Steering geometry and other things.
(c) Tony Foale 1985-2002
In my previous articles on alterative front suspension systems, we considered firstly the stiffness and then the anti-dive characteristics. In this one we shall look at a variety of topics, including steering geometry, ride comfort, spring rates, and anti-pitch bars.
The basic elements of steering geometry are shown in the figure. For the usual telescopic fork layout, there are three main parameters, viz. rake angle, trail and offset. The rake angle is usually between 25 and 30 degrees, and trail ranges from 3.5 to 5. inches, the offset value is of little importance other than to allow for the desired combination of rake and trail. Two alterative combinations of offset and rake are shown, and both these give the same trail. Trail is the important parameter as this gives directional stability to the bike.
It seems reasonable to assume, that just because we change the design of the wheel supporting structure, that there is no reason to change the steering geometry that works with telescopic forks. However a quick look at some of the alternative designs shows that their geometry often does vary. Within the Hub-centre variety, the Difazio and Bimota both have the steering axis passing through the wheel spindle and hence have no offset, thus the rake must be steeper than normal to achieve the required trail. On the other hand some versions of Nessie and the arrange ment used by Malcolm Newell on his Phasars (radical feet forward machines) have the steering axis behind the spindle to give geometry similar to the norm. The double link family also has variation, Hossack, Parker in America and the Elf.e have some offset and near normal rake, although the Elf is arranged with a large amount of trail. My own design for the QL. and that of Didier Jillet in France (similar basic design to the Elf) use no offset and hence a steeper rake is necessary. So there is obviously a divergence of opinion on the best geometry to use, but surely if alterative geometry works OK with alterative structures then it should also work with telescopic forks.
Now I must declare that I have always thought that a steeper rake angle might have various advantages, but before constructing my double link design I decided to put theory to the test. This was done by modifying my own long suffering BMW.. Two bolt on superstructures were fabricated, one of which supported the standard forks at an angle of 15 degrees and with the yokes reversed this gave the standard trail of 3.5 inches. The other gave an almost vertical steering head but with the forks reversed as well this also gave the standard 3.5 inches.
It was with great trepidation that I ventured forth on the initial test run with the 15 degree rake, all the established motorcycle folklore was ringing in my ears reminding me that the great Australian designer "Phil Irving" had stated in his highly regarded book ( Motorcycle Engineering ), that wobbles were the order of the day if trail was provided with a small rake angle ( He had also told me the same thing in person when I met him whilst still in my teens, and quizzed him on this point, as you see I have been curious on this subject for a long time ). Undeterred by this I headed out of the workshop and on to a narrow bumpy, winding country road, perhaps not the best venue for the purpose but it had the advantages of no other traffic and the ditches were soft.
The thing that struck me most immediately was that the bike was remarkably stable on ruts and bumps, there was no tendency to be pushed off line, confidence was rapidly built up and before too long I was riding no-hands cris-crossing the ruts at a shallow angle, something not possible on the standard machine. Subsequently about 2000 miles were done with this setup and with five different riders, the basic characteristics were largely as I expected:
Low speed balance was easier, stability was improved throughout the speed range, and at speeds above walking pace the steering was lighter but not unpleasantly so. The only disadvantage noticed was some fork juddering under braking, this is simply because more of the braking force goes into bending the fork legs with the steeper angle and was expected. This aspect did not worry me because the purpose of the tests was to check out my ideas on geometry prior to building the double link QL. the juddering was not expected to be a problem with the new design.
The almost vertical steering axis was also tried but not for such a mileage, mainly because the reversed forks converted the twin leading shoe front brake into a twin trailing shoe device, which did nothing for one's survival chances in the fast lane. However, enough was done to establish that it was quite ridable, stability was excellent and there were no odd handling quirks. In fact one feature of the tests that did surprise me somewhat was the totally 'normal feel' of the bike in both guises. There seemed to be very little change in stability from the 15 degree headstock to the vertical, the biggest improvement coming from the change from the norm of 27/28 degrees to 15 degrees.
This test programme convinced me that advantages existed in using a steering geometry that used no offset, this gives the minimum steered inertia and the rake chosen would depend directly on the desired value of trail. The double link suspension was then constructed and experiments done over a range of trail values from 2 inches to 4.5 inches. Interestingly the settings seemed less critical than with normal forks, the bike handling quite normally thoughout the range, only with the lower figures did the steering start to get a little lively over ruts, but never more than the standard arrangement with more trail.
In the previous article it was mentioned, without explanation, that constant anti-dive characteristics were bought at the expense of a changing steering geometry. That is to say, the trail increases as the suspension is compressed. The amount of change depends on the detail layout of the links, but is not usually a great amount. But this is opposite to that with telescopic forks, with these the trail and rake decrease as the bike dives under braking, in practice this does not usually cause many problems because it prevents the steering becoming too heavy when subject to the weight transfer.
However, with the link designs we have less braking dive anyway, and so trail change is minimal under braking, the suspension being compressed mainly by bumps. Now, when the wheel meets such an obstacle the tyre contact patch moves forward ( see sketch ) and the effective trail reduces, it may even go negative. This is one reason for the instability and wobbles experienced by some machines on rough roads. So a design of front end that increases trail when compressed by bumps, will partly compensate for this and may not be such a bad thing. All I can say on this is that after about 1900 miles on my prototype double link bike I have not noticed any ill effects from this changing geometry and stability on bumps is excellent, for whatever reason. So if a moderate amount of increasing trail works, then perhaps a large change will work better? It is difficult to arrive at any figures for this by speculation because different riders have different requirements and feel, and the amount of change needed depends on bump shape and bike speed, so the only way to find out is by practical test. If we construct a single swing-arm to support the front wheel, instead of two arms or links, then the angle through which that arm moves would equate to the change in rake.
In the 1920's such a machine was produced, the Ner-a-car, I do not know just how large was the trail change, but suspension movement was small by today's standards limiting this change, but the arms were short which enhances the change from each inch of movement. So the actual variation in practice was probably considerable. The chance to ride one of these has not yet come my way but I know someone who has, and he testifies to good handling and stability. It must be remembered though that this was a slow speed machine by today's standards. My curiousity has been aroused on this topic and so within the next few months I intend to build a single arm machine, the trail will vary from about 2 inches on full rebound to 4.5 inches on full bump. Assuming that it doesn't punch a hole in a hedge I'll report back on the results in due course.
ANTI-DIVE SIDE EFFECTS.
There are various side effects that stem from the anti-dive characteristics of the hub centre and link systems, some good and some not. The good, wheel movement and spring rate selection are not compromised by the need to avoid bottoming under braking, this means that suspension movement and spring rate can be selected purely on comfort and handling considerations. Resulting in softer springs and less movement. The smaller movement allows the front of the bike to be made lower which may help drag and improve styling possibilities. A disadvantage of the anti-dive concerns ride comfort. As was pointed out in the previous issue the wheel moves forward as it moves upward, the greater the anti-dive percentage the greater is this forward movement. Whereas with telescopic forks the movement is upward and rearward, just the opposite. When we hit a bump the line of action of this force is from the contact point with the bump up through the wheel spindle, i.e. up and back, we can see therefore that a harshness will be given to the ride by those systems that cause the wheel to move up and forward. Telescopic forks thus have good potential for a comfortable ride, unfortunately this can not be fully exploited, because the stiction in the sliders dulls the response to those bumps that do not cause the force to be aligned with the fork legs. In practice the lack of stiction and the softer springs possible with the pivoting systems give a ride no worse than teles. and considerably better over the very small and very large distubances, provided that the anti-dive percentage is not excessive.
ANTI-PITCH BARS (APB).
Ride comfort could be improved even more if we could devise a design where the wheel was allowed to move back and up (i.e. a low anti-dive percentage) and still reduce the dive under braking, and while we are at it we might as well get it to reduce he squat under acceleration as well. Well, the car boys have used such a device for years, but they use it to limit roll in corners, it is called an anti-roll bar. It usually takes the form of a torsion bar across the car the two ends being connected though levers and links to each wheel, in such a way that as one side tends to compress its suspension the other side is forced to compress also, hence keeping the car more level than otherwise. Because of their greater length cars suffer less pitch change than bikes and so even though the idea behind the anti-roll bar could be used as an APB to limit this also it is seldom if ever done. Car designers content themselves with anti-dive and anti squat suspension geomtry at each end if warranted.
With the use of telescopic forks the fitting of such an APB would be of great benefit, but unfortunately would be difficult to engineer, but this is not the case with the pivoted link designs. In fact I have fitted such a device to my own test bike, comprising of a torsion bar running down the length of the bike. Let's see how it works.
Reference to the sketch shows that as the front wheel tends to compress (say due to braking) a torque is applied to the torsion bar which must be resisted at the rear end, the links here are arranged so that this tends to compress the back end also, hence limiting the amount of pitching. It works the other way around also, under acceleration the rear end squat is reduced. This sounds great but what are the disadvantages. Basically they concern the performance under the action of single wheel bumps, most bumps are like this although a bump at the front wheel will later become a bump at the rear when that catches up to where the front was. As the front wheel responds to a shock the anti-pitch bar must become twisted thus resisting the compression of the suspension, in other words the effective wheel spring rate is increased. The solution to this seems easy, just reduce the main spring rate in the first place, but herein lies another problem. If the suspension at both ends is compressed at the same time, such as under the action of cornering loads or when levelling out at the bottom of a fast dip, then the anti-pitch bar will turn freely in its mountings and offer no resistance to the suspension loads. And so if we reduce the basic spring rate to compensate for the stiffening up effect under single wheel bumps , then our suspension will compress more under cornering conditions, reducing the amount of movement left to deal with bumps and may also result in ground clearance problems unless the bike is built higher to start with. It is possible though to reverse the connection of the anti-pitch bar at one end and then compression under cornering can be reduced, but then it will become a pitch enhancing bar, and will still stiffen the suspension for single wheel bumps, anyway.
So with these potential problems has the anti-pitch bar anything to offer motorcycles? Well, to illustrate this I worked out a simple test case. If we assume an elementary bike with equal weight distribution front to rear and decide that we wish to reduce the squat under acceleration to half its normal value. Then the effective spring rate of the torsion bar at the wheel must be 50% of the normal suspension rate. But the increase in the effective wheel spring rate to single wheel bumps is impossible to specify exactly because it is a dynamic problem and the answer is dependent on bump shape, road speed, sprung to unsprung ratio and damping, but it is possible to calculate the outer limits of the increase. For the case being considered this worked out to be an increase of between 33% and 50% in the single wheel bump effective rate. Now to achieve this same pitch reduction effect by the alterative method of simply increasing the actual basic spring rate, would require an increase of 100%.
So if pitch reduction is desired then the anti-pitch bar is obviously worth considering,as it allows for a smaller anti-dive percentage and thus the improved ride comfort that goes with it. As with all engineering design compromise is inevitable and by clever selection of the basic parameters it should be possible to arrive at a design which balances the sometimes conflicting requirements of pitch reduction, ride comfort, and cornering loads. This balance will be different depending on type of machine, a road racer is cornered very hard, and so suspension compression under these conditions assumes greater significance than on a heavily loaded tourer bouncing along on an Australian back road. In this case pitch reduction and good bump response are all important.
After testing my development bike with the APB, I can report that at first I was unsure of any real difference in the ride or handling, and began to think that it might be of academic interest only. However, after several miles I disconnected the bar and continued on, to my surprise my riding started to get a bit ragged and several times I found myself using more of the road than intended, reconnecting the APB cured all this. Basically, when scratching around bumpy bends on English country lanes, the bar just steadied the bike up a bit, allowing an easier faster ride. I wouldn't be with out it now, despite the power deficit the old BM., with the QL suspension and APB, has never been passed in a corner by any of the new race-replica rice-burners. In heavy traffic with the continual throttle opening and closing, the usual yo-yoing is no longer noticed, giving a much nicer ride. Of course if we ever get active suspension then none of this will be necessary, but that's another story.
This concludes this series on alternative steering/suspension systems, I hope some of the mystery about them has been removed, and some future possibilities shown. Perhaps some of you will be inspired to come up with even better designs. It is generally thought that within the next few years the major manufacturers will be turning their efforts in this direction. I hope so, because after experimenting with a variety of these designs over the past few years, I can testify to the handling and stability benefits on offer.