and/or fit stuff. But I will tell you one thing: designing a unit around angles or trail is one path I never took. That method made no sense to me even back when I did have a protractor in my shop atmo.

PS I continue to believe that the sport is the best classroom and test bed. Ride lots. Find/exploit alliances in the racing community. Study what works there. Lather, rinse, repeat. Whatever learning curve you want to reconcile will become a straighter line faster if you take this free advice.

[Luke]

Whatever learning curve you want to reconcile will become a straighter line faster if you take this free advice.

We are fortunate to have your experience to absorb, more fortunate that you share it so easily...oozing atmo our direction.

[Craig Ryan]

The Hydra fixture presents a different perspective, and I would love to work with one/see it in action for the process. With my current fixture I must establish numbers first through a drawing, either CAD or not, then mimic them on the fixture. When I make a change to the front end I have to go back into my drawing and recalculate. This is not to say I don't like making drawings, it takes time though. I've adopted 590 as a target, but struggle to achieve it with some riders reach. What Richard has written clarifies much and makes me a strong believer. Perhaps less attention to trail gives a few mm's which are well placed.

[jamaris]

We are fortunate to have your experience to absorb, more fortunate that you share it so easily...oozing atmo our direction.

Yeah reading this stuff, makes me feel like I am cheating. Kind of you to share and thanks again.

[Clockwork]

Thanks, Richie.

[Chauncey Matthews]

I've adopted 590 as a target, but struggle to achieve it with some riders reach. What Richard has written clarifies much and makes me a strong believer. Perhaps less attention to trail gives a few mm's which are well placed.

Hi Craig,
Are you saying 590 is the target for all frames or just the ones in a certain size range?

[Craig Ryan]

Hi Chauncey, what I meant is I like to use 590 as a minimum target, and get all frames up to that measurement. I wasn't referring to going over 590, that's when it's easy. Riders who have limited reach make it hard to hit the 590 mark, and I've found myself compromising. I believe FC is of primary importance, but I wonder if there is a point where the trade-offs made to preserve FC have diminishing returns. Interestingly I made that 590 mark in my mind after reading a discussion on the frame email list around 7 or 8 years ago. it stuck with me at the time because two builders with high cred agreed on it as a base minimum number. And those two never seemed to agree on anything otherwise.

[jamaris]

Both - fit and handling are inseparable atmo. I should also add there it's very easy for me to be as cavalier about this as I am. Over many years I have used many bicycles and made many, many more for others. Sooner or later something makes sense, and the natural tendency to want to fit stuff into neat little compartments passes. What's left (or next) is the time in which everything just gets done without thinking about it, or asking why.

There really is no formula or magic bullet to this frame design and/or fit stuff. But I will tell you one thing: designing a unit around angles or trail is one path I never took. That method made no sense to me even back when I did have a protractor in my shop atmo.

PS I continue to believe that the sport is the best classroom and test bed. Ride lots. Find/exploit alliances in the racing community. Study what works there. Lather, rinse, repeat. Whatever learning curve you want to reconcile will become a straighter line faster if you take this free advice.

So I may not be thinking through this enough, but when someone wants the bike to have faster steering, what do you do to the fc?

[shand]

when someone wants the bike to have faster steering

I suspect that doesn't happen often

[e-RICHIE]

So I may not be thinking through this enough, but when someone wants the bike to have faster steering, what do you do to the fc?

You add more rake to the fork atmo.

[jamaris]

You add more rake to the fork atmo.

Ok, that makes sense. Thanks.

[jamaris]

I suspect that doesn't happen often

Haha, sorry cars on the brain, had the car at the shop today.

[benson]

For me, on every frame and order, front center in mm is a critical measurement and I will alter other elements if the f.c. doesn't resonate with me atmo.

If front centre is the critical measurement, why do you vary fork rakes at all?
A 5mm range of fork rake has much the same effect on front centre as adding or subtracting half a degree of head angle.

[e-RICHIE]

If front centre is the critical measurement, why do you vary fork rakes at all?
A 5mm range of fork rake has much the same effect on front centre as adding or subtracting half a degree of head angle.

I use more rake on smaller frames and less rake on larger frames.
No matter what size frame I make, leaning the head angle more forward not only reduces the front center, it increases the reach.
The converse is true if the head tube tilts back further.
Maybe I don't understand the question atmo.
The most critical measurement is the anxiety level; I don't let any single detail indenture me to it.

[WadePatton]

huh huh huh huh.

i incidentally made my roadie 579. noted that that was too short, but didn't "design" by FC. I looked at standard established HTA's and Rakes and Trail ranges-and made sure i stayed "inside the lines". Turns out it's a bit short for my flip-flops (handles fine), so i made sure not to get any shorter on the next one:

So i go looking at my current frame-in-progress cad file...and the number is:

593. (triangle is cut, fork can still be manipulated-but i don't thinks i will).

and i'm soaking in some of this linear stuff. thanks richie.