Gallery, Projects and General > The Design Shop
Hossack Motorcycle Front End
sebwiers:
BP- yeah, I looked at steel, and could easily make it work for just a bit more weight (and a bit less money, and maybe easier fabrication). However, I think aluminum is a good choice IF you are comfortable working with it and paying for it (which I am - I'm not planning anything fancy, and I'm looking at well under $100 in tubing, which isn't out of line with other costs; carbon fiber was WAY to spendy for the dims I needed). My impression is you can get a stiffer, stronger structure from the same mass of aluminum IF you are able to use fatter, larger tubes (or deeper girders, etc). If I end up needing tubes over 3" od, I'll probably go with steel, but I'd like to keep the fork weight at a minimum to reduce unsprung mas.
For equal lengths and loads, it looks (from those same online calculators) like 2x1x.125 rectangular steel is pushed closer to its limits than a 3(od)x.25 aluminum, and they weigh about the same, so it sounds like I'm on track. I'm not sure what bits would NOT be subject to fluctuating loads - I assume you mean the frame itself, maybe the bushings & pivots of the control arms? For me those may go either way, since their weight doesn't influence the suspension behavior as much. Steel looks appealing for the control arms just because it is easier to weld, and costs less (so I can afford to build multiple generations to improve the design). The idea is to have a nice, light, strong fork that is adjustable enough (ball joints go into "triple clamp" type setup) to allow me to set the geometry after the fact.
Brass Machine - The duo lever design is hard to replicate (compact construction w/ monocoque and cast parts) and doesn't lend itself well to adjustable geometry, but otherwise is a Hossack setup (according to both BMW and Hossack). Cosstinos setup looks quite similar to what I have in mind, and is much like other prototypes (including Hossack's own racing bikes and BMW adaptation) I've seen photos of. His geometry looks a LOT like what I had in mind- steep steering angle, a few inches of trail.
The brake mount in his later designs is interesting- it goes at the bottom of the fork, under the axle. Not sure why he did that- I'm guessing it moves the brake as close as possible to the center of the steering axis (which is actually why its in back on normal forks). Looks trick, might be worth a shot.
The main differences between all those designs is the length / mounting locations of the control arms, and the mounting location / actuation method for the shock. Cosstion looks to have some sort of rising rate linkage going, which I had considered. I've got the full rear shock and linkage from a GSXR to play with, so the details would be a bit different, but much else looks similar (same rough control arm length, rod end pivots for control arms, ball joints sandwiching fork bridge). My setup is just gonna have a hell of a lot beefier fork, since I'm working on a bigger bike. That, and it will be bolting to an existing frame with an existing headset, so the steering will work differently.
bp:
All fair enough so far. Are you looking at a street bike or a competition/road race bike??
My point about fluctuating loads is that steel has a far longer fatigue life than al. alloy. So steel bits, as long as they are stopped from rusting, will have a much longer life.
I designed mine from first principles....established what the likely loads would be, applied a factor of safety, then calculated all the bits. At the early stages of any design exercise its far better to not make any assumptions but try and identify and quantify some reasonable extremes.
From memory the braking load is the largest normal load and works out to about 6.5g. The shock loads (hitting a kerb for instance) can be much higher.
Keep going!!
cheers
bp
sebwiers:
Would be a street bike. Normal daily rider with sporting pretensions. I could get fine use from the stock fork, but... meh. Some folks build raked out choppers, this is what I want to ride.
6.5g seems crazy high for brake forces on a fork - how does that happen? Do you mean the stress to the brake mounts (which has some leverage) or the load pushing on the fork?
I understand your point about fatigue; I've been a bicycle mechanic and have seen (and created) a few frame fatigue failures. From what I've seen, both fail from fatigue when pushed to similarly low weight, and do fine (fatigue wise) when built stiff and strong. I suppose that's a compelling argument for steel, since its cheaper...
Honestly, I don't really know the "first principles" to go from - that's why I was asking here. I just looked at some numbers I'd seen recommended (for steel) and figured out what that mass would let me do in aluminum, then ball parked the resulting stiffness / strength. Seemed OK... but that's hardly a ringing endorsement! I figured to do the build the Russian space program way; build the best thing I can with the materials and design I have, then test it destructively! In this case, that would mean building a test rig to load the fork with the simulated loads equal to double the highest braking / bump loading forces I'd expect to see. Doesn't let me know if I could go lighter, but it would tell me if I went to light...
Hmm, seems the logical thing to do is to prototype in steel, then shoot for similar strength in aluminum once I'm happy w/ the function. Would give me time to get my fabrication skills up, etc as well. 4x1.5x11ga steel rectangle tubing is maybe 50% heavier than I was hoping for, but strong as hell and easy to work with. Cheap, too- enough so that I can build another if I change brakes / wheels / etc rather than bothering to make it adaptable.
Brass_Machine:
I don't know if you have read Tony Foale's book, but you probably should. It has a lot of information you might find handy. He also has some analyses software for FFEs. A lot of information that man provides on motorcycle suspension.
Eric
sebwiers:
I've looked at getting those, yes. Seems spendy (especially if I want the paper book- I'm in the USA), but probably worth buying BEFORE I purchase materials / do fabrication, rather than after.
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