Fork offset/trail, front-center, etc.

[wwtsui]

Was following davids' thread (http://forums.thepaceline.net/showthread.php?t=11361&page=1&pp=15) from earlier this week with great interest, as I've been puzzling over why my current bike feels "twitchier" than my old Lemond Zurich: both have 73 degree head tube angles, and 45mm fork rakes (based on Zank's post in that earlier thread, I think the Wound-up fork on my current bike is 45mm -- see picture below).

My (admittedly basic) questions:
What is "front-center", and how does it affect handling? It sounded like it had something to do with weight distribution, and perhaps cockpit length. If that's correct, the effective top tube difference is 7mm (new bike is 53.8cm vs. old 54.5cm), which I think is largely explained by the difference in seat tube angle (73.25 vs. 74 degrees on the new bike).
What is "race height"? I have no idea what the measurement is for the Lemond fork, so no idea whether that would help explain the difference between the two bikes.

The difference between the two bikes doesn't really bother me most of the time; however, I find I have to be more careful when I reach for my waterbottle & take a drink, or am looking at something as I ride by. Also, if I'm pedaling hands-free, the bike does weave a tiny amount, which is a bit disconcerting (I'm sure it's yet another indication that I need to keep working on evening out my stroke :crap: , but still...)

I'm considering changing forks, primarily because I'm planning to do the PanMass Challenge next year (190 miles, 2 days), and figure that I'd rather not worry about winding up in a ditch while doing my first really long ride. Using Dave's formula from that prior thread [R/tanH - (offset/sinH)], switching from a 45mm to 40mm offset fork would increase trail from 55.1 to 60.3 mm (assuming I'm doing the formula right). How noticeable would that be? Any recommendations on forks?

Would appreciate any thoughts. Thanks.

[Smiley]

Increasing the trail is always a good thing if your wanting to slow the front end down a bit . Just keep in mind that both forks must have as close a SPAN as possible or if the new fork is taller ( span ) then your artifically changing the head angle and changing trail while your doing this . I'd say its a safe bet that you'll be happier with a 6 cm trail then a 5.5 any day . Also use an Ouzo pro fork cause this is a sweet fork too boot from the Wound Up your showing .ps: they do have a 40 mm rake fork in the Ouzo Pro as when i considered making my change I should have picked this fork over the 43 mm rake I used . Good luck .

[Brian Smith]

can we see the stem and bars too?
while switching to less fork offset would increase your trail to a more stable number, the reduction in front center, unfortunately, could serve to destabilize you a bit. IMO, this would especially be cause of worry if you were already on a long reach stem and bar combo. Without seeing the stem/bar setup, my thoughts would be:

1) mass market forks (such as the Lemond) are notoriously off spec and could actually be different from the Wound Up, even though they are both "listed" as the same offset. For that matter, many carbon forks are notorious for being out of spec too. Taking an accurate measurement is not easy without specialized tooling.

2) the 7mm cockpit length differrence is not great, but it's almost free to close that gap a bit more with an extra stem centimeter, compared to the cost of trying another fork. WWtJD? (What Would the Jerk Do?)

3) even with the same steering geometry, the more forward saddle position of the new bike could be putting more of your weight onto your front wheel, causing you to exaggerate your steering correction movements, making the steering seem more unstable.



"race height" probably refers to a measurement of the fork's length from the front hub axle to the installed position of the bottom of the headset called the "crown race." It is not something that directly affects steering feel.

[tch]

is the distance measured from the center of the bottom bracket directly to the center of the axle of the front wheel (or fork dropouts). As I said in my post earlier in the thread referenced, I think MY issue had a lot to do with front center. If you have a short front center relative to other dimensions on the bike, you overload the front wheel and get the sense that it is too much underneath you. It leads to a very unstable feel. My own, admittedly unscientific and personal reaction is that I am happier with a longer front center and a trail number close to 5.8, rather than a shorter front center and a longer trail.

[wwtsui]

Brian -- Jerk would definitely not approve of my stem/bar combo -- it's a 10 cm +17 degree setup, resulting in relatively little drop (I think 2 or 3 cm) and a decidedly unfashionably upright position -- both are concessions to my middle-aged (bordering on geriatric) back, and were fine-tuned by Fitwerx this past spring. Given how short the stem is, I'm guessing the ~5 mm reduction in front-center (thanks for the explanation, tch -- I guess I should have been able to figure that out from the BikeCAD drawings) is unlikely make a huge difference, right?

Thanks again for all your help.

[Dave]

One thing that's been neglected in this discussion is that fact that larger frames will almost always have longer front-centers than smaller ones.

Larger riders with long TT frames have never ridden a bike with a truly short front-center. They get this idea that every one needs the shortest front-center they can get and a 140 stem to weight the front end. This might be true for them, but it's not true for every size of frame and rider.

In a recent discussion, I measured the weight balance on both of my 51cm c-c bikes and found them to have a substantial 45-46% of the weight on the front, with a moderate 110mm stem and average reach handlebars. The saddles are not in an unusually far forward position to achieve this weight balance. If I choose to position the saddle 2-3cm further back, then I can get the weight on the front to be as low a 42%.

To answer one poster's comment about a front-center being too short, yes, it's possible, particularly if the TT length is only 52-53cm. One approach to increasing the front center is to use a numerically smaller (slack) HTA. It's quite common for smaller frames to have slack HTAs and more steering trail to reduce increase the front-center and slow the steering a bit. The weight balance during standing pedaling can't be altered except by body position. Personally, I've never noticed the difference (while climbing) on any of the frames I've owned. It's not like handlig is critical when you're going 10 mph up a mountain.

Personally, I wouldn't worry about a 55mm trail being too little for adequate stability, unless you've already experienced problems. The fastest steering bike I every owned only caused me a problem when I encountered a severe wind gust. Knowing that severe winds can come up quickly in the mountains, I chose to sell this frame. It was advertised as a quick-handling bike, probably well suited to crits, so the new owner would not get an unpleasant surprise.

One other question that went unanswered is the definition of "race height". It's what others call fork length, which is the distance from the front axle to the crown race, measured parallel to the steering tube. It's rarely listed by anyone selling replacement forks, which makes people think they are all the same. A "standard" length would be nice.