Sunday, November 27, 2005

Geometry Class: Engine Mounts! Part III

Welcome to the third installment of "Engine Mounts". We discussed the effects of seat tube length and angle in a "bio-mechanical" sense last time. This time we are going to start looking at handling characteristics. We also will begin to understand that all facets of the frame layout are going to be affected by any changes made in one area. I touched on this a little bit last time. No less than frame building guru Matt Chester has this to say, as well: "In the real world, all aspects of frame design work in concert with one another. Adjust one, and the others are affected." Try designing a simple frame using a "stick-man" drawing approach. Try keeping your wheel base constant while you change the seat angle. See what happens? Several erasers and crumpled pieces of paper will result!

Okay, so one thing leads to another. (Hey, that's an old song....isn't it?) So, what does this have to do with handling? Well, nothing and everything! You see, you will want a certain type of handling characteristic, let's say you want your bike to climb well. Okay, steep seat angle and short chainstays, right? Well........can I say that you've been reading too many magazines! While it's possible to build a great climbing bike that way, it's not necessary to do that to get a great climbing bike. And besides, you probably would like your bike to go downhill pretty well too, right? There is more than one way to get to where you want to be, and probably better ways than you know.

What I'm trying to say is that the normally accepted geometry numbers do not necessarily mean all bikes handle poorly if they fall outside of these "norms". For instance, your "typical" seat tube angle is 73 degrees. Sometimes you see a 74 degree angle. This is supposedly the "best" angle for a mix of climbing and decending on a 26 inch wheeled hardtail. It's easy to combine this with a chainstay length that keeps the "chainstay freaks" at bay. Some people tend to obsess over this number. You see, if you keep the wheel base constant, and slack out that seat tube angle, your chainstay length gets longer. This is because you have to keep the seat tube straight and clear the rear tire. A resulting gain in length in the chainstay will raise the eyebrows of the average cyclist, who will mistakenly surmise that the bike won't climb well, when in fact, the weight of the cyclist will be over the rear axle even more, giving the rear wheel more bite. Of course, all this happens within reason, and it does affect other aspects of the frame design as well, such as the top tube length, and reach to the handle bars. Remember too that you need to consider the riders most efficient posistion in all of this. A comfortable rider in an efficient posistion will always be faster for a longer time than he would be on a "standard geometry" frame that didn't work as well for him.

Discussing chainstay length for a minute, I was always wondering how the early pioneers of the sport were able to climb anything with those 17 to 18 inch chainstays. Shouldn't have been possible! I even had a bike at one time that had parallel 72 degree seat and head tube angles. I still consider it the very best handling hardtail 26 inch wheeled bike I ever had. I don't know what the chainstay length was, but I'm sure it wasn't short by any stretch. Once again, Matt Chester has something to say on this matter: "Chainstay length is subjected to incessant analysis. "Shorter chainstays are better for climbing!", crows the internet discussion forum sage. Utter nonsense! No single aspect of frame design will enable you to predict a bikes handling characteristics." Thank you, Mr. Chester, now.....back to your torch!

The handling characteristics of a bicycle are also largely affected by the way the bike steers, obviously. The next installment will leave engine mounts behind for awhile and we will start to delve into "The Long and Winding Trail"! See you then! Class dismissed!

No comments: