I'm in the process of ordering a new custom ti frame (Enigma Eulogy (http://www.enigmabikes.com/eulogy.htm)) and would like it to have a little more trail than my Cervelo SLC-SL (54). I'm extremely happy with my Cervelo, and feel I've got the fit dialed in. Enigma's fitting process includes a series of body measurements and the numbers from my current bike. I expect most of the frame geometry will follow quite close to the Cervelo (given its good fit) but I would like to see a little additional trail in the front end.

I've made an attempt to measure the trail on the Cervelo and come up with about 50mm. I've inquired on the Cervelo forum for the official number.

Does anyone have an idea whether my measurement is accurate and thoughts on what a good target would be for trail on the Eulogy?

Slightly more slack head tube angle and/or fork with less rake?

::Michael

thanks for any suggestions.

The head tube angle on the Cervelo is 73 degrees, so all you need to figure the trail is the fork rake. The formula for trail is R/tanH - rake/sinH.

If the rake is 43mm, the trail would be 57.8mm. With a 45mm it would be 55.7mm, assuming a 336mm tire radius in both cases.

If you want much more trail, then you should alter the HTA, so you can use a readily available fork rake.

55-57mm is usually called neutral. I prefer 60mm with a low BB myself. I can put up with a little more wheel flop/wander going up a mountain for more stability on the way down.

Thanks Guys,

I knew it. Sooner or later that high school trig was going to be useful. I was counting on a reply from C-40; since I was reading another bike fitting thread and you had some great advice. After I posted I found that you answered this question way back in 2003. Should've checked first.

The Cervelo fork has 43mm of rake, so with a little lattitude for tire radius, I'm currently between 57 and 58mm of rake.

If I kept the same HTA and spec'd a Reynolds Ouzo UL fork with 40 degrees of rake (a standard size), and everything else remained equal I'd have 65 mm of trail. Would you both agree this is too much?

I think I'll ask the designer, Mark Reilly to aim for 60mm of trail per Rene's suggestion, with a Reynolds Ouzo UL fork with 43mm of rake. That will mean a slightly slacker HTA.

C-40 do you agree that's a decent trail target given my objectives vs. the Cervelo

thanks

::Michael

A 72.5 degree HTA would give you about a 61mm trail, which isn't a big increase. A 72 degree HTA would produce 64mm.

Thanks for the follow up C-40, I'm going to suggest the 72.5 HTA and 61mm of Trail figure to the designer and get his thoughts. Looking forward to the riding the Eulogy and the Cervelo in 2008.

Happy New Year and good riding.

I'd assumed your tire radius number was likely to be standard, but after measuring my Topolino AX2.0 wheel with Continental GP4000s tire I get a radius of 345mm

345/tan(73) - 43 x sin (73) = 64.4mm of trail

obviously the deeper the tire, the greater the trail, but the continentals don't strike me as unusually deep.

Based on the nuances of millimetres that have been discussed, if my effective trail is 64.4mm I'd expect the Cervelo's handling to be very mild mannered.

Did I get something wrong?

A conti 700 x 23 will runout at about 2110mm, so the radius is 336. This is only relevant if you plan on running different tires from one bike to another.

C-40,

when you say runout, do you mean the effective radius of the wheel+ tire combination when the rider is compressing the tire?

336 millimeters from hub center to compressed outer surface of tire?

Just trying to follow along. I think my math was right, just need to understand the assumptions behind it.

when you say runout, do you mean the effective radius of the wheel+ tire combination when the rider is compressing the tire?

Runout is the circumfrence of the tire. It will vary slightly with rider weight during the rollout. 2110 mm divided by 2 (Pi) = 336 mm.

A conti 700 x 23 will runout at about 2110mm, so the radius is 336. This is only relevant if you plan on running different tires from one bike to another.


I get 338 with a Michelin 23c tire, which is lower profile than a Conti 23.

Nessism,

It sounds like you and I are actually measuring our wheel + tire. C-40 and Kerry are deriving the radius from the circumference. I'm confused.

Can you guys explain how direct measurement is not the way to go? The tire is fully inflated to the recommended pressure.

Consider my previous question about the effective height of the tire compressed by rider weight; that's the only explanation for the difference I can come up with.

Good discussion though

::Michael

Nessism,

It sounds like you and I are actually measuring our wheel + tire. C-40 and Kerry are deriving the radius from the circumference. I'm confused.

Can you guys explain how direct measurement is not the way to go? The tire is fully inflated to the recommended pressure.

Consider my previous question about the effective height of the tire compressed by rider weight; that's the only explanation for the difference I can come up with.

Good discussion though

::Michael

Direct measurement is the proper method, better yet, measure with a rider on the bike to compress the tire slightly such as will be the case while riding.

It's downloadable...

http://www.anvilbikes.com/

My real point was that you're comparing the design of the new bike to existing ones and wanting more trail. Unless you know that the new bike will have a different size or brand of tire than the others, the tire radius doesn't matter that much.

Most of us buy a frame and get whatever trail the manufacturer thinks is best. Some riders might use a smaller 700x 20 tire and others a large 700x28, but I don't see people wanting to change the fork rake to make up for the change in trail. I swapped from my regular 700 x 23 Michelins to some 700 x 28 Conti 4-seasons and the rollout is only increased by about 13mm. It only changes the trail by 1mm.

While it's tough to do, rolling out a tire over several revolutions while loaded and aired to the normal pressure will give the most accurate tire circumferance, which is converted to a radius by dividing by 2 x pi (6.283). I assume that "direct measurement" means using a scale and measuring from the floor to the center of the axle. The tires still need to be aired properly and a rider on the bike to get the most accurate measurement. The wheel must be also be sitting absolutely vertical and the scale held vertically, or errors will occur. Either method has it's possible errors. I checked a 700 x 23 Conti GP3000 with no load and got about 338 by direct measurement. It's certainly not 345.

Regarding your new custom frame, I'm always skeptical about a builder's ability to make such minute changes and verify those changes with sufficient accuracy. It all depends on the precision of the builder's frame jigs and how much distortion may occur during welding. The builder also has to account precisely for the fork length that will be used. Fork length affects the HTA. Stock CF forks vary by as much as 9mm from one brand to another and that can change the HTA by .5 degree. A lot of people buy a different brand of replacement fork for their frame and never consider the length. Some builders even offer a choice of fork brands. I'd be surprised if they properly compensate for length differences.

I know some custom builders do compensate for fork axle to crown length and headset height, though as a practical matter the headset measurement is probably overkill.

Direct measurement is the proper method, better yet, measure with a rider on the bike to compress the tire slightly such as will be the case while riding.

You're right that there should be a rider on the bike, but "direct measurement" is not necessarily any better that circumference measurement. The wheel must be absolutely vertical and the scale use to take the measurement held vertically, or errors will crop up.

Measuring several revolutions of the tire, with a rider on the bike should produce the same value for the tire radius. If not, then one of them was done incorrectly.

A suggestion for the OP: rather than trying to specify the details of the frame geometry for your builder, try instead to compile a list of objectives. Consider the handling and ride qualities you desire, your position (or range of positions), tire clearances, cable routing, aesthetic details, misc fittings (chain hanger, pump peg, fender/rack mounts), acceptability of toe-overlap, etc.

My point is, free the builder to build a bike to satisfy your objectives rather than hamstringing him to a set of design parameters; a good builder should be able to satisfy your objectives. When you're flying down the road, you care about how the ride feels beneath you, not what trail you rollin' with.

Good luck and enjoy the process.