swingarms?

[fintip]

Could someone educate me on swingarm mechanics? What makes one swingarm better than another, from a performance point of view? Skymasteres mentioned a dramatic increase in rear wheel 'tracking' when he swapped his out to a thunderace unit.

Case in point, I put a Thunderace swingarm on my bike. I did it because I wanted to shorten up the wheelbase a little and stiffen things up in the rear. Well, since I'd already modified my stock swingarm to let me move the wheel ALL the way forward on the adjusters, I had already achieved the same effective wheelbase as the Ace arm. (Well, chain length at least. Since I raised the rear at the same time the wheelbase is actually shorter)

Just shortening the wheelbase with the stock arm had a slight impact on handling. Made the bike feel like it hunkered down a little more with heavy throttle application in corners. Other than that I can't really say if there was a direct impact.
The F4i shock made a HUGE difference in the handling on the bike. The rear was a lot softer, but it follows the road amazingly well. Even when riding as aggressively as I can the rear was tracking well. Part of this perceived improvement was due to the fact that I was replacing a stock shock with 120k miles on it.

The comment in the RPM shock thread about "The best suspension you know is the best you've ridden" was made abundantly clear to me when I put the Thunderace arm on the bike. I did not realize just how flexible the stock swingarm was until I got on the bike with the Ace arm installed. Holy crap that made a HUGE difference. That stock swingwarm is just wandering all around in comparison. The difference is so pronounced that when I had Lilly on the back her first reaction when we stopped was "What have you done to the bike?" I hadn't told her that I had put the Thunderace arm in and asked her what she meant. She said it felt more stable, like it was planted and not dancing around when I'm pushing it. I figure she feels it more than I do being further out on the subframe that I am. But the difference is definitely noticeable.

I can understand wheelbase making a difference, but he is saying it is something else. I had previously assumed it was mostly a cosmetic and weight difference. Is the material itself flexible, though, and adding rigidty accounting for that change? 

Thanks.

[1tinindian]

You said it, "rigidity"!

[Pat Conlon]

You said it, "rigidity"!

Yep, my wife agrees.....

[skymasteres]

Could someone educate me on swingarm mechanics? What makes one swingarm better than another, from a performance point of view? Skymasteres mentioned a dramatic increase in rear wheel 'tracking' when he swapped his out to a thunderace unit.

Fintip, what makes one swingarm better than another depends on what you are trying to change. My goal was to shorten up the wheelbase slight and get a more rigid arm on the bike. (It's a heavy bike and I'm over 200#'s and ride with a passenger often) If you are looking for a lighter swingarm that's stiffer than stock and slightly longer you should try the R-6 swap that's been documented here.

As for the why, it's about making the bike ride better. I put an arm in that maintains nearly identical suspension linkage geometry so the shock performance is predictable. For me the rigidity of the new arm was the main benefit over the stock arm. If you look that the box structure of the stock arm compared to the thunderace arm you can see just how much taller the box is. This is like having a taller web in an I-beam. If provides much greater resistance to bending loads than the stock arm.

The main point is, the stock arm is just fine if you're not riding the bike hard. If you've put a bigger 180 tire on the bike and start getting over to the edge on it, you might want to think about stiffing it up. You won't know how much the stock was wondering until it isn't anymore.

[andyb]

To be a good swingarm, it needs to be:

Straight.  They can twist, bend up/down, or sideways.  Having things not straight leads to some obvious problems.

Strong.  Prevents it from becoming un-straight.  Also prevents the shock knuckle from tearing out, things like that.  Bad welds can break under stress.

Light.  Ideally, anyhow.  Less unsprung weight is pretty much always a good thing.

Fit.  Needs to have a lack of slop at the mounting points so that it locates things fairly precisely.  Also when you start customizing, it's a lot easier when you don't have to move everything under the tail because it's longer/shorter than you really wanted.

Length.  Depends on what you want.  Longer means a bit less cornering clearance and a bigger chance to spin the tire, but less chance of a wheelie.  Shorter means it'll turn quicker, gain some clearance, and have more issue keeping the front end down, but wheelspin is less of an issue.  The other thing to think about is if you change the length of the lever, it effectively changes how the shock sees the world, and if you do real extreme things with the length, a respring/revalve/swap may be in order.

Then there's the usual stuff about cost, looking nice, having an internal air tank (if you're dragracing with an airshifter), and so on.

Pat:  They make pills that can help you out there.

[fintip]

Thanks. That explains it all.