Yup, we're thinking the same way Claude. I'm a pretty experienced welder/fabricator...safety is the most important. Plug welds with a low heat dress up weld to keep water out of the joint is the plan. I'll get a thread going here soon and I'll have the front end build in there. Fox Racing air shox limited with bump stops...
If you compare the unloaded axle position to the fully-loaded axle position -- you'll notice that the trail is about the same. Its just during the brief mid-travel period that the trail is somewhat shortened (discounting what the rear suspension is doing at the same interval)! You are correct about the rigidity of a short swingarm which, requiring less material gives less moment-arm for flexing, and on the whole means less weight and less pendulum effect side-to-side on the steering. http://www.matus1976.com/akira_bike/content_currentprogress.htm (Akira replica with raked leading link) 60 million Honda Supercubs . . . . leading links over 43 years! . . . . . . . . . . . . . Compare the /2 Earls Fork to the current Ural leading-link! The Ural design is better for rough terrain, while the /2 Earls is mainly a tarmac design. The Earls will have more pendulum effect side-to-side on its steering but that's not a big problem on smooth roadway. (Still, if you do have some significant mass its better behind the axle than in front. My '74 Moto Guzzi 850-T came with a single leading position brembo caliper. After the upgrade to twin-disc front end, I swapped the telefork lower legs to position the TWO brembo calipers to a trailing position. . . . that meant the mass of the calipers had a smaller side-to-side arc when the forks were turned lock-to-lock. I ended up with crossed brake hoses but there was no stress on them.)
The Earles fork is a specialised case of the leading link design where the pivot point is aft of the wheel. It is an important point because it is that variation that allowed Ernie Earles to patent them. It should also be noted that a great many off-road bikes were built with Earles forks. The design is well triangulated and very rigid and can be quite light though production considerations often caused them to be quite heavy. The Ural leading link was designed to mimic the Earles fork within the design constraints imposed on the engineers. It had to be cheap and use the same shock absorber as used at the rear and sidecar. The engineers simply took their usual motocross sidecar fork and modified it have the same rake and trail as the telescopic forks. It should be noted that Ural triple clamps are raked 5 degrees at the steering stem. I've seen several versions that the engineers would have made if released from those design constraints and I can say that the peeny-pinchers have a lot to answer for. Almost any type of front end can be designed to handle well and all can be poorly designed and incapable of handling well at certain or even all speeds. A good place to talk about designs is http://micapeak.com/mailman/listinfo/mc-chassis-design It is frequented by such greats as Tony Foale and Ian Drysdale so the quality to noise ratio of discussions tends to high. Over the years I've ridden bikes and scooters with girders, leading links, Earles forks, Diafazio HCS, trailing links, single sided trailing link, single sided leading link, Hossack steered upright and telescopic forks. All have advantages and disadvantages and most are in truth only theoretical. All can be set-up to work well and all can be badly designed. A lot of compromise goes into design and a lot comes down to what considerations are given the greatest importance.
There are two designs that impress me the most- The design from Norman Hossack, and my variation on Hub Centre systems that I label Trailing Hub Centre. I grasped the mechanics and advantages of the Hossack design when I first saw it in a motorcycle magazine back in the late seventies or early eighties. Norman testified that he was surprised to encounter people labelling his system as a Girder Fork: The simple fact is that the swinging links on a girder fork are outboard of the steering head and swing side-to-side with the turning of the handlebar. Hossack's suspension A-frame arms (wish-bone) support the steering bearings (ball joints) at their forward extremity, and do not turn but simply accomodate the up-and-down suspension movement. I encountered the same sort of misunderstanding during correspondence with Craig Vetter! Initially he considered Hossacks to be Girders! Norman had difficulty getting any motorcycle builders to try his system, and only BMW seemed to appreciate its advantages. Then, BMW's main version was only half-Hossack until the K1200S with its (full Hossack) Duolever front end. . . . . . . . Often Hub Centre systems present problems! Multiple (sometimes vague) steering links to the handlebars, a lack of self-centering effect, and the problem of retaining enough steering lock. I feel that the placement of the steering axis, running through the centre of the hub is the major mistake. The forces that make the average spinning motorcycle front wheel act the way it does, precession, have a lot of that effect because more than 53% of the spinning mass of the wheel is ahead of the steering axis. If the Hub Centre 'type' system is built with the steering axis behind the actual axle, by as much as 20-40mm, then that Hub Centre 'type' will exhibit the same strong precessional (self-centering) effects as the usual fork-mounted front end. As to that steering lock problem, if only 47%-42% of the wheel is swinging in the arc behind the steering axis then this version will have greater steering lock when the side clearance displacement is equal to a more 'standard' true Hub Centre. In addition, a full 'U' shape doublesided leading arm design (where the suspension pivot passes BEHIND the wheel) as the Difasio version has, will have a shorter (and lighter) pair of arms in a Trailing-Centre Hub build. Usual Difasio Hub Centre diagram where 50% of wheel is ahead AND behind the steering axis. When the steering axis lateral support (like an 'axle') is positioned 35mm more to the rear than the true wheel centre, the leading arms can be shorter and more steering lock is available for an equal side displacement ('elbow') in the shape of both arms.
The Britten is often described as having a girder front-end even though it's clearly a Hossack/Fior design. The BMW Telelever is a Saxon-Motodd copy and operates quite differently from the Hossack. In the Saxon-Motodd design, suspension is still a function of the uprights (telescopic forks) but suspension loads are input into the chassis from both the A-arm and steering head. In the Hossack design the upright is merely a steered component and all suspension loads are carried by the two A-arms. The Hossack system should be ideal for sidecar useage and if combined with a steerable sidecar wheel (about 50% of the front wheel input) and a torsion bar interconnect to the rear wheel (a concept well-proven by MZ-Stoye with the Superelastik) would create a delightful outfit. Unfortunately the Australian ADR width limits tend to make a steerable sidecar wheel VERY difficult.
Well, This thread went quiet as a few months back I had a friend with a set of 'Unit' leading links at the right price. I called him the other day and said give me your bank account details. he said what for, I said to pay for the Units. He said Oh %@#&, I sold them last week. I thought he was having a stir, but no, he forgot he said they were mine. No hard feelings really. He is a good mate albeit a little bit forgetful. So now I am back in the designing stakes. so keep posting pics and ideas. Centre hub has its merits but for off-road work would the unsprung mass be a little higher?
I'm very interested in hub center, but worry about wheel travel mostly. Strength seems to be there..LL is easier to build. Maybe we do need a hub center thread?
When it comes to sidecars, Hub Center Steering is the bomb. Steering effort is featherweight since you're not fighting the tire, it tracks -so much better- and the tire does not go up on edge when you turn it. Your contact patch remains flat during the entire turn of the bars, lock to lock. Not only that, But since a hub center bike moves it's axle straight up and down as opposed to in an arc, your numbers are constant throughout suspension travel, you can get your chassis -perfect- with Hub Center Steering. Oh, and let's not forget ease of service. Knock off one nut, or four lugs and your wheel is dropped off the carrier. Access to your brakes, spindle, bearings, everything... in about five minutes. As opposed to leading link which is a PITA to simply remove the wheel to get access to anything if you're running a meaty front rim like I am. All high performance sport touring or serious performance sidecars with wide front rims should be running hub center steering front ends, it's the only way to fly. I'll be thrilled when we switch over to an LCR chassis next year and I have the luxury of knocking off one nut and pulling a wheel right off our double wishbone front end.
Sounds like the go. So where does one find info on setup ect? I have seen talk of using a car front wheel drive (Civic) hub. home.planet.nl/ ~dreuning/mygts2.html [/COLOR]
Personally... you'll want to start by talking to the guys at the HP Sidecars Forum. http://hpsidecars.com/forum/ Look at the front end on this week's feature rig... sexy. I even see a concentric bottom adjuster for changing the caster. There are several threads right now just on hub center design specifics and theory. Here's a new one for example... http://hpsidecars.com/forum/viewtopic.php?t=538 And here's a cantilever A-arm setup on a Hayabusa HPS These parts look heavy, but you can't think in 'motorcycle' terms any longer, you have to think in 'race car' terms now, since once you slap three big wheels on a motorcycle you've essentally turned it into a very light and fast car. And if you do that... these components are pretty light weight for what they do. Aside from that. Look at the photos on LCR's website and other professional Formula 1 sidecar builder websites. I'm in the process right now of desiging my own F1 sidecar chassis in SolidWorks driven by the inspiration from dpcars.net. I'm going to use all the drivetrain (minus engine) wheels, and suspension components on my monocoque chassis design sourced from the Radical SR4 and SR3 mass production club sports race cars. Why redesign the wheel when you can order exactly the components you need right off the shelf from someone else ready to go and fit them to a chassis of your own design? Everything is there in the 'Suspension System' catalog to build a 100% complete turnkey Hub Center Steering front suspension down to the bushings for the A-Arms. That'l give you somewhere to start. -Mike-
Thanks Mike. That gives me a LOT to work through I have joined the HP sidecars site and will start reading
Looking at the two rigs above, it looks like they both have quite limited suspension travel. That's probably fine for a street rig, but for dual sport or off-road, you would definitely want a system that gives more travel than those. The other negatives are the un-sprung and overall weight. A leading link setup looks much better in those respects. Again, for anything going where there are bumps, weight is definitely bad. The one thing that I like about the center hub system is that the tire stays level while turning. That's definitely nice if you have a wide and flat profile tire up front.
As I am looking more towards the all road front end, the simpler LL's is the lightest way to go when considering travel. Enjoying the thought process of Centre Hub steering too, and it would look a bit out of place on an old Airhead
It's a trade off type of thing it seems. I could go with a hub center front, shock with 4 inches of travel or so and a big bouncy tire I can air down. Or LL with 8 inches of travel and a lower profile tire for better street manners. I'll probably go with a LL for a little better dirt performance on this project. Thanks to you and Bad Cat for the info, very helpful.
