Basic parameters of open cockpit FFs
including basic 'cut'n shut' conversions
"It all comes down to a few inches, a few degrees, and a few pounds" Jack Difazio
A single track vehicle with a seat base less than 20” above ground level at ride height, fitted with a seat back capable of supporting the rider. The front suspension should not be steered.
This is a guide to dimensions and settings for new constructors and for trouble-shooting on 'legacy FFs. It has been derived from empirical testing and figures given are best estimates based on available evidence. The rider is assumed to be near the centre of the 90% euro male ergo size range.
1. Tyre selection.
Tyre size, type and often actual tyre are usually selected first in a clean sheet design. This allows various other parameters to be established. Estimated weights for home-built FFs are given below and the tyre speed and load ratings needed are the starting point for tyre selection.
Conversions of motorcycle, or scooters usually use the original tyres and this has worked well in practice, with one proviso. This is that many FF features, such as increased control authority, weight distribution and the increase in overall weight of most conversions, all increase loading on the front tyre.
Scooter conversions like the Cmax have not reported trouble as a result. The 2010 Cmax was run on Southern European motorways extensively without front tyre temperatures becoming excessive. It will be wise to monitor front tyre performance on a new conversion and be aware of front tyre load rating, actual load and maximum pressure. The front tyre should never run hotter than the rear and maximum pressure should not be exceeded.
HCS adds load on the front tyre. HCS-equipped, heavyweights like the Voyagers and Quasars also run into tyre availability problems because their ancient shaft drives limit rear tyre width to cross-ply sizes. this requires matching cross ply fronts and there are very few suitable. Rear tyres patterns work best on HCS front ends and fortunately there is an Avon rear pattern 'universal' 16"/18" cross ply with a rear tread pattern that is commonly used.
Designers of modern HCS equipped vehicles, able to use modern radial rear tyres, will similarly find just one or two sizes of radial front tyre with adequate load ratings but no 'universal' radial types that would allow a rear-tyre tread pattern. If a front tyre must be used it should have minimal tread pattern to limit 'squirm' under heavy braking.
Tyre selection may influence wheel size, gearing and rim width.
2. Seat height and CG.
Once the seat height gets much below about 400mm (16") the improvement in handling, although detectable, becomes increasingly irrelevant in terms of actual, real-world roll-rate requirements - the vehicle will roll fast enough for all reasonable purposes. This means other factors become worth considering.
Safety. Many experienced FF users agree that eye contact with car drivers, at similar seat heights, is much enhanced and this leads to better conspicuity, earlier recognition and better relations. For this reason it is worth considering setting the seat height close to average car heights in your region. This may be as low as 350mm, although good eye contact is reported at 450mm. (Open face helmets, permitted by good weather protection, also improve this.)
Comfort Several prototypers have reported difficulty getting out of very low FF seats and this must be considered. It can be as simple as standing up off a stool, from a squatting position, to see what height is found comfortable. This may limit minimum seat heights to around 300mm, but depends on exactly how the ergonomics are arranged. A race FF, obeying 2011 E-GP rules could get down to about 275mm, but the open sided cockpit imposes structural limits beyond that.
350-425mm has proved to be a good window for seat height in general road use.
CG. Even If a seat height is selected on the basis of the safety and ergonomic reasons mentioned above, it remains the case that lowering the CG by other component location, will always improve handling.
Mass should be concentrated along the roll axis. This is a dynamic line that will always be somewhere between the line of the contact patches and the line of CG height along the vehicle.. It moves between the contact patch line and the CG line during various stages of roll dynamics. In practice this means, as usual, that heavy components should be placed at the bottom of the vehicle, as close to the centreline as possible.
Fore and aft mass centralisation is less important. Moderate distribution of mass to the front and rear will improve ride quality by damping pitching.
CG height can vary at different positions along the vehicle, according to where mass is placed.. If the CG line, along the vehicle, rises slightly but steadily to the rear, then the roll axis will also tend to be higher at the rear. This is desirable as it gives early load on the front wheel during turn-in and reduces load on the rear slightly more, also aiding turn-in.
Maximum traction is the only conceivable reasons for limiting low CG. Pre-electronic calculations, including rear wheel torque indicated that a total vehicle CG height of 400mm above the ground will generate 100% rear wheel weight transfer, with 1525mm wheelbase.
This is not an achievable CG for a road vehicle. Traction will not be a problem. Even Malcolm's 'SEV'' Z1300 FF would not sustain wheelspin, just wild acceleration.
Many Legacy FFs have very long wheelbases. These have demonstrated that figures beyond 1775mm are quite usable, if any expectation of traffic penetration is abandoned. In countries where lane splitting is illegal and PTWs are required to behave as cars this may be acceptable.
Very high speed use, including racing, may also use long wheelbases to add stability in drift but this has yet to be established.
Wheelbases above 1650mm do reduce manoeuvrability. Where wheelbase has not been limited by physical design factors it has been found that a good 'window' for a full-scale, two seat FF is around 1525mm.
Shorter wheelbases may not ride quite so well, due to increased fore and aft pitching. Longer will not turn quite so fast, specifically in low speed 'Elk Tests' As with seat height, reducing wheelbase, will produce a faster reacting vehicle but below 1475mm it becomes questionable whether the increase is relevant.
I have placed this item before geometry because it will be easier to lay out suspension with these features included.
Dive and Squat are less relevant on FFs than on motorcycles because of the reduced weight transfer.
This allows a motorcycle transmission, usually set up with substantial anti-squat, to be lowered in the chassis, reducing CG height.
Zero anti-squat has been used without trouble on chain-drive FFs. - that is with the rear wheel centre at the same height as the rear fork pivot centre.
At the front the Voyager HCS is set up with 10mm of anti-dive - the centre of the 'Bottom' ball joint in the wheel centre is 10mm below the fork pivot centreline. This is rather a high figure. Applying the front brake alone at speed causes the front to rise slightly, both brakes generate a slight drop. 5-8mm is probably more suitable for these 1600mm wheelbase 300kg. FFs.
High anti-dive and squat figures will interfere with suspension quality. In any case these figures can be adjusted in development by changing spring platform position and hence ride height. Zero anti-dive and squat is a good starting point
5 Steering Geometry
If using telescopic forks or some other form of steered suspension, it is unwise to alter the stock geometry substantially as it is needed to stabilise the steering and suspension in the absence of any significant structural or steering stiffness. The wheel more or less has to look after itself.
The FF seat back increases control authority to the extent that 'over-controlling' stock geometry and handlebars is normally reported until users get used to it. Some users reduce rake and trail slightly, by the usual method of running the forks further into the yokes, and then use the extra control authority to cope with the occasional wobble.
The result is acceptable; reasonably stable steering that is slightly ponderous and heavy.
In the case of HCS, specifically one with the high torsional stiffness provided by a top wishbone,. and where the suspension is not steered, none of this is really relevant.
It has been shown repeatedly that there is no problem of stability, with too little momentum in the steered assembly and too much stiffness in the structure, to permit wobbles. This allows the geometry to be set at whatever figure suits low speed handling effort.
In practice geometry has been run down to car-type figures on HCS equipped FFs - 10 degrees of rake and 44.5mm (1.75) inches of trail. At these figures the front wheel stops 'self-steering' at above walking speed - which is undesirable - although there were no other problems.
Default' geometry on the HCS equipped Voyagers has settled at around 55-60mm trail and 14-15 degrees of rake.
It should be noted that these figures have been used to achieve pleasant, low effort, steering 'feel', rather than anything else. More trail just means heavier steering for no reason, 15 degrees of rake is ample to make the steering turn at low speed.
These figures are running on 300 Kg Voyager FFs, close to the upper limit for leg support. Lighter FFs will run smaller tyres and slimmer contact patches at the front. This may allow less trail, if desired, with adequate low-speed self-steering that would be lost with the heavily loaded, fat Voyager front tyre.
Actual rake and trail figures define the quantity of steering feel. How heavy, how responsive. The relationship between rake and trail defines the quality of the steering by setting the balance between the speed-related stabilising effect of trail, and the fixed de-stabilising effect of rake.
Most HCS systems have a fixed rake/trail relationship - although some can be altered slightly at the machining stage. Designers of such devices therefore need a guide to selecting a relationship early in the process.
Because it is the relationship between rake and trail, rather than the actual figures, that define handling quality it is possible to choose an existing vehicle with a desired quality, define it's rake/trail relationship, transfer it to an HCS system with radically different rake trail/figures and get recognisably similar handling quality
Simply dividing the rake, in degrees, by the trail, in whatever units suit, produces a number that represents that rake/trail relationship.
Initial FF rake/trail relationships (001, 002) were set by selecting a relationship that seemed pleasant on chosen motorcycles then applying that relationship to HCS geometry with a much lower trail figure (Exploiting the stability of HCS to give light low-speed steering).
Subsequent development has led to figures selected specifically for Voyager HCS on Voyager FFs, but this was an effective and useful way of starting out.
6. Suspension settings
Lack of dive and squat and reduced weight transfer means that the vehicle can be set closer to the ground than a motorcycle. This is the cheapest way of lowering the CG.
It will also be rolled as fast as it's secure driver chooses so keeping the wheels on the ground is important.
For both these reasons the suspension should be set much closer to full bump than would be done on a motorcycle. The heavy Voyagers run 50% travel to full bump, lighter FFs have been run at 70% to full bump.
This also reduces the ground clearance needed. In testing at '50% to bump' settings, 95mm (3.75") proved lightly too low on a 1525 (60") wheelbase, while 100mm is fine - in terms of clearing kerbs, speed humps etc.
Selecting suitable springs at the design stage usually requires some guesswork and this will invariably be optimistic. In practice this is good as it means that initial spring settings will be too soft and this is safer than too hard. The weight of the vehicle and rider is needed for accurate spring selection and this is usually done in the vehicle development stage.
For initial setting assume that a home-built FF with bodywork will weigh 200-250 Kg if it's a sporty single seater and 275-300 Kg if it's a two seater like a Voyager.
7. Weight Distribution and spring selection
Aim for a 50-50 weight distribution with the vehicle normally loaded but without passenger. This avoids 'snatch locking' the front brake and provides solid, stable handling. Adding a passenger and luggage give a rear bias, bit increases load on both wheels Taking either the estimates above, or actual figures, derive spring rates as follows:
Divide weight by two. This gives each end loading
Divide by two again if using twin shock set ups (useful FF packaging option)
Now you have weight per strut.
Measure distance from full droop to selected ride height (see above). Divide the weight on that strut by that distance. You'll get something like 24Kg/Centimetre or 15lbs/inch etc.
You have to add to this any leverage ratio between the actual wheel and the strut.
For instance, at the Voyager front end, the distance from the fork pivot to the wheel centre is 30% more than the distance from the fork pivot to the spring pick up on the fork. So you add 30%. Leverage ratios can be quite large on multi-link systems. You can also derive this ratio by measuring actual wheel centre movement against actual strut movement if convenient.
You've now got the spring rates for each strut.
8. Ground clearance template
A slightly complex clearance template has been developed which avoids ground contact before slide.
There are two modes in which ground clearance becomes marginal.
First is in relatively low speed, tight corners, typically urban, where in conditions of good grip a vehicle can be decelerated hard into a tight turn with fairly moderate lean angles. This mode depresses the suspension, due to both cornering and decelerative loads, more than a steady lean and can cause grounding.
The template developed for clearing this is a line from a point directly below the wheel rim on the side being considered (tyre at nominal rolling radius) at 45 degrees to horizontal, with the suspension on full bump.
The second mode is the high speed maximum lean condition. To clear this, take a line from the same point, under the wheel rim, at 55 degrees to horizontal, with the suspension at ride height.
This produces a template where the bottom corners of the 55 degree/ground clearance 'box' are cut off by the 45 degree line.
9. Hand control.
Although strictly part of a specific vehicle design the height of the hand controls above the seat are a relevant part of the overall vehicle setting. This is due to the relatively high G loadings (more than 1G) that can be generated by an HCS-equipped FF. To cope with loads the hand controls needs to be higher, and more robustly supported, than steering itself might require.
If the hand controls are too low or too flexible they will compromise rider security and stability under heavy braking.
To establish a good height setting for the hand controls on a specific vehicle, kneel on the floor, on hands and knees. You will naturally place your hands in the best position to support your upper body. Measure your position and transfer it to the vehicle design. Taking a photo of the kneeling rider and turning it through 90 degrees gives a good guide.
10. Basic Cut'n shut conversions.
The above parameters give an idea of a 'centre-of-envelope' window for an uncompromising FF that will carry two people at any reasonable speed. Although some of the dimensions, and HCS in particular, present technical challenges to constructors, the performance available in this window is ample reason for overcoming them.
These parameters are relevant to any FF design. Where an FF project is not a clean sheet design, but the conversion of an existing motorcycle or scooter, it is important to understand what can be compromised for practical reasons - and which compromises might render a project pointless.
There are a growing number of 'super scooters' or motorcycle where the engine layout appears ideal for the FF layout. This is usually referred to as the 'Linto' layout, after it's originators. The Linto consisted of two Aermacchi 'laid down flat' singles, coupled into a 'laid down flat' parallel twin. This has been used by Yamaha and Suzuki scooters for a while, with BMW and Honda about to launch similar layouts.
This is a very convenient engine layout for FF projects and the task is made easier as most of these vehicles also package the rear suspension so that no structural alteration is needed. In England this means no alterations that have to be notified to the registration or insurance agencies. Footrests, seatbacks and bodywork are of no interest to them. Anyone considering an FF project should study these scooters.
There are two 'Cmax' FF conversions on bikeweb, the 2010 version in particular being a full two seater similar to a Voyager. This conversion took a little more than eighteen months and used none of the original bodywork which could therefore be sold. As a general rule these conversions work best if the rider is moved forward as far as possible, to avoid a rearwards weight bias.
The Linto layout makes achieving a suitable seat height easy. The Cmax seat base is lower than the Voyager unit. However all these vehicles have telescopic forks and these have been retained in the Yamaha and Suzuki conversions that have been done. As noted above the result, with stock geometry and handlebars, is acceptable but ponderous steering. Braking dive is reduced, with the weight transfer. This is an acceptable compromise given the ease of conversion and the otherwise good fit with 'centre-of-envelope' parameters.
In another millennium, while discussing cut'n shuts, I said that telescopics were usable on FFs, chiefly as a way of demonstrating their failings. This remains the case. Problems may be minimised by removing any non-essential items from the steered assembly and fitting a fork brace. Spring and damper rates may need to be changed
There are other downsides of these conversions. For some the deal-breaker is the common use of automatic transmissions, apparently demanded by scooter riders. There is some hope on the horizon in the form of Honda's 700S 'Linto', a version of which has a conventional sequential motorcycle transmission.
Another issue is the low output of most of these Linto types. The Cmax owner reports running at full throttle on motorways and although these modern engines should cope better than some cut'n shuts from previous era's they will be hard used and this will reduce life.
These low outputs place emphasis on controlling weight, if acceleration is to be retained. Basically, the simpler the conversion the more difficult it is to avoid adding weight with the footboxes, seatback and bodywork, the minimum FF components. As more of the original is replaced this weight gain can be reduced by good design but a cut'n shut FF will be heavier than the donor. Keeping the weight gain below 25Kgs would be cause for celebration.
Constructors who must have more power than provided by a Tmax or similar 'Linto' scooter, have the more difficult task of converting a larger motorcycle. In the past this was done with great abandon by Malcolm Newel, the Quasar project leader. His interest was in fitting the largest engine possible, culminating in the 1300 Kawasaki open-topped "SEV" and the Quasar bodied "Thirteen". Both had HCS however and were not really cut'n shuts. They also compromised ordinary drivability to a heroic degree.
The first difficulty is usually finding a layout where it is possible to get the seat base to 20". As this was a 'best guess', based on limited information by a few people some time ago, it is relevant to ask if this figure can be compromised for practical reasons. The answer is that a better guess, made more recently by more people, suggests that 18" would be a better place to draw the line. In terms of roll rate, which is what lowering the CG addresses, any increase in seat height raises the heaviest part of the vehicle and decreases safe roll rate. Period.
However. Reducing any seat height also reduces frontal area, and hence fuel consumption. And it increases roll rate compared to any higher seat. For people prepared to travel at moderate speeds these are advantages worth having, and they always have been. Every motorcycle I have ever owned or built worked better when I reduced the seat height. It's just that you need to get below 20" to actually keep the wheels on the ground...
It's a similar situation with wheelbase. One of the easiest ways to get the seat lower on a motorcycle is to increase the wheelbase, making some room between the machinery and the back wheel. There are two difficulties with this. One is, obviously, that it increases the wheelbase. The other is that it implies the rider sitting at the back of the vehicle. This can be done, but is less desirable, ergonomically, than sitting in the middle of the vehicle - another advantage of the 'Linto' scooters. It also makes good weight distribution difficult.
Some of these problems can be moderated. A high seat can be offset to a degree by lower components. Weight distribution can also be changed by component location. Wheelbase remains wheelbase...
It is not easy to produce a truly effective FF by modifying a motorcycle and few have achieved it. Generally, the bigger the motorcycle, the more difficult it is. Invariably, at some stage in the process, constructors will consider how much easier much of it would be if starting from scratch.
This last is true even of the 'Linto' scooters. However they represent a quicker, cheaper route to an FF than any that have been available up until now. A conversion can range from an absolutely minimal 'quick and dirty' and take a few days, like the first Cmax - or discard almost everything not holding the wheels on and take months, like the second Cmax. The difference is largely about fit, finish and specification. Both are a huge improvement on a Tmax and unequivocally modern FFs.