Tuesday, December 01, 2009

The following was posted by me at a forum, but is worth repeating.

I'm going to step on some toes here. It's not my intention, but hard to avoid given the subject matter.

Anyone, including Brendan, is entitled to his/her opinion and to share same.

ATMO, not too many folks really know all that much about frame design, from established (even relished) handbuilders to product designers at major bike manufacturers. Don't get me wrong, they know enough to make a decent bike. But let me offer a few illustrations.

Years back, when I participated more on the framebuilders list, virtually no one could speak to their rules of thumb for determining the right combination of tubing diameter/wall thickness. Richard Sachs and Freddy Parr could offer insightful input on this topic, but no one else then participating could. At least not beyond the opinions that might be offered up here by riders (as opposed to builders). Mind you, all the builders here weren't participating then on the list: so please don't make any assumptions about any particular builders. Anyhow, the lack of input really opened my eyes up a bit - especially when I saw the range of solutions that recieved glowing reviews through various online/print venues.

In another example, its hard to find anyone who can really speak well to the pro/cons of different front geometries re handling. That includes builders and other industry participants who too often bluster their way through the topic. Let me illustrate some of what I mean, but first a warning that a lot of facts about specific designs are hard to find. For example, look at EPS geometries on Colnago's website: head-tube angle, rake, and trail aren't listed. It's a deep secret.

But, some secrets can be unraveled. Using a combination of resources (BikeCad, Anvil Trail Calculator, Excel), and keeping to a limited range of sizes (55-56CM top-tubes), it's possible to make fairly accurate projections of how these key measures vary by brand. I picked the size range based on it being toward the mid-point of sizes, and presumably less succeptible design anomolies that might be present at the ends of the size ranges.

NOTE: there are precision errors in published data - plus some companies may intentionally misrepresent specs. Anyone who can, is welcome to improve the accuracy of the numbers that follow.

Colnago EPS HTA=72 Rake=43 Trail=64.
Dedacciai Scuro HTA=72 Rake=44.5 Trail=62.
Cannondale's 'comfort' fit HTA=72.5 Rake=44 Trail=59.
Trek's 'performance' fit HTA=73.5 Rake=40 Trail=58.
Trek's 'comfort' fit HTA=73.5 Rake=40 Trail=58.
Cannondale's 'comfort' fit HTA=73 Rake=44.1 Trail=56.
Specialized 'performance' fit HTA=73.5 Rake=43 Trail=56.
Specialized 'competition' fit HTA=72.5 Rake=40 Trail=56.
Pinarello Dogma HTA=73.5 Rake=43 Trail=55.

We see the EPS with a trail of of 64mm while the Dogma has a trail of 55mm. Both are considered great handling bikes. An easy out would be to suggest that the Dogma is a crit bike - but I don't think Pinarello considers it such. Moreover, if there are different handling paradigms for different events, why don't more builders/manufacturers offer clearly deliniated models with different geometries for different events?

Instead, many lines carry the same geometry through broad portions of their line. Others that don't, make no attempt to suggest differing uses for models with different geo's.

In fact, looking at most of the literature, the debate re designs-for-events focuses on the question of weight versus stiffness. ATMO, that's an artificial distinction, but that's what prevails in the market today.

Meanwhile, I can come up with quite a list of venerated existing/retired builders who will simply say to keep the trail between 57-58mm and everything will be fine.

Going back to the sometimes deified Colnago, I believe that they stick to a 43mm rake throughout the size range. With the variations of HTA that occur across sizes, this will lead to different handling characteristics between their smaller and larger frames.

Where an I going with all this? The thoughtfulness and care provided by your builder is more important than tenure in role. And that's ATMO.


Jim G said...

Trail is dependent on tire size/width. Did you assume a constant tire when you calculated the trail values for the sample bikes?

And, trail under 50mm is delightful, especially with a bit of a front load.

Rick Guggemos said...

Actually, trail is not dependant on tire width.

However, among the bicycles listed in the examples, we can assume a tire size of 700x23. A few folks will go down to a 700x21 or up to a 700x25. Nonetheless, this is such a narrow range as to be of little consequence.

Regarding low trail designs: there success is dependent on tire size, weight distribution, and HTA. Generally, close to 50mm falls in a bit of a no-mans land between normal and low trail designs, but can be made to work with the right combination of the factors above.

David A said...


Couldn't agree with you more. I have looked at this exact same issue in the past and came up with the same conclusions with respect to most large MFG and some hand made bikes. I always get a kick out of how a Colnago with 64mm of trail will be lauded as "quick handling", "responsive", "intuitive", & etc. in the exact same way that a Dogma with 55mm of trail will in the bicycling press...but, then again, those articles are about advertising dollars, not reality.

As you say " The thoughtfulness and care provided by your builder is more important than tenure in role."...Amen.

Dave Anderson
Anderson Custom Bicycles