Richard Pollock owns Mule Motorcycles. He has been building custom bikes and street trackers since 1975. He is the go-to guy in Southern California. He has an international reputation for building spectacular custom motorcycles. He is intimately involved with motorcycles as a motocross, flat track and road racer. His mechanical background includes cars, bicycles, motorcycles, airplanes and missiles. In a distant way he’s a competitor, but he builds jewels and my focus is on the big bang/low buck level for do-it-yourselfers. When Richard speaks, smart bikers listen.
In describing Sportster flat track handling I glossed over technical issues. Richard knows a whole bunch more than I do and has shared his knowledge with us. So read on and learn from the master.
The foundation of handling: Rake/offset/trail
Rake
Let's start with “rake”, which is the angle of a straight centerline through the frame’s steering head in relation to a vertical line 90 degrees to the flat surface a bike is sitting on (the ground). I’ve seen this range from 20-35 degrees depending on the style and intended purpose of the bike.
Offset
Next is triple clamp offset, the distance between a straight line going side to side through the center of each fork cap and the center of the steering stem. This can range from zero to 3 inches.
Trail
Trail is a measurement derived from the above while being affected by several other factors. When the dust settles, Trail is the number that determines a lot of what you feel, or how the bike reacts. The important takeaway from this is that changes you make in other areas of the bike have an effect on Trail, and, if you want to change the Trail, there are other areas of the bike that can be tweaked to change it. Sorta like riding along in your car and it gets too warm inside. You can turn on the A/C, open a front or rear window, drive faster or if it’s a convertible, you can put the top down. All will affect the temperature inside the car while causing possible side effects. Messing up your hair, more noise, kids too cold in the back seat or possibly letting in the rain. Same with trail and handling.
For the moment, let’s focus on Trail. Here’s the best way to explain it.
- Trial results in a certain level of stability (also known as tracking or the bike’s ability to maintain, on not maintain, itself in a straight line).
- More Trail equals more stability.
- Too much trail equals hard-to-turn, heavy steering, but also feels like you could ride one-handed across the desert at 150mph without any wobbles.
- Too little trail makes a bike turn super easy, steering feels very light, exceptional maneuverability in a parking lot, but you have to hang on tight at 60mph! Wobbles or gets head shake at highway speeds. Scary to ride on the road and NOT confidence inspiring!
For a road bike, 95-100mm (aprox 4”), of Trail is the most common goal. Good turning for the amount of effort and acceptable stability at a wide range of higher speeds. At Bonneville or on a dragracer, trail numbers should go up. On a trials or flattrack bike, the TRAIL number drops. Ease of turning (or control) is desired.
First thing to do is start by measuring the trail your bike actually has. Pretty simple to do! I do a full scale drawing on the garage floor with a Sharpie. For example, I use a tape measure to get the distance from the front axle center to the ground while a friend holds the bike vertical. Then the fork angle with a bubble type or gravity type protractor/angle finder. Now measure from the axle center to the top of the top triple clamp where the top of the fork tube ends with the bike sitting on its weight (again with a friend holding the bike vertical). Lastly, the triple clamp offset from the steering head center line. Now you have four simple dimensions. Axle center to the ground, steering head angle, fork length and triple clamp offset.
Transfer these dimensions to a full scale drawing on the floor. See drawing.
Note, the more accurate and repeatable these measurements are, the more precisely you can document changes or results. You will find that many things on the bike will have an effect on this seemingly insignificant dimension. But when it’s right, you’ll feel the difference!
Other factors - There are several things that effect trail. Maybe you’ve changed one of these things before and the bike felt “Better”, but you never really made the connection. A few of those things would be lowering the front end (raising the forks in the clamps), raising the front end, raising or lowering the rear end via shock length or sag, changing tire sizes or wheel diameters, swing arm length or maybe the triple clamps or the complete frontend.
What do you want for trail and what changes do you make to get it? For the sake of this discussion (a flattrack bike), and whenever you want to make handling changes on a bike, it’s best to try and not be a copycat if you don’t what you’re doing or why you should or shouldn’t be doing it. Take your time, learn what works and make gradual changes. If you’re not willing to do that, you’re probably not this far into this explanation anyway, so I’ll keep going.
So, we’ll take a Harley Sportster (being converted to a Hooligan racer), which was not designed for flattrack racing, but can be modified to work very well. Tons of improvements can be made on the entire bike, but for this discussion, we’ll stick to rake and trail. The issues preventing proper application (flattrack), are the shocks are too short, the forks are too long and the steering head angle is kicked way for a very relaxed, slow steering ride. Kind of a mild chopper stance. When doing our drawing on the floor, we’ll find that the triple clamp offset is substantial. Probably 60+ mm. Why? Because with more head angle you have more trail (stability). In this case, with a 29+ degree head angle, the bike would be impossible to turn without increasing the triple clamp offset. Huh?? Yes, an increase in offset reduces trail, loosening up the steering that got heavy with the “Kicked out” steering head angle.
Let’s Review:
Trail equals stability.
More trail equals more stability.
Less trail equals less stability.
More steering head angle equals more trail equals more stability equals harder to turn.
Less head angle equals less trail equals less stability equals easier to turn.
However…!
Less triple clamp offset equals more trail equals more stability.
More triple clamp offset equals less trail equals less stability.
*Adding or subtracting 1mm of triple clamp offset has a 1mm direct effect (up or down) on the trail number.
*Adding or subtracting 1 degree of head angle however, can change trail as much as 10-15mm
So on the Sportster example above, the head angle is so relaxed for the “Chopper” look, that Harley had to go to a lot of triple clamp offset to restore some semblance of light steering. The stock trail they ended up with is still a very high 130 mm! So, this is 30-35mm OVER what would be considered a sensible trail number… for a good, neutral handling street bike.
What I would suggest is to do some of the basic set-up on the bike, wheels and suspension wise and then take another group of measurements. Since this is more of a flattrack/Hooligan bike/goal for primarily very short tracks, I’ll assume you’ve done a (Phil Little) 19” rear wheel conversion and raised the backend of the bike somewhat. Comparing that to the stock bike’s initial dimensions, you’ll probably notice the head angle got a bit steeper and you trimmed the trail number down a bit. The added bonus of this reduction in head angle is a reduction in wheelbase, which also has the effect of moving the motor forward within that wheelbase putting a bit more weight on the front wheel.
How long should the rear shocks be? Personally I like a degree or two of downhill rake going forward on the seat rails or frame cradle tubes under the engine and after establishing the forks I’ll use, match the shock length accordingly. At this time, don’t worry about swing arm angle, but that should be covered in another discussion.
The problem now is that without going to some crazy long rear shocks, your steering head angle and trail numbers are still too high. Cutting the steering head off and resetting/re-welding the angle would be ideal except for the cost, complexity and if you’re running the current Super Hooligan class, per the rules the main frame cannot be cut or re-configured. Going back to steering head angle and its relationship to trail, when the bike is bent over and sliding into the corner, the steering head angle relative to the ground, lays way back, increasing the trail considerably!!
Add to all this, what’s required for a well set-up, easy to control (on the throttle), flattrack bike, is a trail number of around 60mm. Get on a half mile or larger track and a trail increase will be necessary for sure. On a shorter tracker, if the trail number is high, (95+mm), you’ll find that the front sticks like glue but the rear will feel like it’s on ice when you touch the throttle. Or in other words, in a style of riding where traction and throttle control are everything, you will have very little. Conversely, decreasing trail improves throttle control, allows you to turn in quicker and is less work to ride hard. Remember that almost everybody can come flying into the corner one way or another. It’s being able to get onto the throttle early and exiting the corner smoothly that separates the good riders from the masses. Set your bike up for control.
If only there was a way to reduce the steering head angle without modifying the frame! The Phil Little steering head adjustment kit!!!
Richard Pollock
mule@cox.net