|Let's talk about rake and trail.||bill|
Sep 10, 2001 7:04 AM
|In searching around to figure out in the abstract how these variables will affect bike handling, I've come across directly contradictory information. Some sources say that longer rake = quicker turning and longer trail, more stability, some say the opposite, and every combination in between. |
What makes sense to me is that a longer rake will give a shorter trail (according to the diagrams, which I think that I understand), and that a shorter trail will give less stability at speed and more stability at lower speeds (I don't understand why it shoud be more stable at lower speeds, but this seems consistent among the sources that otherwise make sense to me). I'm still trying to sort out whether a shorter rake is supposed to turn quicker or slower or even precisely what that means in terms of the way that the bike feels, because a longer rake is going to increase the effect of a small turn in the bars (I think), but I'm not sure how that matters to the ride when you factor in lean. All very complicated.
My new bike has a rake a cm longer than my old one (4 cm vs. 5 cm, as best I can measure). I believe that the head tube angle is the same (73 degrees), which should mean that the new bike's trail is less than my old bike. The ride difference is noticeable, although I'm still trying to break it down and qualify it. The longer rake appears to require more leaning and less bar turning to effect a turn. It's a little harder to maintain balance (very difficult to ride no hands, for example). At speeds of 25 mph plus, it still tracks very well (in terms of responding to bumps, etc.) but requires a little more attention to hold a line. The steering isn't scary, and I can take turns at the same speed (greater, probably, because the bike does track very well, with little bouncing around), but it feels maybe a little sluggish compared to the other bike. The old bike, on the other hands, sometimes felt like it was too responsive to small handlebar movements (that the bars moved too easily). The new bike sort of requires a little pushing of the bars to get them to move, and you end up just leaning. I'm still sorting it out and trying to decide whether one is better or whether they are just different.
The other main difference in geometry is that the new bike's seat tube is noticeably steeper (the old bike is supposed to be 73, but it's a little less than that), but I don't believe that this should have any effect on handling.
|semantics||alex the engineer|
Sep 10, 2001 7:50 AM
|Many people consider greater offset to mean "more" trail. This is why things get confusing. Since trail is definable by a number, it is logical to think that more trail NUMERICALLY means GREATER trail. Unfortunately, there are people of the "old school" who consider trail to be something counter to numbers, and consider a lower numerical value to be MORE trail. It's a tradition which makes no sense.|
|Well, whatever we're calling it, how does it affect handling? nm||bill|
Sep 10, 2001 8:14 AM
|I like lots of both ...||Humma Hah|
Sep 10, 2001 8:22 AM
|The cruiser has both a slacker head angle than is generally preferred these days, and a generous forward kink to the forks. It is as stable as a rock, the smoothest, most precise-handling bike I've ever ridden.
My MTB has far less rake and trail, and about 4" less wheelbase. It is a twitchy, almost frightening bike to ride fast in technical sections. It takes full concentration to make it follow a chosen line. The cruiser is much the superior bike in, for example, "baby-head" rocks. The cruiser only turns if ASKED to.
Does this mean the cruiser is hard to turn? Absolutely not! I can turn tighter on it than I can on the MTB (I have lots of fun doing U-turns on a 6-ft wide bike path on the cruiser). It turns fine, but only if asked.
|he said TRAIL not TAIL....nm||rib-eye|
Sep 10, 2001 9:24 AM
|related question on custom build||Dog|
Sep 10, 2001 8:59 AM
|If someone wanted to build a bike for exclusive use on long rides on the aerobars, being as stable as possible as well as comfortable, what geometry would be used? I'm talking about a bike that you could nearly fall asleep on and have it continue in a straight line, that could hit ruts and potholes and just barrel along. Probably looking at a longish chain stay, too. Any ideas?
|From what I think that I understand, you would want a long||bill|
Sep 10, 2001 9:31 AM
|trail (slack head tube angle) with a medium rake (4.5 mm). You'd get a longer wheelbase, a good amount of high-speed stabilizing trail, and nothing too finicky about the steering. A too long rake would reduce your trail, defeating the generosity of that variable. A too short rake not only would turn too easily -- you could turn without lean, which may be destabilizing in the aero, far forward position, but I would think that too much of your weight would be over the contact point of the front. I guess you'd want a low bottom bracket, as well. |
But, what the hell do I know? I did a google search last night of "rake trail handling" and came up with loads of interesting stuff, a lot of it about motorcycles. Can't say I understood more than a little of it. I may go back to it again.
|The motorcycle connection is a good one, though ...||Humma Hah|
Sep 10, 2001 11:42 AM
|... I've owned a couple of off-road motorcycles and a street bike. All have had more rake and trail than most bicycles I've tried. Yet, we'd race the dirt-bikes, typically about 230-pound machines, thru the woods at 18 mph or faster, the bars trimmed down to 24 inches to better fit between the trees.
We considered these machines to be plenty agile.
Any single-rider conventional bicycle will turn just fine. You should not be afraid to try a bike with a relaxed rake and trail. It may feel a little different than you're used to, but I guarantee the sucker can be made to turn as nicely as you care to, with surprisingly little effort.
|I believe that...||alex the engineer|
Sep 10, 2001 9:21 AM
|...according to a couple different framebuilders, 55mm is the minimum needed trail for decent high speed stability.|
|Yeh, I came across that, too. I guess what I'm looking for out||bill|
Sep 10, 2001 9:39 AM
|of posting here is to talk about how these dimensions relate and the benefits/drawbacks of the generosity of these dimensions here or there, what people like or don't like about having more of this or less of that, or whether it's a personal thing that matters mostly to how you ride the bike. For example, does height, weight , weight distribution of the rider matter in how these variables are experienced? It's something that I didn't even think about before feeling the difference, but maybe it will help people buying on-line or help people to distill the qualities they desire in a dream bike. It may help make sense of bike comparisons, help choose forks (or at least understand what a fork is going to do to a ride). |
I'm going to try to measure the trail on my bikes. Might be interesting.
|Seems to me that. . .||Trent in WA|
Sep 10, 2001 10:36 AM
|Pardon the musings of a fairly clueless fairly newbie, but IF you're looking for the most stable and "well-mannered" ride you can get for an ultra event, wouldn't it make sense to use a bike with a "touring" geometry built up from lighter materials and componentry than is normally used on touring bikes? Something like, say, a cross between a Trek 520 and a Trek 5200?|
|it's called an audax bike||alex the engineer|
Sep 10, 2001 10:53 AM
|or a radonneur bike. These are made mostly in Europe, and are essentially a long wheelbase road bike, capable of accepting 25mm or 28mm tires, as well as fenders and a lightweight rear rack. Some even have dynamo front hubs.|
|Pardon my ignorance.........||Len J|
Sep 10, 2001 10:42 AM
|I know what rake is, but What is Trail?|
|search previous threads (nm)||alex the engineer|
Sep 10, 2001 10:54 AM
|Did, couldn't find anything pertinent(nm)||Len J|
Sep 10, 2001 11:00 AM
|I've never successfully searched prior posts, so I can't tell||bill|
Sep 10, 2001 11:54 AM
|you how to do that, but I can tell you what trail is. You draw an imaginary line down the axis of your steerer or head tube to the floor. Mark that intersecting point. Then, you find your contact point of the front wheel on the floor (draw a line straight down from the axle). The distance between where the imaginary line hits the floor (which is ahead of the contact point) and the front wheel's contact point is trail. A slacker head tube angle means more trail. |
It almost seems counter-intuitive for a longer rake to mean a shorter trail, because it would seem that more rake (more forward bend to the fork) would be the same as a slacker head tube, but it's not the same as a slacker head tube angle and it does lessen the trail. More rake puts the contact point farther forward. Voila. Less trail.
To fathom the significance of trail, recall something that we've all experienced -- supermarket cart wheels. Sometimes, they're hard to push because the wheel is turned around. The trail makes the wheel, set up on a bit of an offset, stable only when the offset is in the right direction -- pushing the wheel forward of the point where the wheel is attached to the frame, just like a bicycle wheel.
Some of the articles I found talked very specifically about how rake influences lean, which is something that I am trying to understand. If I'm visualizing it properly, imagine a fork with no rake. Twisting the steerer causes the wheel to turn on the contact point. Now, add rake. The wheel not only turns on the contact point, but the bend in the fork causes the wheel on the end of the lever that the fork has become to lean over. Try it with a paper clip (we're all at work, right?). Balancing all of that has a lot to do with how the bike handles, but I still haven't figured out exactly what happens, other than that more rake certainly would exaggerate the turning of the wheel, because that's what levers do.
|I'm not sure that I'm glad I asked.....||Len J|
Sep 10, 2001 12:09 PM
|but I do appreciate the time you put into the response.
I understand that more rake causes a lever action (as you describe). Does that mean that less trail causes quicker steering?
Maybe what you are missing is that more rake equals longer wheelbase & therefore a larger turning arc for the same steering input (as compared to less rake)?
I'll have to reread your original question & think about this more.
Thanks for the lesson (& the headache(LOL))
|Sheldon Brown says:||Dog|
Sep 10, 2001 12:15 PM
The "rake" or "offset" of a fork is the distance between the wheel axle and the extension of the steering axis. This may be accomplished by bending the fork blades, or by attaching the fork ends to the front of the blades, or by tilting the blades where they attach to the crown.
Rake is one of the three factors that affect the trail of the bicycle, which has a considerable influence on the handling qualities.
Trail is the distance from the contact point of the front wheel with the riding surface to the intersection of the steering axis (head tube) with the surface. The trail is a function of the head angle, the fork rake, and the tire diameter. Trail has a major effect on the handling of a bicycle. More trail increases the bicycle's tendency to steer straight ahead. A bicycle with a largish trail dimension will be very stable, and easy to ride "no hands". A bicycle with a smaller trail dimension will be more manuverable and responsive.
Joshua Putnam has a good discussion of trail and Bicycle Steering Geometry in general on his Web site.
|Anyone who wears hats like that is O.K. with me! nm||MB1|
Sep 10, 2001 12:18 PM
|many responses, little information....||C-40|
Sep 10, 2001 1:09 PM
|Gee, it looks like almost everyone is confused.
I've seen people confuse the results produced by changing rake or trail, but the two terms are distinctly different and shouldn't be confused.
Rake and offset are most often used interchangeably. Rake or offset is the perpendicular distance between two parallel lines, one through the steering axis (center of head tube), and one through the center of the wheel. It's difficult to accurately measure the rake of a fork without taking it off the bike and placing it on a layout table, where precision measurements can be taken. A plumb line could also be used, with the fork off the bike, but a high degree of accuracy is difficult to obtain. Fortunately, most manufacturers provide this information, so rake measurement is generally unnecessary.
Trail is the horizontal distance between the tire's contact point on the road, and the point (on the road)about which the tire is rotated. The tire is always rotated about a point that is in front of the tire contact point. Trail can be considered to be a lever arm or tiller. A longer arm will react more slowly, but be easier to turn due to increased mechanical advantage. A shorter arm will react more quickly, but takes more effort to turn.
The formula for trail is quite simple. Trail = (R/tanH)- (rake/sinH), where R is the radius of the tire, and H is the head tube angle. The first half of the equation (R/tanH) is trail using a fork that has no rake (offset). The second half of the equation (rake/sinH) reduces the trail by the horizontal component of the rake. If you run a sample calculation with no rake, a 336mm-tire radius and 73 degree head tube angle, trail will be a very large 102.7mm. Steering on a bike with a 73-degree HTA and no rake would be extremely slow. Increasing the head tube angle to about 80 degrees would reduce the trail to a "normal" 60mm, but wheel-to-toe overlap problems would be created and the wheelbase would be substantially reduced.
Adding rake to the fork will reduce trail and make the steering quicker, without creating a wheel-to-toe overlap problem. Recalculating the example with 40mm of rake reduces the trail to 60.9mm. Recalculating with 50mm of rake further reduces the trail to 50.4mm. As you can see, the more rake, the less trail. As the lever arm (trail) becomes shorter, the steering response will be quicker.
Those who write about high speed and low speed "stability" confuse the issue, IMO. I've never seen anyone define what's "low" and what's "high" when they make these statements. I've never worried about it either. If you want a twitchy criterium bike, which responds to the slightest input, reduce the trail down to about 50mm. If you want a slower responding bike that won't wander off the edge of the road easily, increase the trail to 60-70mm. My C-40 for example, with its 71.7-degree head tube angle and 43mm rake has a trail of 65.8mm. It's designed with slower steering for European road racing, not criterium quickness.
Sep 10, 2001 2:21 PM
|My C-40, 54 cm, always felt like it had heavier, more stable steering than my 55cm Bianchi EV2. Now it may make sense. The EV2 has a 73 degree head tube angle and 45mm of rake. Hmm, the larger rake would put the contact patch further forward with respect to the steering axis, but the steering axis is steeper. How's this work again? Which "should" feel quicker/less stable? This is a difficult concept.
|Bianchi trail only 55.7mm...||C-40|
Sep 10, 2001 4:49 PM
|Compared to 67mm on your Colnago, assuming that the geometry chart posted on the trialtir-usa.com web site is correct. The Colnago will have slower steering with it's larger trail, which should make it hold a straight line more easily.|
Sep 11, 2001 6:14 AM
|or should I say desertation.
One comment, it has been my experience that as fork offset gets out to 50 mm or so, the front wheel tends to flop or tuck under at low parking lot speeds. Maybe this is what some people are noting as "low speed". I think this tendancy may be why most framebuilders don't use this much offset. Of course, Richard Sach sure has built a lot of frames with 50 mm + offset...and we all know how well respected he is.
|offset without head tube angle means little....||C-40|
Sep 11, 2001 9:23 AM
|You can't lump all bikes with a 50mm offset into the same category. If one has a 72 degree HTA, and the other has a 74 degree HTA, the resulting trail and steering response will be entirely different. Larger offsets are often appropriate with HTA's around 72 degrees, to reduce the trail, and speed up the steering response. The the HTA of 72 degrees or less may be used to increase the wheelbase and reduce toe overlap.|
|"Ideal" angle and offset for maximum stability?||Dog|
Sep 11, 2001 9:45 AM
|It seems that if you move in directions from ideal, the bike becomes less stable. In other words, there should be a point at which, for a give sized wheel/tire, stability is maximized. Any idea how to figure this?
|Also, trail alone does not determine steering characterists.||Nessism|
Sep 11, 2001 2:42 PM
|A frame with a 72 degree HTA and 5 cm of offset may have the same trail as a steeper HTA and less offset frameset, but the handling will be somewhat different at low speed. In my experience, the slack HTA/large offset frameset will tend the flop the wheel at low speed. I learned this from personel experience since my first homebuilt frameset was configured this way. I learned that there is a reason that most builders try to stick to 4-4.5 cm of offset and HTA's between 73-74. It works without the drawbacks.
Of course, special considerations must be made for fitting riders of non-standard proportions. In these cases, convention must be thrown out inorder to find the best solution.