I’m doing a bit of R&D for a possible eCruiser project. I’m an old man now so sportbikes aren’t my goal and dirt bikes will have to wait until I move from the bay area.
The last bike I was riding, a 2014 Honda CTX700, was an awesome step to the old man world of motorcycles. A relaxed upright but low position, swept back bars and small forward floorboards. It was so comfortable. More, the DCT and ABS were a joy. Sadly, that got totaled in a crash at the Golden Gate Bridge toll plaza when I was cut off by a crazy driver a year and a half ago. I’ve been off the motorcycle since and really need to get back for my sanity. Buying a beater Shadow makes sense. Making something cool is more exciting.
This is helped in that I have a K8 GSX-R1000 front and rear end that I can use. This is leftover from a 20 year old zombie project. I had to dig around in the garage but I managed to find all the parts. The parts are so nice there’s little reason to use anything else. I just need to optimize them for this use.
Frivolously, I also obtained a L/M rear end to play with as the linkages are arraigned differently allowing different solutions. I’m investing more into getting this right than I was initially expecting.
This post focuses on the rear end kinematics.
I modeled the parts of the system in CAD. Nothing too elaborate in detail but precise in measures. This would allow me to figure out the best locations for the chassis trailing arm ground node, tension link ground node, and upper damper ground node. I could go down two paths, a motion study or a calculator. I prefer calculators.
The math is similar to what I did for the Transition Smuggler in 2023. The shame about this method is that it goes from the wheel to the shock and I’d prefer the other way. That is hard. Let me know if you have a way.
Getting some valid parameters starts with the CAD sketch for extended and compressed situations. This proves that the locations I’ve chose work geometrically for the defined parameters. I’m moving some things around for space and decreasing travel from 130mm to 125mm, so optimal fit is hard. I also don’t have a lot of influence on most of the system. I would prefer more travel but would need a longer shock, but I’ll save that for revision 2.
The difference in the K and the L/M rear ends is primarily in the linkage arraignment. The K is far more compact and allows for more geometry range. It’s just a pain to make the parts when I do custom stuff. The L/M is a ton easier to make parts for but it takes up more space and has some limitations. Using the L/M will force a bent tension link.
From here, I can enter them into the calculator. These are the driving parameters and a few fixed calculations. It’s a challenge to figure out the correct naming conventions to use for this work. I want to find the right way to discuss this so that folks can understand precisely what is being said. If you see a name that should change, let me know.
So many steps to get to the end. This takes focus and time to figure out. I’m still cleaning all of this up. I wish I was better with math and serious engineering kinematics.
I can now get some visual understanding of the behaviors. The ‘curves’ as they say.
The wheel path. It climbs up and back then forward. This is obvious since the trailing arm node is only just above the wheel axle at initial. This isn’t desirable, it would be better if the wheel never had to move forward but getting a low saddle is forcing some packaging decisions.
A simple plot of the shock to direct travel of the system. It’s always funny how straight that this looks compared to the leverage ratio but that’s to be expected. The leverage ratio curve plot is designed to make very small changes look big.
The leverage ratio. I want some progressivity in the action but not crazy. The bike should feel cozy and smooth with some margin of safety for bottoming when I’m on dirt roads and rallying hard. 2.1 to 1.9. Not linear but not super progressive. There is a bit of brute force and reality of special relationships so it takes a while to get things working well.
Because there is little discussion about the behaviors that are changed by any one change in this system. I do have to guess and hope. Anyone with a better understanding of this stuff should reach out to me. I have a lot to learn.
Next task is to calculate the countershaft sprocket location with the anti-squat values that I anticipate. I probably could have gotten this done for this post but I was a little burnt on math for right now.
