Ironhead Upside Down Monoshock IH - A Longterm Project

[Paul Kl]

Nice project Paul and a very good result. I am exactly doing the same thing with my Sporty 78. I have an Kawa front end including the wheels and brakes. Only difference is that I use the stock rear swing arm with an ZX front wheel. Also the rear end will be an shorter version. Thanks for the inspiration. Greetings Paul

[84rzv500r]

very nice.. I like the look and the improved suspension...

[paulc]

Originally Posted by Paul Kl

Nice project Paul and a very good result. I am exactly doing the same thing with my Sporty 78. I have an Kawa front end including the wheels and brakes. Only difference is that I use the stock rear swing arm with an ZX front wheel. Also the rear end will be an shorter version. Thanks for the inspiration. Greetings Paul

Good to hear my project provided some inspiration. My swingarm has the original pivot but from there on back it's new. Your build sounds interesting, you should start up a build thread, it would be good to see.

Originally Posted by 84rzv500r

very nice.. I like the look and the improved suspension...

Thanks. The rear suspension so far doesn't really seem better or worse, just different. I do really like the look though. The front though is definitely far better.
Paul

[Ferrous Head]

The deficiencies in the rear swing arm don't show up at slower speeds. The early round tube forks (I just don't know how strong the later square section are) just aren't strong enough at high speed.
The rear suspension has 3 deficiencies from an engineering standpoint.
1. the arms are made with "too thin" tubes allowing some flex.
2. The pivot point is too narrow.
The problems show up at high speed when precessional forces are strong enough to come into play complicating the whole thing.
If you stand behind the bike and imagine what happens when you twist the fork. The tire contact point will move. When this happens while your cornering it means the front wheel and the rear wheel are now traveling on two different arcs. That's not great for a start. But when you tilt the rear wheel over precessional forces change the lean angle of the bike. The bike will begin to "self correct" as the front wheel hasn't changed it''s axis.

The net effect is the bike "wallows" trying to find an equilibrium between the two competing forces.

At slower speeds there simply isn't enough force being generated to twist the arms. Except of course when you hit a big bump or pothole. A perfect swing arm will just absorb the shock and the only sideways "slip" will be fur to lack of traction as the rear wheel is unloaded.

Some will see this as "technobabble" but it's what happens and as long as your arms have been made strong enough it will improve the suspension at higher speeds.

The Sportster (K Model) frame was accepted practice in 1950. But shortly thereafter the McCandless Brothers invented the Featerbed frame making the Harley frame obsolete.
To be fair it wasn't until the 1980's that other manufacturers caught on and we got Deltaboxes and Trellis frames.