Increased fork offset & impact on handling
medium el rey. going from a Reba with a 38mm offset to a WB with a 44mm. Just wondering what kinda handling impacts there might be...
i have the same question.
med el rey here also.am considering a WB or F29.curious as to how the handling may change,not to mention the increase in front end stiffness.
Speedgoat recently posted this excellent explanation:
INTERBIKE BREAK: 29ER FORK RAKES AND HEAD TUBE ANGLES
Q: Howdy... Getting a serious itch for a new bike, good chance it will use bigwheels, and I'm leaning toward FS. One complaint my more discerning riding friends have long had about bigwheels is that suspension forks are configured wrong. Seems like the new model from Fox may go at least some way toward fixing that. You'all seem to know as much about bigwheels as anyone. And I see Mr Curry briefly mention the topic on one of his blog entries. But am curious to learn a little more about what installing an F29 might do to something like an El Rey or Sultan. Make a good thing better? Make
it too twitchy? Or maybe not much of a diff to the typical rider?
Do you think folks like Vicious and Sycip will go so far as to change their front geometry?
A: I've noticed a lot of confusion about 29er geometry and handling out there, and of course there are many opinions about what a bike is supposed to ride like. No one mentions that, but the riding in Oregon is different from the riding in Southern California is different from West Virginia is different from the Swiss Alps. You get the idea. So first lets dispense with "right" and "wrong" as quantitative data. What works for somebody might not work for somebody else. Easy answer, the Fox'll speed up your Ventana or Turner, assuming it had a Reba on it.
Real answer? Man, I hope you're sitting down.
The "configured wrong" bit you describe refers to the RockShox Reba and early White Brothers 29er forks that had fork rakes that were under 39mm. While I've even seen useless debate about the definiton of "rake," the bottom line is that the bicycle industry refers to rake as the distance between a hypothetical line drawn straight down from the center of the fork's steerer tube (in other words, your head tube angle), to the center of the dropout of the fork (or where the center of the wheel's axle will be). Do all companies take this measurement as perpendicular, with the axle center on a presumed parallel to the steerer center or head tube angle? Probably, though they don't go out of their way to say. In fact, call a few manufacturers of mountain bike forks and ask them what the rake is on three random models. Fork rake itself isn't common information, let alone methods of determing it. In fact, I've had more than one fork manufacturer initially give me the wrong rake information when I asked about a 29er fork. Muddy waters, these.
Rake in and of itself doesn't mean much, but it's a factor in the calculation of something called "trail," which can loosely (and inaccurately) be described as your bike's foremost virtual contact point on the ground. Actually, you determine trail by dropping a vertical line down from your axle, following that hypothetical head tube angle to the ground at a second point, and then measuring the gap in between. Anybody interested in learning more about the relationship here should Google up the terms "bicycle rake" and "bicycle trail" and pay particular attention to the relationship. Generally, a bike with a larger trail number is more stable, while one with a shorter trail is more squirrely. Like everything else with bicycles, it's all about finding a happy place in between.
Most people don't notice this stuff until it's really obvious. Install a 100mm fork on your stock bike that came with an 80mm one? You just completely altered everything about your trail measurement--but maybe you'd like it more. Don't panic. Like microscopic bacteria, math is all around us. And mostly it's benign.
But here's where people start to screw this stuff up. Yes, steepening the head tube angle of a bike will decrease trail and thus quicken the steering. But increasing fork rake will also quicken the steering. I've included a handy graphic to demonstrate why this occurs.
Notice in the graphic that if we were to increase our fork's rake--basically dragging the fork to the right of the screen--our trail measurement of 82.01mm would shrink, and steering would get quicker. The common fallacy that's in place regarding 29ers is that they steer too slow because of some ******** voodoo magic you can feel with the larger wheel, and so we all need steeper head tube angles and so fork manufacturers are now making forks that work with those steeper angles. Tripe, I'm here to tell you. In fact, the exact opposite is true.
Fork manufacturers have been pressured into creating forks with more rake primarily because of toe-overlap. Until now, 29er framebuilders couldn't actually produce frames with much less than a 23.5" effective top tube, which can make it tough to hit that 5'8" demographic, let alone smaller riders. Only solution was to slacken the head tube angle until you kick the front wheel out away from that pesky rider's toe you see sticking out from the left side of our graphic. This is what Fisher has done with their sub-70-degree head tubes. But when you slacken things that much with a sub-40mm rake as on the first generation 29er forks, you end up with huge trail measurements, and thus sluggish handling.
Because you're constrained as to how slack you can really make the head tube, you need a different method of moving that wheel forward. Thus, the rake increase. How much rake is good? Obviously, it depends on what your head tube angle is, and they're all over the place, making it tough to decide. Personally, I think the new Fox is perfect at 44mm of rake. It will play very nice with what I've referred to as "original" full-suspension 29er geometry that had head tube angles in the 70-71-degree range, but it won't completely scare the piss out of you on those 72-73-degree frames for people without toes. But this goofy disconnect out there, where people seem to think the new forks are made for steeper head tube angle 29ers? Completely backwards. I don't care if you have a Vicious, a Sycip, or a one-off Parlee 29er (Tristan, I'm selling my car--let's do this!); the newer forks with more rake prefer slacker head tube angles.
Why the crazy huge response? Well golly, it's just the kind of meticulously brilliant service you get at Speedgoat, where you should purchase all your 29ers forevermore. But it's also my personal feeling that a 29" wheel can handle better than a 26" wheel bike for most riders 5'10" and up. A bigger wheel's a ***** to haul around, sure, and it has its own inertia (or what, as a die-hard 29er geek, I'd like to refer to as "gravitas"), but really analyzing the handling on a 29er involves so many other things, like wheelbase and chainstay length, that those feeling they're going to "solve the fork and head tube angle handling problem" aren't really looking in the right place to begin with. We're getting there, and a 29er fork with a rake in that 43-45mm range is a great start, but there are plenty of other things that need to happen with 29er handling. I could tell you what those are, but of course I'd have to kill you.
Last edited by The Squeaky Wheel; 11-17-2007 at 08:15 PM.
That is a kick-ass (and informative) response.
I've run both the WB 100 Magic, and the Reba set @ 100mm on my 19" El Rey and I prefer the handling, etc with the Reba. As for geometry changes, you'll notice that the front of the bike is lower with the WB - the axle to crown dimension is 10mm shorter [unless you run a Magic 110], which steepens the head and seat angles.As much as I wanted to like the WB more, I prefer the Reba.
Originally Posted by jrm
"Like microscopic bacteria, math is all around us. And mostly it's benign." That's so far from the truth.....yes math is all around us but it's not benign. All it takes is for an engineer or a designer to misplace that little thing called a decimal point and your little world could come tumbling down. Great illustration on 29er geometry.
That statement really got your goat, didn't it?
Originally Posted by mtnbiker4life
No it didn't get my goat! I actually think it is a well stated explaination of how fork offset, rake, and head angle effects ride quality.......making statements like calling math benign is the quickest way to lose creditability. I just hope my son doesn't read anything like that then all my math drills will seem pointless and unwarranted.
it utrns out that with the fluid 110 im adding some 7mm in a to c to that increase in rake. So im hopefully that the reduction in trial will have more of an impact then the additional length..
I also found this graphic depiction..