Friday, June 30, 2006

Squirrelly


The word sometimes used to describe a rider’s style but more often it is used to describe the way a bike feels or handles.

Typically a bicycle that is “squirrelly” has a steep head angle and will deviate from a straight line at the slightest movement. Some people like the lively feel of a bike like this and I often hear quotes like, “People who call a bike squirrelly don’t know how to ride a bike.”

Agreed if you put a novice on such a bike he is a danger to himself and others; put an expert on the same bike and he can ride it safely. However take the same novice and put him in a bike that handles well and is easy to ride and he rides safely, and the expert on the same bike becomes a brilliant bike handler.

Why build a bike with a steep head angle anyway? Track bikes designed to be ridden on a banked velodrome have steeper head angles, because it is necessary to be able to change direction quickly to dart around an opponent, but the banked track has the effect of the rider traveling in a continuous straight line. At speed the rider is still theoretically 90 degrees to the track surface, and actually centrifugal force is pushing him down on the surface making it harder to deviate from a straight line. Hence the steep head angle to compensate.

Steep head angles on road bikes were in vogue in the mid 1970s and some builders carried it on into the 1980s. I never followed this trend and for a while was I was out of step with most other builders. Let me take you back to look at the history of bicycle design and to explain why this happened. In the 1940s an 1950s when I started riding the standard road geometry was a 71 seat angle, with a 73 head angle. The shallow 71 seat angle can be traced all the way back fifty years earlier to the Ordinary or Penny Farthing.

Early “Safety” or conventional design bikes as we think of them today also had shallow head angles, but by the 1940s it had been established that 73 degrees was the ideal head angle. I happen to believe that is still true today; however bikes of the 40s and 50s had 3 inches or more of fork rake. There was a theory back then that the steering axis, a line drawn through the head tube, reached the ground at the point where the wheel contacted the ground.

In other words there was zero trail; the theory was if you had trail the steering would be sluggish and heavy. The 71/73 angle design made it convenient for framebuilders to make frames in various sizes. With the head and seat angle going away from each other as the frame got bigger (Taller) so did the top tube become longer. The heavy steel cast lugs used back then gave little scope to build frames with different angles.

Moving on to the 1960s there was a huge slump in bicycle sales world-wide as economies boomed and in many places working class people were buying cars for the first time. The cost of a frame hardly rose from the late 1950s to the early 1970s even though earnings and the price of everything else had.

Framebuilders had to look for ways to cut corners and build frames in less time. No braze-ons was one cut back; and another was the parallel angle frame. First 72 degrees parallel because riders were not ready to jump from 71 to 73, but in a short time 73 seat and 73 head became the norm.

Framebuilders were using jigs for the first time for speedy assembly and the parallel design made it possible to make a simple fixed jig. Tubes were pre-mitered all the same length and to build various sizes of frames they simply raised or lowered the top tube. You could have any length top tube as long as it was 22 1/2 inches. Rather like Henry Ford; any color as long as it’s black.

By this time the fork rake had become a little shorter, and to have trail was considered okay, overall wheelbases had also shortened and everyone realized that these bikes were a lot livelier than the bikes of the 1950s. By the 1970s the framebuilder’s lot had improved with frame prices starting to catch up with the rest of the economy; helped along by a bike boom starting in America.

Remember back in the 1950s with the seat angle 2 degrees shallower than the head angle and how it made it convenient to build various size frames with varying top tube lengths. Nobody wanted to go back to 71 seat angles so frames began to appear with 73 seat and 75 head angle. Riders also discovered that these steeper head bikes felt livelier, especially when you got out of the saddle to sprint or climb.

About this time I was doing my own experimenting with frame design and I realized that it was not the steeper head angle that gave the bike its lively feel, but in making the head steeper they had moved the front wheel back directly under the handlebars. When a rider gets out of the saddle to sprint or, climb the bike is going to sway from side to side whether intentionally or unintentionally.

With the handlebars directly over the point where the front wheel contacts the road the bike can swing from side to side keeping in the front wheel in the same straight ahead plane. I had remembered back in the 1950s with those long fork rakes and short handlebar stems; the bars were way back behind the wheel’s point of contact. When you got out of the saddle the bike felt sluggish and heavy. I named it “The wheelbarrow effect.”

As the bike swayed from side to side the front wheel was turning to the left and right, but the gyroscopic action of the spinning wheel was trying to keep the bike straight; you had two forces fighting each other. To demonstrate this to yourself place the end of a straight edge on the ground and hold it 90 degrees to the ground. Swing the top of the straight edge side to side and you will see it moves in the same plane. Now hold the straight edge at an angle to the ground and move from side to side; you will see the top end where your hand is swings in an arch. If this were your bike you would be turning the front wheel as you swayed.

I never liked the twitchiness of the steeper head so I stuck with the 73 degree head angle and to get the handlebars over the front wheel I did two things. I shortened the fork rake; this actually gave my bikes a little more trail than average and made the bike very stable and gave it certain self steering qualities. The other thing I did was shorten the top tube and use a longer stem. The result was a bike that felt just as lively but without the inherent twitchiness. A rider’s weight is mostly in his shoulders and upper body so by pushing the rider forward as I did I moved the mass forward greatly improving the bike’s handling especially when cornering and descending at speed.

Most frames built today have a head angles around 73 degrees; a degree either way is no big deal. It is when a head angle gets to be 75, 76 or steeper would I consider it steep. So to sum up; what makes a bike squirrelly? A steeper head angle has less resistance to turning, so any slight movement of the bike will cause it to deviate from a straight line. Also steeper head angle means less trail which is the castor action that helps to keep a bike tracking straight.

5 comments:

sydney_b said...

"A rider’s weight is mostly in his shoulders and upper body so by pushing the rider forward as I did I moved the mass forward greatly improving the bike’s handling especially when cornering and descending at speed."

Would you change anything when considering the distribution of a woman's weight?

Dave Moulton said...

A framebuilder is often limited in his options when building for a woman; especially if she is very small. Even if she is not it is probable that she has long legs and a short body so the last thing needed is a longer top tube. Whatever the gender and the weight distribution of the differing body types I still found it advantageous from the handling aspect of the bike to have the weight moved forward.

Jim G said...

Two of the bikes I have (one w/72-degree head angle, the other with 73.5-degree head angle) behave very differently in cross winds -- the bike with the shallower HA feels like it's "blown off course" moreso than the other bike. Could you talk about this? Thanks!

Dave Moulton said...

Jim,

One possible explanation could be; the bike with the shallow head angle probably has a shallow seat angle also, especially if the top tubes are the same length. So your weight is maybe more over the back wheel with less weight on the front wheel. Like a VW mini-bus it will behave badly in a cross wind.

Stand the two bikes side by side with the wheels lined up and see where the saddle and handlebars are in relation to each other. Or it could just be with the shallower head angle there is less weight directly over the front wheel; look where your handlebars are in relation to the center of the front wheel on both bikes.

Dave

Jonathan V said...

I was interested to read this piece as I have just had a frame built with a 71 seat and 73 head angle. I needed the shallow seat to get the right distance behind the BB: I'm surprised it's nor more popular but then it's all about fashion isn't it?