I am not an expert on motorcycle design but from what I have read things like the angle of the forks, trail and swing arm length are very important in determining how a motorcycle will handle. Major manufacturers use a test mule that allows them to adjust all these parameters and make sure a new motorcycle will have decent handling.
From what I have seen on shows like Orange County Choppers, when they decide to design a motorcycle the immediately start with the look. How do they make sure the motorcycles are rideable?
Do they just not show this part? Do they show it and I missed it? Do they have a couple of starting frames that they know will work? Do they not care about handling?
If they don’t change the geometry, then they can hang pretty much anything on it and it will behave in a reasonably civilised manner. There is every chance that these bikes would ride better as they often cut out a lot of weight.
A lot of it comes down to experience. These are not knife edge machines designed for high speed direction changes, and for the most part they are not for racking up huge mileages, or three figure speeds and that reduces the criticality of the chassis design somewhat.
AFAIK, no chopper style bike is built for handling round corners. They are built for looks first and a comfortable riding position second. Eg you sit on them like you do in a lounge chair and can cruise in a straight line for hours on end. It’s not hard to build a bike like this.
Sports bikes are entirely different, your crouch over them and they are carefully balanced and controlled as you say. But try riding 500 kms in one day on a sports bike and you’ll suffer.
define “rideable”. Two wheels and an engine in almost any configuration will stay upright and go in a straight line and go (very slowly) round corners.
A lot of chopper custom shops use Harley Davidson frames and then just extend from there, looking at OC Choppers designs they may use their own frames but they use Harley engines etc I believe done’t they?
One bike they did several years back was clearly difficult to control. Senior took out for a test spin and said that basically it was a good looking bike and in the future it should be looked at and not ridden.
They later seemed to use more stock frames, which is definitely a good idea.
They also sometimes run into issues with safety from too much weird stuff on the bike, especially on the bars. There was frequent “discussions” on the show about some of these back when I used to watch it.
The term “chopper” came from the guys back in the '50s and '60s that took what would be considered military bikes and “chopped” off weight from the fenders and other non-essential body components. The front forks were extended to raise the frame, since many of the back roads at the time were rather rough. You could take the bumps without the extended forks, but you’d have to slow way down. With the extended forks, you could take the bumps at-speed. The “sissy bar” resulted from the fact that a passenger could very well be bounced off the seat when taking the bump at-speed, so a back-stop was needed if a passenger was to ride.
From what I understand, the most important factor is trail (i.e. the distance between the steering axis and the tire’s contact patch). Three factors affect trail: tilt angle of the steering axis, the rake (offset) of the fork, and the diameter of the tire. So, for a given fork and wheel/tire, you can easily calculate the “correct” steering column angle.
Of course I’m over-simplifying, but just making a point that the basic math for a reasonable steering is fairly simple and doesn’t require prototype testing. I imagine many builders can do this by rule of thumb or experience rather than actually doing any calculations.
They having building choppers for about 60 years or so. What angles work and the one that don’t work are not top secret.
While the bikes that OCC builds won’t be sate of the art handling wise they will go down the road.
What always mystifies me is how they’re able to rummage around in a box of pipe, hold bits up and decide where to make cuts, bend and weld them together and get an exhaust that not only physically fits and looks how they want it, but actually works efficiently.
Again, not exactly rocket science, but they seem to have no concept of gas scavenging, and depend on the motor’s brute force to overcome excess restriction, excessively large pipes, ill-timed pulse reflections, etc.
There was a recent show where OCC and PJD (the Teutul’s) competed in a three way build off with bad boy bike builder Jesse James. It was interesting watching that show as Jesse shed unflattering light on their design process. He clearly stated that they did not design bikes for riding, but for display purposes.
I’ve seen many of the Teutul’s bikes with sharp things sticking out ready to spear you in the slightest collision, etc. They aren’t something I would want to ride to Sturgis, but I’d sure love to have one in my garage to look at and brag about. As works of art, they are awesome.
A frame bought from a major manufacturer has one big thing going for it - a VIN number. That helps a lot when you get ready to register it for riding on public roads. As already mentioned, a lot of the radical customs are meant to be ridden only from trailer to the showroom.
VIN Number? Really? VIN will suffice. Sorry, but it grates on my redundancy nerve!
As for steering geometry, it’s as easy as plugging numbers into a calculator like this one:
and you can come up with a drive-able machine.
FWIW, most of those custom bikes use S&S engines. Similar to Harley - but available in bigger displacements and developing more power. The exhaust usually as no engineering put into it to maximize power or efficiency. Drop a 150 hp engine in a 500lb bike and you have no need to get the most out of your engine. As long as it’s shiny and makes obnoxious amounts of noise, the designers and customers are happy.