Suspension Geometry Question - Position of Dogbones/Linkage

Been thinking about this. Referenced to this pic below:



Current position of the dogbones (blue) is the jist of how an Aprilia RS250 is setup. If you were to move the mounting point on the swingarm in the exact arc of its natural travel, so that it mounts below the swingarm instead of thru it, but the dogbones were the exact same distance from the linkage as they are currently, the linkage is in the exact same position unloaded, no lengths change on anything...

Would the suspension act the exact same way? Leverage ratios wouldn't change right? As long as the dogbones don't bind or hit any part of the swingarm from that position? Is there any changes in actual bike behavior from doing this?

The swingarm is fixed mounted to the frame. The shock is fixed mounted to the frame. The linkage at the front is fixed mounted to the frame... so really the only changes possible by moving or changing dogbones, would be affecting the angle of the shock bottom/linkage, or position of the linkage... so if I also want to keep THAT consistent with its current state, I feel like nothing should change at all? But i'm not sure. i think i'm over thinking this. Or does it now not work because of the way the linkage pivots and moves as the shock is compressed and the swingarm moves?

And to answer before it comes out... doing this would help with overall packaging. That's the only reason.

 

I think that you need to account for the location of the swingarm pivot location related to the dogbones mount point. I'm no geometrist (I think that I made that up), but I would assume that you need to assess the rotation point of the assembly to determine reactions.

 

Nope aint gonna work. Thats going to reduce or lock up the rear suspension travel if the swingarm pivot is where Im guessing. If you made a new linkage to account for the geometry change there may be a chance.

 

No. The dogbone pivot at the swingarm end moves in an arc defined by the swingarm pivot. Reorienting the dogbone about the pivot on the rocker does not maintain the same relationships. Also as drawn the dogbone(s) could go overcenter at some point in the bump travel. Well it woudn't go over center, the suspension would just lock at the point where the rocker pivot is in line with the dogbone.

 

yea thanks, i knew something felt off. but then next question... if i wanted to still get them down into that position, could i find a mounting point that stayed in relation to the swingarm pivot in the same arc? or is it some kind of compounded math equation based on the movement of the linkage AND the pivot of the swingarm? would it simply just be that any point along this circumference would result in the same leverage ratio as long as nothing is binding or hitting? or not that easy and any change there would ultimately result in a complete redesign of linkage shape, shock length, spring rate, dogbone length, and mount points?




as an alternative... also just considering CNC'ing some bones in an arc themselves, like so:



something like this kinda stuff:



we'll see when the swingarm comes back to me from Harris. but trying to think of ideas as a possibility of how to proceed.

 

Tony Foale made some software that allows you to play with this. Additionally his book is an excellent read into the mechanics of suspension. If you are serious it is worth the money. If you are going to machine parts, definitely worth it.

And to answer the original question, YES it is a major change. You are playing with the leverage ratio based on the center of rotation of multiple points. Think lots of centers and lots of circles that are connected. You mentioned length but did you think about oblique angles? Cardboard cutouts and pushpins worked for us back in the 80's and even into the 90's because it was cheaper and easier than an engineer or a computer.

Have fun, things like this got me into the industry in the first place. Britten rules FYI!

 

I think theres only one other point on that circle that gives the same suspension action. rotate the dogbone about the linkage pivot and fix it to the circle on the other side. I think that would give the same result. the dogbone would have its forces flipped, tension instead of compression and vice versa. of course, its not feasible to mount it there.

 

i will look into that book. also i just had an idea for a bracket that may make any of this unnecessary haha. ain't that always the way? i AM interested in this kind of stuff just for the sake of learning. i'm pretty good on compression/rebound/preload/rake/trail/offset and how changing those affects the bike and its handling characteristics, but actual design of the rear suspension itself, which design you would choose to use, how that all plays together i am NOT up on. its definitely fascinating though.

 

You could mount dog bone where you want but you would have to make new link.
What design you use makes no difference, you can have million links moving around but only thing that matters is how much shock moves vs rear axle through travel range.

 

@SPATT has been looking into this stuff while trying to come up with a linear link for the new SV. For our application, we are restrained by the rules package which mandates us keeping the stock mounting points on the frame and swinger.

 

I started with modeling it all in 2D sketches in solidworks and putting restraints to build models in excel to see what it does to motion ratios and leverage on the swingarm. It's fun stuff similar to visiting the dentist. Motion ratio isn't the only thing to be concerned with but also wheel force. Its all about compromise and working in the envelope available.

The math you are questioning is called Kinematics.