Monday, February 7, 2011

And now for something completely different

After working on all these engine parts I got a bug to shift focus and to make some inroads on the pile of chassis parts that need to be made.  My first choice was to make the upright legs for the front suspension.  These legs are central to the variable flex idea that is incorporated in the front suspension.  Instead of following the current ideas on overall bike flexibility which uses stiff telescopic forks and a slightly flexible chassis, I prefer to have the flex where it is needed-at the wheel.  Telescopic fork equipped bikes don't have much of an option here- the forks needs to be relatively rigid, otherwise they would bind up when flexing under braking load, preventing smooth suspension movement.  With the linkage suspension I am using it is possible to have a very stiff chassis and a-arm structure to allow smooth suspension action and accurate wheel location yet have flex built into the upright legs to accommodate small bumps and irregularities in the surface when leaned far over.

From the years spent campaigning my single cylinder racer I have a good idea of the directional stiffness that will produce good results.  No, I won't go into specifics here on what the stiffness ratios are.  As expected, maximum stiffness is needed in the braking direction and much less needed in the transverse direction.  The initial uprights I designed worked well, but was a bit complex to make quick copies of.  I then moved on to a much simpler design using rectangular steel, some with welded on stiffening plates to tweak the stiffness as testing dictated.  The latest version of the front suspension uses some old and some new to result in a low friction assembly with easily characterized controlled flex characteristics.

First upright design using suspended headstock design:

This design worked very nicely but was a bit complex to make variations on.  The suspended headstock and all ball bearing upright mounts provided smooth action and good feel but at a weight penalty to later, simpler designs.

Second design style using spherical bearings (bearings not shown):
This design had the benefit of being easy to replicate with subtle stiffness variations but I found the spherical rod end bearings reduced feel and required frequent replacement in order to maintain optimum performance.

Final version for the V4 project bike:
This version separates the parts into a bearing carrier and legs that bolt on.  It enables easy modifications to the stiffness by making new leg parts with varying dimensions yet keeps the bearing mount arrangement stiff.  The weight is in between the first two designs, a good overall compromise in my opinion.  The upright legs will be machined today.

As with most billet parts I started with a large plate that both parts could fit in.  I bolted this directly to the CNC table and machined the mounting pads and axle holes as shown in the following image:

I then machined a fixture baseplate that would locate on these premachined features and allow me to complete the part:

The fixture locates on the machine table with dowel pins allowing easy and accurate setup for making more parts.

I then cut out the individual parts leaving extra stock around the perimeter and bolted them to the fixture plate as shown:

Next came the pocketing and chamfer operations:

Resulting in a sweet looking part:

Mirroring the toolpaths allowed me to create the 2nd part with a minimum of hassle resulting in a very stiff looking assembly:  This design also makes use of the latest radial mount Brembo billet calipers for the ultimate in progressive braking feel.

I don't mention it often enough so a big thank you goes out my shopmate to Peter H. for allowing me essentially unlimited use of his 4 axis CNC mill.  Without access to his machine I'd be making much lower quality parts on my old BP 3 axis relic.  And I'd still be figuring out how to fit the engine castings on it!

In the next few days I hope to move on to the upright bearing housing and handlebar mount, both of which need the 4th axis and trunnion table.

Until then.....


  1. Really Cool! Can you explain a little more about the flex of the fork and rigidity of the frame? (Excuse my ignorance for I come from a motocross back ground and not a road racing back ground.) If you have a more rigid bike under upright breaking then all initial forces go into the tire. Meaning it might be quicker to slide unless a more skilled touch on the front break. Once leaned over I see a big benefit of the flex for the fork is thinner in the side. However bike rigidity is need to make quick changes in bring the bike up and back to over in turns and it seem like the sideways flex would not be helping you. I know that the moto gp bikes are very rigid and that is why they are not very forgiving. My understanding of the Ducati moto gp bike struggles with this causing it loose the front end quickly. Am I incorrect on my thoughts? Why do you like your fork design better than a front swing arm design like Vyrus. Can you talk more? Cool concepts you are working on.

  2. Sure. First off, I am painting with a broad brush and there are details and balances to be maintained but I feel like they are met with my designs.

    A flexible frame under braking does not decrease the force the tire experiences. The same force transmits through the tire to the frame regardless of how the frame flexes. A stiffer frame would increase the initial rate of loading to the tire but as long as it is not too abrupt it may be a benefit in loading the tire sooner, providing more initial grip at turn-in. What it does do is stop the front wheel from oscillating back and forth under braking load, which is what riders are complaining about when they talk about instability under braking. Yes, too little lateral stiffness would not produce a crisp handling bike but my experience and those of all the GP race teams show that less lateral stiffness than longitudinal is good. How much is the $1M question. They need to do it in the frame because they use round forks which have similar bending resistance from any direction. My design bypasses this structural limitation. Ohlins does make variations in the shape of the aluminum upper part to try to reduce lateral stiffness but have limited room to work with. What Ducati is fighting is too much stiffness under high lean as the rider gets very little warning before it lets go. Their CF front subframe is small and it may be difficult to design in the proper stiffness variations in a small structure. The aluminum framed bikes have large headstock areas and long front engine mounting spars and 2 large side beams that all can be tweaked to vary stiffness. Apparently this seems easier to tune than Ducati's 'frame', or maybe the Japanese manufacturers just have a lot more experience doing it with aluminum that Ducati does with CF.

    Vyrus have their reasons for using a hub center setup as I have my reasons for using a Hossack-style setup. I don't know their reasons but think their steering arrangement is more contrived then mine and the new design with a hydraulic actuator may not be any better. My opinion only, it may work great. I prefer the arrangement I use as it is easier to characterize flex and does not have steer lock limitations that hub center setups can experience. One of my main concerns was to keep the force paths as direct as possible- steering through dual pushrods directly from clip on mount to upright and direct mount shock. Front end feel is of prime importance in motorcycles and the fewer pivots the better. The hub center arrangement may work better with an I4 engine as it does not need any structure where the cylinder head/cam cover is which makes packaging less of an issue. Packaging is one of the main constraints of motorcycle design and not a trivial exercise.

    Hope that helps.

  3. I noticed that on your second suspension design you had the radial calipers mounted at the bottom of the legs, yet on the latest design they are mounted in a more conventional position.

    Is there any particular reason you chose to place the calipers where you did?

    On the subject of Hossak font ends, BMW seems to not be using the Telelever suspension on any of their new models (it may not be a true Hossak design but still) my thoughts are that the public have not been to accepting of a different suspension. Do you think projects such as yours will demonstrate the benefits of such suspension designs sufficiently so as to convince more people? Sorry if its off at a tangent.

    The finished products do look amazing!

  4. Shaun,

    The lower caliper mount position was cool but the first lowside it hit the curb and wiped the caliper out so I then went back to conventional positioning.

    Race success will definitely vindicate alternate front ends although at some point the industry does need to advance in technology. There has been several bikes doing well in European national series in the 70s and 80s but then the Japanese juggernaut steamrolled all the low budget efforts. People will buy what they think is cool and maybe my bike can help move FFEs from weird to cool a little quicker. BMW was a huge letdown for me by using forks for the S1000RR. If they had a FFE and ran outside the top 10 everybody would still be impressed. Now we're onto the 3rd season with their GSXR replica with no wins and they still can't make it as good as a Suzuki. They said they want to follow the sportbike market so instead of innovating like they usually do they followed the pack and on the track they are still following the pack.