Frame Brace Thoughts

[ME 109]

I think the best way to brace the swing arm pivot points against forwards or backwards movement is similar to the hpn type plate, but much bigger. Big to the point where it would need to be removed, at least on the left side, to remove the gearbox. That would require threaded bosses to be welded to the frame to accept a precision fit plate.
There's enough room to do that sort of brace. Good for a racetrack perhaps, but a lot of mucking around otherwise.

[Nourish]

Which is taking us back to the feathebed frame again.

http://www.andover-norton.co.uk/images/ ... 20text.jpg

[ME 109]

Which is taking us back to the feathebed frame again.

http://www.andover-norton.co.uk/images/ ... 20text.jpg

Quite so, it would appear.
I don't know much about Norton frames. Blast, they beat me to it.

[Nourish]

I'm sure you know all this but I've read that BMW styled their frame on the Norton Featherbed frame but decided on a single top tube and so the way the tubes meet the headstock differs.
The rear engine plates connect to the frame at the top and bottom of the gusset plates via two cross tubes (the top one being removable) and to the engine and then the engine and then onto front down tubes. The head stock is then supported via a plate to the cylinder head all to make a very succesfull handling package.

http://www.classicsolutionsengineering. ... 095-01.jpg

Of coarse BMW just attached their engine/gearbox to just the bottom rails to make a less succesfull handling package!

[ME 109]

I'm sure you know all this

Who me? Far from it. I wouldn't know how to describe trail and rake.
The featherbed frame in your link shows superior lower head stock bracing for sure. The less supported bottom of the airhead head stock is where our fork flex extends to.
Once again, only my theories.

[Kurt in S.A.]

I've been thinking a little about this and was trying to visualize the loads that are creating the reason one would want to stiffen the frame. The way I see it, you have this large more-or-less rectangle that is the main frame loop. Due to weaknesses in the corners, the frame may have a tendency to became less rectangular in shape. If I'm looking at the right side of the bike, the upper left part of the frame moves towards the lower right. The opposite direction also responds by the upper right corner and the lower left corner move away from each other.

I see two main load inputs to the frame. Through the steering stem which based upon loads to the front axle, that's going to create a moment around the steering stem, which to me, would more of a local problem and of course change in steering geometry...not good. Local gussets already welded in are designed to counter this. But any dynamic motions of the front axle will result in the second load input, that of the inertia of the engine. Under situations where large upward force on the front axle or say bottoming out while deep in a turn, gravity will try and pull the engine out of the bottom of the frame. Seems like this load input would go a long way to changing the rectangular shape of the frame loop.

If that's what's happening, the long cross brace from lower left to upper right seem like it does the most bang for the buck. Getting that positioned and sized right, it becomes a virtual rigid link. Not sure which way would be best for this single brace, but it may be difficult to go from upper left to lower right given where the cylinders are.

But to add stiffening in other areas other than this cross pattern seems like they will do very little for a lot of work or it has to be so beefy as to add weight and become cumbersome to work around. That single cross brace is the most efficient method for stiffening the frame.

And if my premise is right in the primary way the frame distorts, then the brace from the lower left to the airbox area is primarily going to be in tension, trying to pull that bolt out of the engine case. It doesn't seem like that's the beefiest of locations for this.

Kurt in S.A.

[steve]

I think I understand what Kurt is saying about the distortions that may be occurring in the mainframe. He is suggesting there may be a sort of "flattening" of the rectangle under certain loads, in which the upper left corner (as viewed from the right side of the bike) is forced closer to the lower right corner.

It makes sense to consider the possibility of this type of distortion, because the big open rectangle of the main frame looks like it may be weakest in that direction. I hadn't really considered that "rubber cow" may be the result of that kind of distortion, because the descriptions I have heard mostly sounded like a sort of snakelike lateral (right-left-right-left) wobble during high speed turns. The distortion Kurt describes sounds like it would result in a sort of slight up-and-down "bucking bronco" effect, as the frame flattens and recovers, and flattens and recovers, etc.

I heard one airhead mechanic say the flex comes from the lack of adequate bracing above the swingarm pivot. He thinks the swingarm twists a little, due to the fact that one side of the swingarm is stronger than the other, and this applies a slight twisting force on the swingarm pivot. There is a cross brace above the swingarm pivot, but it's not strong enough to prevent the swingarm's twisting force from transferring up into the subframe, and the rear part of the upper mainframe. This is the weakest area of the frame, he said, since it was left open and unbraced to make room for the tool tray, airbox, and battery, and because the subframe is bolted on, and not welded on. If his theory is true, I suppose it would account for a kind of weave or wobble in the midsection of the frame, (which might develop into a self-perpetuating repeating wave, due to twisting inputs from the swingarm and forks), resulting in a rubber cow feeling.

BTW can someone tell me exactly what "rubber cow" feels like? I mean, exactly what (unwelcome) chassis movement do airhead riders experience when they push the limits of their bike's handling? (I am new to airheads, and haven't even finished putting my bike together, so I have no personal experience with the ride and handling yet.)

I imagine it would be difficult to get many riders to agree on just what rubber cow feels like: the matter is complicated by the fact that some of our bikes are suffering from various maintenance shortcomings, ill-advised modifications, bodged repairs, old tires, loose fasteners, and so forth.

[SteveD]

I had read many years ago about the "rubber cow" or gummikuh effect. It was described as the BMW effect felt under acceleration where the rear is felt to rise instead of lowering. It describes how a cow stands from lying, where it stands by using the rear end first.

Mostly though, the term seems to be used to describe other things relating to handling, weave or frame flex?

A very distant acquaintance had a ride of my R100RS way back in the 1980's, whilst I had a go on his GSX. His comment immediately after getting off the bike was how disconcerting it was to feel the seat rise when accelerating. He had never ridden an airhead and was considering buying one. He didn't after that ride! Too weird he said. The GSX was very well known for a monster motor in a too weak frame causing instability.

[Toga]

I had read many years ago about the "rubber cow" or gummikuh effect. It was described as the BMW effect felt under acceleration where the rear is felt to rise instead of lowering. It describes how a cow stands from lying, where it stands by using the rear end first.

Mostly though, the term seems to be used to describe other things relating to handling, weave or frame flex?

A very distant acquaintance had a ride of my R100RS way back in the 1980's, whilst I had a go on his GSX. His comment immediately after getting off the bike was how disconcerting it was to feel the seat rise when accelerating. He had never ridden an airhead and was considering buying one. He didn't after that ride! Too weird he said. The GSX was very well known for a monster motor in a too weak frame causing instability.

A similar story from the same period, my mate had graduated from a CX500 to a Ducati SD900 so he was familiar with the seat rise feeling but when we swapped a couple years later, me onto his Duke & him onto Betsy my '82RS, his comments were a bit more condemning in pointing out frame flex, compared to the SD900. He hated the RS from then after.

Ive been off the road for a couple years and never really was a crazy racer type, but ive pushed Betsy hard on many occasion... and so my motor skills have a feedback memory along these lines ;

Wobbles & Weave- usually a tyre, fork alignment or s/h bearing issue where the front leads the movement and the rear follows / attempts to. Generally along the horizontal plane L/R/L/R

Frame Flex in general- Medium/high G-force corners gave the feeling of snaking in both, horizontal & vertical planes. Again with the rear following the front. I recall being much more comfortable with this feeling than that of wobbles, weave & tank slappers... it always felt 'controllable'.

Frame Flex at Max - High G-force corners / heavy load would give a similar flex feeling as per above but the rear would kick out at times, as if it were releasing built up / compressed energy from the swing arm pivot / sub frame / rear shocks region. This would cause the front to want to follow the movement of the rear. Completely different shit your self feeling, IMO.

All the different types of 'movements' give a totally different kind of heart beat in the mouth type feeling.

I might be well enough next week to continue some resto work on Betsy, I think ill give extra frame bracing a miss.

[Nourish]

Ok, I started to make up the bracing but couldn't get the tube to miss the air box outlets and the battery tray - so a no go!
Back then to where i started - how about adding another brace to the bent tube?

I think the gusset plate needs to be bigger.
The only trouble is that the bent tube has to be moved inwards to clear the petrol taps - is this a weekness?