FFuture

"Funny" front ends - another view

The October edition of the BFF newsletter arrived and I was interested to see an article extolling the virtues of "Centre Hub" steering. This is just one of several systems that avoid using forks and generally separate the steering from the suspension. Now before we get too carried away, I'd like to give an alternate view. Although I have some thoughts on these systems from an engineering point of view, I'll confine myself to bikes I've actually ridden or owned.

First lets look at what is wrong with forks. Try riding something like an old Honda CB250RS or CB250N. This had a spindly pair of forks with a single disk. After 5000 miles, there was almost no damping. Put the front brake on hard and you can feel the forks twisting, while the suspension goes solid with the stiction caused by the bending stanchions. Meanwhile, the lack of rigidity in the frame makes the whole bike sway around the steering head. Toss the bike into a corner hard and again you can feel the forks twist making the steering vague. Take your hands off the bars at 30mph and the bike is quite likely to break into a wobble from the high steered moment of inertia. Take a sweeper flat out and you'll get that wonderful corkscrew motion as the wheels, frame and forks fight it out.

If you're building an FF, forks have other problems. The steering head and top of the forks are getting pretty close to your line of sight, making it hard to get that smooth line from the top of the front wheel to the top of your head. The steering head is also a long way forwards and exactly where you want to put the headlights. Somehow you've got to get a linkage from the steering back to your hands. And finally, the frame has got to connect that high steering head to the swing arm pivot.

Now let's look at a few alternatives.

Jack Difazio's designs used spherical bearings in several drag links to locate the top of the uprights and provide the steering. It's hard to get the geometry exactly right and these fight each other at the extremes of bump and steering. Consequently, they wear out and need frequent lubrication. Most examples lack steering lock, while the lubrication system for the main wheel bearings meant that it was almost impossible to keep oil off the disks. Of the examples, I've ridden, my own Suzuki GT750 kettle had a ludicrously under engineered sub frame to support the front swing arm and the sway under braking was quite frightening. The White Elephant had very heavy steering and a scary tendency to go straight on if you touched the brakes in a corner. The Banana was quite good with clean new ball joints, but as they were never new or clean, the steering got pretty vague at times. Jan Nelder's KL600 used a variation of Royce Creasey's steering column with UJ. This added a perceptible looseness as the UJ wasn't properly shimmed. The complete lack of dive under braking made it very hard to judge and very easy to lock the front wheel.

The Twin Head Phasars had an amazing system with 6 conical bearings and 6 Mini ball joints in the steering path. This, combined with the badly supported top head stock produced lots of steering stiction with some unpleasant dynamic effects. Their owners may disagree but the LC250 and 350 I tried were virtually unrideable.

The Voyager used a Bob Tait inspired design. This has two large spherical bearings as main support with 4 smaller ones in the steering linkage. The demonstration machine I rode had terrible steering stiction as the ball joints were all new. The geometry was not quite right and the bike wanted to stand up the whole time.

Now how about FFs with conventional forks?

I built and owned the VT500 "White Shark" for about two years. The forks were exactly as good (or bad) as on a standard VT500. There was a bit more dive than I'd like and it wandered a bit in WFO corners (The "Veranda" anyone?) The drag link used two mini ball joints but after I'd worn them in with an electric drill, I never noticed them again.

The BSA B50 FF had a drag link with two rose joints. When I rode it they were worn out and you could hear an audible click as the slack was taken up. The rest of the bike was so horrific that front suspension performance was not really an issue. John Bruce's CB400N racer had cut down forks with an S&W shock. As the front suspension didn't work at all, I can't comment on how well it worked.

The CN250 we all know and love wobbles down the road with gay abandon. How much of this is the fork linkages and how much is the fat tires, weak frame, high bars and suspended engine with no shock damping is open to interpretation. But it doesn't really matter, does it?

Now go and ride a recent Japanese or Italian sports machine, preferably one with upside down forks. Amazing! They work. The bike steers quickly. The suspension works under braking. They don't dive much. They're stable at whatever speed your mind can stand. The wheel seems to float over small bumps and yet you can feel exactly what the tire is doing.

Now if you feel I'm being unfair in my comments above, go back and re-read them and consider how many of the faults mentioned are fundamental to the designs, and how many are due to a lack of development. Is centre hub a better solution to steering the front wheel of a motorbike than forks? Frankly, I don't know.

However, I do know that the best forks I've ridden on have been a 100 times better than the best alternative designs I've ridden. If I was building another FF, I would use the best standard components I could find and afford and at the moment that has to be conventional forks with a contemporary beam frame.

Julian Bond
© 1996, Julian Bond

Jeremy Replies

It's really interesting to get an honest assessment of alternative suspension designs- especially in a FF context. However, I feel that the problems presented are a reflection of the nature of home-brew machines. It is very difficult to match the refinement of current suspension design with something built without the benefit of the engineering resources of a manufacturer. Granted, the current telescopic fork (even in the now popular inverted form) has some inherent problems- I think that it works as well as it does because of the decades of effort put into making it work, and not because the design has any special merit.

I think that there is still progress to be made with suspension design, and that there are a couple of examples that work at least as well as the tele-fork, and in some cases, distinctly better. For example, James Parker's RADD design (adapted for the Yamaha GTS1000), has the benefit of hub steering, and has only minor drawbacks like limited steering lock, and slight heaviness. Another interesting design was recently featured in the Nov. 96 issue of Cycle World. This design is different from the RADD front end in that it lacks a front swingarm, and provides suspension action via a spindle that rises and falls on rollers that surround a rhomboid plate that is chamfered on the leading and trailing edges. The plate pivots to provide steering around a hinge pin mounted on a rigid frame frame extension on the left side, and the plate is steered by an upright on one side that pivots, in turn, in a conventional, roller bearing steering head. The spring and damper mounts on the right side of the spindle, neatly tying to the opposite side of the steered upright. From the front, it looks a lot like a tubular bicycle fork, but with a coil over damper for a right leg, and a tubular subframe that extends from the left front of the motor and wraps inward to the centerline of the dished front wheel. This design offers the advantage of a steered hub but steers the wheel directly, and doesn't suffer from stiction or seal friction. I'll have to see if there is any way I can post a picture to my future website- it is really cleanly realized in the ridable example that Alan Cathcart reviewed. I wish I had a better way to describe it, and I hope that I've made some sense. The prototype was designed and built by Hyperpro and Troll Engineering, which were both founded by ex-WP guys. Hyperpro makes progressive rate springs, and both companies are based in Holland, I think.

What I have been thinking about lately is a FF design similar to the Peraves Ecomobile, but shorter and lighter. Here in Wichita, KS, we have a fair amount of composite aircraft construction, and I'm interested in using aluminum honeycomb/carbon fiber sandwich construction for the monocoque. I think that it would be easiest to use a GTS 1000 suspension and drivetrain (subject to availability, of course) rather than try to formulate something myself. This is all rather academic at the moment, though, as I don't yet have any research materials gathered on carbon fiber layup. The main thing, though, is to centralize the mass. Some of the FF adaptations of conventional bikes seem very tall, and machines like the CN250 use small wheel diameters to offset this. I want to be able to use conventional diameter wheels (17"/18" or so) to retain high speed stability and better bump reaction.

What I think that your letter shows most is that in order for a suspension design to retain the feel and precision of a conventional tele-fork, any alternative suspension should avoid using spherical end rod links, and all pivots should have bearings. When the mono-shock rear suspension made its' debut, the suspension action was degraded by spherical bearing rod ends and their inherently stiff action. Also, articulated linkages between the wheel and the rider are tricky to make work- making them slop free but not heavy or 'sticky' is a major challenge. Some kind of steering upright on tapered roller bearings seems to be the best way to aim the wheel- the trick is to incorporate suspension action and hub steering. The RADD design by James Parker uses a steered upright, but the upright is telescopic with a ball-in-groove arrangement to transmit steering input. Because the upright doesn't have to handle the majority of the braking load, stiction isn't a problem.

More Comments

There have been a couple of Difazio style front ends that used a conventional steering head and a scissors arrangement to the top of the upright (like aircraft front wheels) It should be possible to build a design that used only ball or roller bearings in the steering path, like this.

An FF still has the problem of the distance between your hands and the steering head. Royce has solved this with a UJ and steering column. Properly shimmed this can work. This has a nice side effect that braking loads from your upper body travel down the shaft. I found a problem with the VT of needing to be conscious of putting equal force into each handgrip when braking to avoid steering off the road!

The Radd/Yamaha design is interesting but Yamaha had to really over engineer it as they could not afford any failures. As private individuals, we don't have that problem!

Don't get me wrong. I'm very interested in alternatives to forks, but I'm much more interested in getting more people to try an FF. It's worth doing a quick and dirty conversion to a standard bike to see what it feels like while you design the ultimate. Don't let your desire to get it perfect keep you from turning ideas into metal.

Web Resources - Funny Front Ends

Andi McBurnie's FF Site
As part of his FF site, Andi is compiling details of all the non telescopic fork motorcycle front suspension systems.
Michael Moore's special builder's site
Michael has a lot of pictures and detail about specials. These include several bikes with unusual front ends.
Royce Creasey
Royce can provide complete front hubs as well as help you build a bike round them (at a price)
Tony Foale
Tony probably knows more about Motorcycle dynamics and FFEs than anyone around