Re: [CR]steering geometry


Example: Framebuilders:Norman Taylor

From: "dave bohm" <davebohm@home.com>
To: <rocklube@adnc.com>, "Steve Freides" <steve@fridayscomputer.com>
Cc: <classicrendezvous@bikelist.org>
References: <20010521.162610.-266283.11.richardsachs@juno.com> <3B098973.7195835B@fridayscomputer.com> <3B0A1A52.79B5@adnc.com>
Subject: Re: [CR]steering geometry
Date: Tue, 22 May 2001 09:11:56 -0700


I design using trail measurement almost exclusively. A good understanding of these principles gives a fairly easy way to design frames for different riders and still maintain a similar steering feel and stability index.

Not surprisingly this leads to the same general principles that Brian mentioned. That is that bikes with less rake will handle better than bikes with lots of rake for most of the riding that the people on this list do. MTB's, touring bikes, and other specialty bikes require slightly different steering geometries.

To break it down simply, I look at trail as a measurement that corresponds nicely to the overall stability of the bicycle. The adage used before "more trail, more stable" is about right here. The bicycle has a very specific range of speeds that can be used. Because we are basically one horsepower machines at best, we do not need to design a frame that is stable above speeds of 60 or so. This design has been found generally through trial and error but if one looks at the trail of well known bicycles that handle well we will see that they generally have a trail measurement of between 53-60mm of trail. I tend to design for about 58mm ideally.

Surprisingly, or maybe not, some modern bicycles have trail measurements that fall below this optimum range of trail measurements. Take a modern frame with a 74 degree head tube and a 45mm kestrel fork. The trail on this setup would be 49.5mm. This is too low in my opinion and this bicycle would tend to become unstable at higher speeds. The same with Brian's PX-10. Lots of fork rake on this bike coupled with a 73 head tube would result in a trail of 45mm. Harry Quinn, same thing, most likely standard rake with a steep head tube angle give a measurement of about 50 which is too low for decent high speed stability.

So what does work well? Lets see, a standard Pinarello type geometry. 73.5 head tube with a 40mm fork rake gives a trail of: viola 57.8mm. This trail will allow a good level of stability at speed and an excellent steering feel. My definition of that is a frame which does take some effort to initiate a turn but not too much. Bikes that steer too easily tend to feel twitchy to me and when I am in serious oxygen debt, I don't need twitchy.

Lets look at a different design, I don't mean to speak for R. Sachs here and of course I am just generalizing but Mr. Sachs tends to use a little more rake than some. Is this bad? not at all. Just a different way to get to a similar result. So lets say the theoretical frame in question has a rake in the high 40's but you slack out the head angle just a bit at 72.5 the resulting trail is 56.7mm. A very standard trial measurement as you can see.

Track bikes are an extreme example of this. Most think that they need to handle quickly but my opinion is that the banking is doing the turning for you and that a track frame needs a high degree of stability. Is this the case? the resulting trail measurement from a standard track geometry of 74 degree head tube and a 30mm fork rake is 65mm. This is even higher than a standard road geometry and more stable which is necessary in a sprint platform.

Even weirder is the old stayers bikes. The ones with the reversed forks were the guys would go 60+mph. What is the trail on these. Small wheel, very steep head angle, lets say 78 degrees and a heavily reversed fork rake. These types of trails would be somewhere around 112mm. Massive trail, which leads to a very stable machine that can be used at high speeds.

Well, That's about it for now.

Dave (whanabe rocket scientist) Bohm Bohemian Bicycles http://www.bohemianbicycles.com


----- Original Message -----


Steve,

I have taken a somewhat unconventional approach (what would you expect from me; I'm from California) to the issue of "rake and trail" and "fast or slow" steering. It is the product of my past experiences relative to the large number of well respected frames I own or have owned over the past 30 years.

I have never been able to make any sense of the "resulting trail" dimention which is generated by the head angle and fork rake. Everyone has an opinion and a preference apparently; but very few take into account several other key elememts that bear on the handeling charactistics of any given bike.

My earliest experiences are as I mentioned; I had a boatload of different bikes in the early days including the Italian Masi GC, a Colnago Super, and an Eisentraut "A" frame to name a few. I noticed then that bikes handled differently and just sort of took note of what each bike was dimentionally. For the type of riding I did, the Colnago handled the best, but overall I liked the Eisentraut. I remember also taking delivery of a custom built Harry Quinn frame designed for criterium use that sucked so bad I literally sold it two weeks later. The point here is this; one of the bikes with lesser rake was excellent handeling while the other was terrible, and one of the bikes with more rake was ok and the other one was not so good. The Quinn was so poorly built that I attributed it's bad qualities to that more than the rake in general, although that didn't help this frame at all. On the other side I found that the Eisentraut had more rake than I prefered but was accounted for largely due to its' quality construction. That convinced me that a well built bike was more important than the resulting trail for the most part, since both frames were built with 531 tubing. After almost riding over a huge cliff on my Peugeot PX-10 on the decent into Lake Elsinore; and having easily and safely made the same decent on the Colnago, I decided that I personally prefered less rake. I also remember a steep decent from the top of the Laguna Beach foothills to the Coast Highway, where Mike Howard was riding a Colin Cape frame (lots of rake) and I was right behind him on my Colnago as we raced each other to the bottom. Near the bottom of this road were three switchbacks which were connected by srtaight stretches of about 100 yards or so. At the second turn Mike was going so fast the bike simply wouldn't turn and he went directly into the curb at a 90 degree angle. Ordinarilly one would expect an impact at that speed to cause severe trauma to bike, body, or both. Not Mike! Never seen anything like this to this day, but he somehow got BOTH feet out of the toe straps (without either hands, both of which had a death grip on the brake levers) and somehow jumped over the handelbars and landed running (I'm NOT kidding!) on some dirt as his bike hit the curb dead on, then did a beautiful flip or two as it landed behind him ( I think the judges gave the bike perfect 10's for that routine). Furthermore Mike didn't even fall (only bruised his heel a bit) and the bike was ridden home (about 15 miles) without so much as even truing the wheel! No dent in the rim, bent frame or fork, nothing! All due to the fact that Mikes' weight was not on the bike when it hit the curb. Anyway, I didn't have any problem turning and I was almost on his wheel at the time. So no matter how one describes it or what mathamatical formulas are used; I have gravitated to the less rake side as opposed to the more rake side. I have also experienced the rake issue on tandems. The one that Ross Schaffer and I built (2 3/8' rake) compared to my Hetchins tandem (3 1/4" rake) is like night and day. My bike handels almost like a single ( just ask Jim Ogden) and the Hetchins steers like a Mack truck and has severe wheel flop. I've said before I'm very proud to own that bike, but I ride it for show because it doesn't encourage me to to take it out and hammer (actually it's a little scary, especially on a steep and or sharp turn). Maybe some day I'll try forkblade surgery on her. Hate to mess up the paint job though.

BUT, as I mentioned, there are several factors in addition to resulting trail that need to be brought into balance to arrive at a frame that "becomes part of the rider"; which seems to nullify any blanket statement about "more or less" is better, or a certain amount of trail is "correct" and everything else is wrong. Some bikes are built with only one rake available (like a Masi for example) and much of that is dictated by the fact that for all intents and purposes they are production frames or there is only one "model" available. Since I deal in custom built frames, I can build every fork exactley the way I feel is best for the rider, the conditions they encounter, and the use they intend. I build forks with rakes from 3/4" (for track sprint bikes) to 1 3/4" (which Dales' bike is designed with) for randonner riding. If someone was going fully loaded touring I would add some more rake under those conditions. For racing CA style 1 1/2" is minimum, and 1 9/16 to 1 5/8 is more common.

For a bike my size, I have experienced many times bikes with 2" plus of rake not only tend to want to wander over the side of cliffs, but have "wheel flop" which is especially pronounced when climbing while standing; I find this very annoying compared to a bike that proceeds directly uphill without flopping around like a landed fish. Again, it's only my preference, but I've never had anyone complain about the way any bike I've ever built rides or handles. Have had many who claim to be quite pleased, to say the least; therefore if it works, don't fix it. Often small frames with a lot of rake end up with too shallow of a head angle, like the early "fixture frames" at Masi which had a head angle of 70ish degrees on the 48 and 49 cm frames. Bad combination for my taste.

In my quest to test wheather there was any actual difference between X amount of rake as opposed to Y amount, I had an occassion once to remove the forkblades from a fork on a frame I had had for a while that wasn't quite to my taste, unbend them to Y amount of rake and cut off the extra 6mm of length that resulted, then rebraze them to the same crown. Again I confirmed to myself that the less rake made a better handeling bike out of the same frame, for my personal taste.

So the bottom line is that various people like different handeling qualities and there probably is no right or wrong; but I personally perfer the approach I use based on my experiences with a lot of bikes over a long period of time; and ignore the trail thing altogether. Unless and until I have a complaint I'm going to stick with what has been working so far. I still don't know which way is up regarding more=this and less=that. I can't explain exactly how I arrive at the dimentions of any given frame any more that I can explain how to mix paint colors by the seat of my pants to get the effects I end up with. It's not scientific, it's a California "touchy feely" thing. I guess my overworked little pea brain just can't suck in the theoretical part. No worries, it's all good.

I hope that completely confuses the issue. Glad to be of service.

Brian (hey, I'm not a rocket scientist) Baylis > > Let's see if I can do this correctly for once! > > The line drawn from the head tube and main part of the fork intersects the > ground in front of the place where the tire actually contacts the ground. > Making the fork rake larger effectively brings the tire contact patch closer > to the line drawn from the head tube. This "closer" is called reduced > "trail", or the distance the contact patch "trails" behind the imaginary > line. Less trail equals snappier but less stable handling, more trail > equals more stable but less responsive handling. (That more trail is more > stable seems like an easy to make connection to me.) > > Did I get it kinda sorta right for once? > > Steve "on the trail" Freides > wet from riding in rainy Ridgewood, NJ > > Richard M Sachs wrote: > > > > <<I take it from Brian's comment that the reduced fork rake makes > > > the bike handle quickly, no?>>> > > Douglas Brooks > > Canandaigua, nY > > --------------------------------------------------------------- > > reducing the rake produces a slower handling bike, > > not a quicker one. > > e-RICHIE > > http://www.richardsachs.com > > > > _______________________________________________ > > Classicrendezvous mailing list > > Classicrendezvous@bikelist.org > > http://www.bikelist.org/mailman/listinfo/classicrendezvous > > -- > Steve Freides > > _______________________________________________ > Classicrendezvous mailing list > Classicrendezvous@bikelist.org > http://www.bikelist.org/mailman/listinfo/classicrendezvous

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