Chassis Evolution

[Ji Gantor]

This is a typical late 60s to mid 70s frame geomtery.
This is not meant to cover all frames of that period it is just so we are talking about the same thing.

Ji

[Ji Gantor]

This this a typical load path that the frame is designed to.


Ji

[Ji Gantor]

The attached drawing is of a modern aluminium frame.
Notice the differences and the similiarities.

Ji

[Ji Gantor]

This is a typical load path that a modern frame is designed to.

Ji

[JohnnyO]

I think good suspension and handling is the most beneficial for motocross but probably not the cheapest. I'd like to ride a bike 10 kilos lighter, i'm sure that would bring the lap times down as well.

[NSR]

I would go for usable power and good suspension first then weight.  A friend of mine works on the MXGP circuit, He used to have an XR600 with good suspension which He would sometimes have in the truck when he went testing.  I can’t remember who the rider was, but they were testing on a 250 and 500 2T’s after they had finished he pulled out the 600 and did a few laps, the rider then jumped on it so my mate timed the laps.  When he come in my mate thought the 600 was going to melt and the rider said he hated it, they then looked at the times, He was fastest on the 500 then 1sec back was the 250 and 600 was only 1sec behind that. 
Noel     

[Ji Gantor]

The following frame has three truss sections.
The first is the steering head/spine,
The second is the air filter section and
the third incorporates the air filter section and the rear shock.
The members in a truss are subjected to tension and compression and not bending moment.
Small diameter steel tube is excellent at carrying tension and compression loads but not so good at resisting bending moment.
That is why the original tube swing arms had the rear shocks fixed very close to the rear axle. The load from the shocks transferred just about directly into the axle. When the shock was moved away from the rear axle the swingarm members were subjected to bending moment and the tube alone was not adequate. To reinforce the load carrying capacity of the tube swingarm a gusset plate was welded to the top of the tube stiffening the swingarm from pivot to axle.

Ji

[Ji Gantor]

I am not sure this is correct but from very limited research I believe that the 1997 Honda CR250 R was the first motocross bike to have an all aluminium frame.
I will keep checking to see if I can find out the type of aluminium used and how it was welded.
If any one knows this info please post it.

Ji

[Ji Gantor]

If the Mid 1960s to mid 1970s chassis is our starting point we can see the evolution and what influences made the changes.
As we have seen the swingarm was horizontal and the rear shock was almost vertical and fixed to the swingarm near the rear wheel axle. This produced only about 4 inches of rear wheel travel. Riders were getting faster and the tracks were getting more technical so the next round of requirements were more wheel travel, better cornering and less weight. So the bottom shock fixture was moved up the swingarm towards the pivot point or the top shock fixture was moved down the frame to increase the shock angle. Both of these shock orientations positioned the swingarm with a negative angle and produced more rear wheel travel. It also raised the back end of the bike up placing more rider/bike weight onto the front forks. This added advantage gave more ground clearance and made the bike corner better. The first thing the top riding coaches in the States do is drop the forks down in the triple clamps up to 1 inch and then set the ride height. This way the bike will corner like it should.


Ji

[LWC82PE]

I am not sure this is correct but from very limited research I believe that the 1997 Honda CR250 R was the first motocross bike to have an all aluminium frame.

what about the Hagons as featured in VMX mag, do they count?

[Ji Gantor]

Hi LWC3077,
I don't have that issue of VMX mag.
This is great, what year was it made and did it have a full aluminum frame including swingarm?

Ji

[Nathan S]

The Hagon is a monocoque. Not really a 'production' bike though.

Magoo thinks its ugly, but he's wrong.

[Phil]

Beauty's in the eye of the beholder. The Hagon's owned by Klub Kevlars own Alan Jones and even he thinks it's been hit by the ugly stick. I myself think it's cool but we all reckon there are a lot of things on the bike that could have done better with a bit more thought. For instance the integrated welded in tank isn't the smartest idea, especially seeing that the tank now needs some repairs. The main reason Alan bought the bike is to complete a personal trifecta. He's now got speedway, long track/short circuit and now motocross monocoque Hagons. A side bonus of course is that he's now got a good pre '78 or Evo racer to add to the pre 65 and pre 70 stuff he's more noted for. The bike will be getting a superficial rebuild in the near future to rid the bike of the previous owners bodges and rough touches and to refit the OEM 38mm Maico forks.

[Freakshow]

As much as i'd like to join this topic, i got scared off by page 2, just maybe ross was onto somthing here  :O)

[Phil]

Here's another monocoque. The frame was made by CRDC in San Francisco around 1972.