Extract from an article first published in “Fast Bikes” magazine.
© Ian R Cramp September 2000
People in pubs are very quick to condemn telescopic forks, and they usually have any number of perfectly sound reasons to back up their views – too heavy, stiction, lack of rigidity, poor geometry, etc etc. However, the telescopic front fork has been with us in a relatively unchanged form for about 50 years, so there must be something good about it. As is so often the case in engineering, some solutions don't look particularly good, but when there is no "good" answer to be found, you have to go for the one that is the least bad. Maybe this is the case with a tele – it has its faults, but it does the job, and nothing else has threatened it despite the best efforts of whole pubs full of engineers for decades.
I therefore started my investigation into my funny front end from the opposite direction – by having a good look at conventional forks, and working out what were their good points, which had made them so popular for so long. I decided, after a great deal of thinking and research, that the most important thing of all was the way you could mount clip-ons directly to the fork tubes. Being able to feel what the front tyre is doing is crucial to controlling a bike, and any system which has a sloppy linkage between the tyre contact patch and the handlebars is therefore an instant non-starter. The Bimota Tesi was particularly shite in this respect, and so was the first Elf, though the Elf team soon learned from their mistakes and their later versions had a much more direct and rigid steering link which was nowhere near as bad.
Secondly, whilst telescopic forks are heavy, a fair proportion of that weight is sprung. Weight is always a bad thing on a bike, but unsprung weight is double-bad. If you look at many really quite well thought out funny front end designs (like the BMW Telelever and the Tryphonos, for example) you see that they carry much more unsprung weight; things like the steering head, bearings, clamps, and other stuff which are mounted on sprung parts of the bike in a conventional setup become unsprung in these arrangements. Not good.
Another key advantage of forks is their geometry. People can write whole books about offset, trail, and the simple up/down travel of a tele, but the bottom line is that it appears to work better than anything else using current technology. Aha, the pub philosophers say as they prop up the bar, but as you brake and the front dives, so the rake, trail, and wheelbase change. Well, yes, but so what? It seems to work pretty well, so what the hell? It can't be denied that some other systems, with different (they say "better") geometries, which maintain a constant trail or a constant castor or a constant this and a constant that, nevertheless feel rather weird to the fellow holding the bars. Since absolute confidence in the ability of the bike to react as it intuitively should at all times is the most vital part of riding quickly on unfamiliar roads, systems like this were to be rejected out of hand as far as I was concerned.
A big problem with bikes is that they need a lot of suspension movement, and this makes it very difficult to find any suspension system, other than teles, with the right geometry. Modern racing cars have so little suspension movement that there are actually not even any balljoints on the ends of an F1 car's wishbones – the flex of the joint itself is all that's required. By contrast, bikes have about ten times as much movement for each end, and this means big problems for geometry. For example, if a car's wishbone is two feet long, it will always be sitting at pretty much the same angle when the end moves half an inch or so. The opposite applies to a bike. Taking the rear swinging-arm as an example, it must allow about six inches of travel, and as it's only a foot and a half long, it must move through about 15 degrees of arc – therefore having a huge effect on the geometry of the rear end. The longer you make the arm, the less is the angle it must move through – which is why we have a trend towards longer and longer swinging arms, and why the Yamaha YZF-R1 therefore has a stacked gearbox.
Now, as the arm gets longer, the path through which the wheel spindle moves becomes straighter. If you could make the swinging arm infinitely long, the spindle would move through a straight line – just like a telescopic fork! Years ago, there were actually rear ends like this on bikes before swinging arms became the norm – the Norton plunger or the Triumph sprung hub for example. The problem with having straight-line motion of the rear spindle, however, is that the chain tension changes too much with suspension movement. The angularity problem associated with front swingarms also applies to leading and trailing link type front forks, which are surprisingly popular; ten million Honda C50s can’t be wrong, and Moto Guzzi fitted them to their GP bikes for years with some success. I’m not sure about this, but I believe that Bob MacIntire had leading-link forks fitted to the Gilera on which he set the first ever 100mph TT lap (don’t all write in if I’m wrong – the only reason I don’t know for sure is that one of those pub bikers borrowed the book and hasn’t given it back). JPS Norton-Cosworth Challenge bikes had a similar Earles-type set-up, which can be examined minutely at the National Motorcycle Museum, and systems were fitted to TZ350s as late as the 1980s by privateer riders who were only too happy to put the standard teles in the bin.
Nevertheless, in light of what I've already said about steering links, geometry, and unsprung weight, I didn't consider having any sort of swinging-arm or link on the front; and anyway, it's already been done. Designs like the Bimota Tesi and the ASP are shite, but something like the Tryphonos project was as good as these things can get. There was no point in my trying another.
Since I wasn't keen on going for teles, there was only one other choice really – the girder fork, as fitted to millions of bikes before the last war, and still very often seen in 1951, the notional "styling date" for my bike. To be honest, I often wonder why designers abandoned girder forks in the first place, as they're fine bits of kit. My trusty 1929 Velocette with its spindly girders that look like drinking straws is still plenty good enough for a knee-down now that it can be treated to modern tyres and tarmac. The conventional girder fork reached its apogee of development in the 1950s with the Vincent Black Lightning (this is the bike in the famous picture of the herbert doing 150mph on the Bonneville Salt Flats by lying flat and wearing only his swimming trunks). The Vincent’s girder fork legs were aluminium forgings – the perfect material for the job, though I thought I could make steel tube ones which would be stronger and just as light.
The bike industry abandoned girder forks more or less by accident. There was nothing wrong with them at all really, but no-one bothered to fit them with good geometry or good damping (the technology of the time just wasn’t up to it, and with crap roads and tyres, no-one could go fast enough to notice). When Norton came along with their telescopic “Roadholder” forks, everyone assumed that the reason they were better than the girders of the day was simply because they were telescopic. Not so; it was that the package allowed enough space for the designer to get a half-reasonable spring-damper unit on the front end of a bike for the first time. Also, I'm sure that the public were influenced by the idea of buying a bike with what they thought was the same sort of suspension as Geoff Duke was using on his Norton GP bike, and all of the British manufacturers immediately jumped on to the fashion bandwagon. Italians were not influenced by the fashion in the same way, and they went on to kick Norton’s arse with link-type suspensions for several years.
Because girders faded away rather than being superseded, a few enlightened frame constructors have continued to use them in conjunction with the modern damping systems now available, which would have transformed the old ones. Hossack, Cobas, and ROC all had a go with varying degrees of success, but it was the Britten which really put girder forks back on the map – with a system which differs only in detail from the Vincent of fifty years before, with the aluminium forgings replaced by carbon fibre. Ironically, it was the carbon bits that gave the most trouble, with some frightening structural failures occurring before the design team got their act together.
A crucial advantage of girders is that, because their movement is controlled by a couple of links (like wishbones), you can vary the geometry quite easily by changing the length and angles of these links. Rising-rate, falling-rate, anti-squat, anti-dive; rake, trail and offset either increasing, decreasing, or staying the same – all are more or less possible by changing the lengths and angles of a couple of links by a few millimetres or a few degrees. This is known in engineering as a four-bar link set-up, and it has been used to bore the arses off engineering students for centuries.
The four-bar link is basically one fixed axis with a bar on each end, with the ends of those two bars joined by another link. By changing the lengths of the bars, an infinite variation of displacements, velocities, and accelerations is possible – turning the four-bar link into (for example) the double-wishbone set-up of a racing car, or the parallelogram rear suspension of a modern-day Moto Guzzi – or a front girder fork.
Now, it so happens that when I was studying dynamics of mechanical systems at university, I was tortured at length with learning the ramifications of these great variations. I studied the geometries, the displacements, the velocities, the accelerations (both linear and angular) and I derived all sorts of equations for maximising, minimising, and optimising any or all of them. It wasn't boring in the way that watching a girder rust is boring, it was more like being methodically and rhythmically beaten over the head with a rusty girder. I was mightily pissed off, but I continued to wrestle with it manfully because, in my youthful naivety, I really thought that I would need to know all of it to become an engineer, and it would ultimately be very handy. Imagine my surprise when, nearly fifteen years later as I looked at this front end, it actually was – just about the first time that anything I was taught in university (apart from in the boxing club) has ever been of the remotest use.
Because of what I said earlier about forks, I thought that it would be smart (and safe) to fix the geometry of my girders so they moved the front wheel in exactly the same way as a set of telescopics. This would mean that I wasn't sticking my neck out too much, and also that the front end would have the same sort of feel that riders are used to. It would have been an easy job to design such a system, but the large suspension movement of a bike caught me out yet again – the front wheel has to move about six inches up and down, and modern spring/damper units only have about two inches of travel, so that means you need some sort of link in the system – you can’t just bolt the bottom of the shock to the fork and the top of it to the top yoke. This threw a surprisingly big spanner into the works, and I had to study (and reject) many ways of getting round it before I came up with an acceptable solution.
A spin-off from the solution I settled on – using the bottom link as a rocker to act on the bottom of the shock – was that the front end became genuinely rising-rate, that is geometrically rising-rate, without the need to put progressive springs and damping in like they do with some teles.
A good rising-rate characteristic is vital on a front end. It must provide the lightest of springing and damping to cope with keeping the bike pointing in the right direction when hard on the power (so there’s hardly any weight on the front and the tyre is barely skimming the surface), and also it must be beefy enough to absorb bumps even when hard on the brakes, when the back wheel is off the deck and the nose is trying to bury itself into the ground. The conventional solution, largely pioneered in the 1970s by Kenny Roberts Snr, Yamaha, and their Ohlins partners, centres around dual-rate springs (soft for most of their travel, but much harder for the last bit) with the latest refinement being dual-rate damping (ie damping which can be set to respond to high-speed and low-speed movements differently). Obviously, this works very well indeed, and with a set of forks costing eight grand it bloody well should do, too. I was hoping that my scheme could be just as good, but lighter and more rigid, as well as looking the part for a classically-styled bike and being able to use a £600 over-the-counter shock absorber to do all of the springing and damping required.
Even though I had most of the basics of the design sorted in less than a day, I spent more than a week agonising over various aspects, and I’m still not happy about some of them. It makes sense to get your worries in sooner rather than later, because it’s a lot easier moving a line across a CAD screen than it is chopping bits of steel and moving them around when you find out, too late, that you’ve made a cock-up. For this reason, I like to put in a great deal of effort at the design stage, keeping the famous British Army dictum in mind – train hard, fight easy. Cutting metal is when the bullshit must stop – as you’ll find out in the next exciting episode, when my lily-white hands move off the keyboard and get down to some honest toil in the welding shop.
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Originally Posted by Frits Overmars
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