Join Date: Jan 2005
Location: Nine Mile Falls, Washington, USA
I stumbled upon this,, good reading...matthew
BMW Motorcycle neutral steering
by Duane Ausherman
Neutral steering is very important for stress free long distance riding. Just what is it and why is it important? Neutral steering is what makes some motorcycles so easy to ride, so effortless to go through turns and allows the rider to go for long distances. It is the tire profile and geometry of the forks and frame that determine it.
Test for neutral steering
First you must test for straight tracking. With your hands off in the 30-50 mph range, it must go in a straight line. If it does not, then maybe the frame is bent.
It is very easy to test for neutral steering. Make sure that your tires are in good condition and are properly inflated. It is an absolute must to have the throttle lock to keep it at one speed. If you can't hold a steady speed, you can't perform this test. The steering bearings should be correctly adjusted. Remove any steering damper friction. You should also know that your bike has little or no tendency to have a low speed wobble. If you have any question about that, go to my page on wobbles.
Find a curvy road where a fast rider could easily go through the curves at 50 mph, but 30 mph is easy and "normal" for the average rider. Find a time when you have little traffic with which to be concerned. As you go through a turn, sort of lighten up on your hand pressure on the bars. As you lighten up, the bike should stay in the same line as before. If you feel that the bike wants to "fall over" or "go straight" then maybe you don't have neutral steering. As you feel your bike want to stay in the same curve, lighten up even more. If you can get to the point of having your hands completely off of the bars and the bike still stays in the same curve, then you definitely have neutral steering. It may take several "runs" though the curves to do all of this. You should discover that your airhead BMW will go through the turn without your hands even on the bars. One can even put it into the turn and pull it out without even touching the bars. It won't be done as quickly as with your hands on the bars, but it certainly can be done. I have ridden miles through many curves without even touching the bars, just by using my body to steer.
This is where the bike seems to want to fall over into the turn, or turn tighter as you relax your pressure on the bars. If you had a remote throttle, you could accelerate and that would tend to "pick" the bike up. As it is, you have to "grip the bars" in a curve and that is a most unsettling feeling to have. You sort of have to "hold the bike up" to feel confident. Many of the older Japanese bikes were designed this way and it was "normal" for them.
This will cause a change in geometry, basically too short of a wheel base and too little trail. If the front end sits too low, this will be the result. One of the reasons for this to occur on a BMW is to have sacked out (collapsed) fork springs. A common reason is when the Earles fork was bent back from an accident. We saw a few telescopic fork models do this too, but usually they bind up so badly that the owner can't/won't ride it.
Low air pressure will allow the front end to sit low too. A tire that is too small for the front will do it too. One of the most common reasons is that the owner mounted a too large tire on the rear. This sets the front lower by comparison and it will oversteer.
The /2 Earles fork series used the same sized tire front and rear, a 3.50 X 18" and that worked very well. The US models with the "new" telescopic forks used a 4.00 X 18" on the rear and it had neutral steering when all was in top condition. Don't confuse the two types of forks and get some wild hair to use a 4.00 on the rear of your Earles forks BMW if you value neutral steering.
On the /5, and later bikes, it was common for owners to overload the bike with accessories and ruin otherwise good handling. I have no experience with BMWs after 1981, but geometry and human nature probably haven't changed very much.
This is where the bike doesn't even want to lean. It will try to stand up in the curve if you reduce pressure on the bars. This is far rarer than oversteer on a BMW. It makes you really work to go into a curve and stay there. I have seen it from a variety of situations on a BMW. Once a /2 came in with a 4.00 on the front and it was a real bear to ride. Once I found a 3.00 or 3.25 Metzler C on the rear. I didn't even know that they made them that small.
The shape of the tire profile greatly alters how a bike goes into corners. The old Dunlop K81 had a triangular profile and that caused it to give one a scare as it transitioned from straight ahead to medium cornering. After it was in the corner it really held well. Take a look at the profile and see if it has any irregular shape to it. I have seen some new (meaning in the last 10 years) tire profiles that would really mess up an old airhead. The cafe racer could easily give up some smoothness in transition for some other advantage, such as greater traction while in the corner.
The Metzler and Continental tires recommended by BMW really had a profile that felt good. I well know that better compounds exist today, but I would like to keep a regular profile and keep the neutral steering.
The result of neutral motorcycle steering
Neutral steering makes it easier to ride long distances, go through curves with confidence and save your shoulder and back muscles. When a rider becomes really comfortable with a bike like this, the rider only needs to "see a curve" and the bike seems to just "go through the curve." I have often tried to do nothing consciously when approaching a curve and the bike just seems to have a mind of its own and do the right thing. It is this characteristic that causes some bikes to be very easy to ride and others seem to be a wrestling match. On the older stuff, the European bikes tended to be neutral steering and the Japanese were often guilty of oversteer.
Stay with the tire sizes suggested by the manufacturer. I do not know the size conversion to the newer modern metric tire sizes.