Laurence Smith runs SuspensionSmith, a suspension business in Australia. Working on motorcycle forks, shocks and swingarms all day, it shouldn't be surprising he has a few of his own ideas about what comes between the wheels and frame and when he started thinking about why girder front suspensions weren't often found on sport bikes, he decided to look into it. Of course, Laurie was in a better position than most of us to work out answers to his questions and the result is the bike you see here, a 2004 Yamaha R1 with his very own aluminum girder front suspension. He's pretty pleased with the result and thought readers of The Kneeslider might want to have a look. Laurie sent me some photos and explained how it all came about.
Here's what Laurie had to say:
The project began in late 2009. I was wondering why girder forks aren't seen on any sports and race bikes,... usually just on very old stuff like Vincents, Rudges, etc. and on modern bikes it's usually restricted to custom bikes and cruisers, low dynamic performance stuff. Why is this so when girders have all the advantages of link type systems with antidive characteristics, low stiction etc.?
I began by using Tony Foale's FFE suspension software to investigate the working dynamics of how girders function. I then built a rough prototype to test some ideas on geometry and construction, the bike I used was an FZR400 frame with a 98 CBR 919 motor for extra go that I had built some time ago.
This prototype taught me a lot of what I needed to know for the next step. Having mucked around with the FFE software I realized what the problem with girders was. The advantage a link type design offers is the ability to tailor the wheel path and control the amount of prodive/antidive you want to use, which, in this case, means a more vertical wheel path. The girder's big weakness with this wheel path is massive trail loss when braking or when the suspension moves in general.
The reason is the wheel on a girder moves independently of the steering axis with a more vertical wheel path, this leads to an increase in steering offset which leads to trail loss and all the problems associated with very low trail.
On teleforks the wheel moves in parallel with the steering axis and offset is fixed and so doesn't change, so trail doesn't change except with variations in rake angle. Other designs like Hossack, center hub, etc., the steering axis moves with the wheel/suspension and again offset is fixed so trail remains more constant.
So how do I fix this? Very simply, steepen the rake angle so that the rake angle closely follows the wheel path and offset and trail remains more constant, it's not perfect but miles better than what has been before.
I chose a 16 degree steering rake angle and I was very confident of doing this because Tony Foale has long put forward the advantages of steep rake angles and these kinds of angles are commonplace in bicycles, from mountain bikes to road bikes, and to do this I needed to modify an existing frame to achieve this angle. I found the 04 R1 to be the best candidate for this type of modification, which was just a matter of cutting out the standard steering head and welding in a custom longer one.
With that done I now designed and made the link setup with rearwards offset link pivots, the advantage of this design is it allows the use of very long links but still keeps the design very compact for good steering clearances and a very low steering inertia. Another advantage is the natural balance the steering now has, a telescopic fork has most of its steered mass in front of the steering axis so its natural balance is to fall away from center, my girder fork has its steered mass behind the steering axis so its natural balance is to return to center and stay centered, that's gotta be good for stability and feel.
A very important feel, one of the biggest advantages of a telefork is the direct feel it gives to the rider and as riders that's what we are used to. My girder retains that kind of feel with benefits. So I did a first prototype in wood to test some ideas.
Then made the final design in 6061 aluminium and a TTX30 front shock from a quad bike.
Wasn't long before the bike was ready for its first road test.
Overall it feels brilliant, it has exceeded my expectations. The steering isn't twitchy or sensitive, its uber stable and ultra precise, the suspension works very well over very rough surfaces and braking is a revelation, I have dialed in some amount of initial brake dive but it is very controlled and confidence inspiring.
Thanks, Laurie. Well guys, that's how you do it. You get curious and then you get busy, you answer the "I wonder" with action. The Yamaha R1 seems to be a good candidate for this conversion, but there may be some other possibilities, too. How long before we see more of these? Great work Laurie.
Related from The Kneeslider: Another R1 girder