Radian Arm, rear shock question

baniels · Posted: October 29, 2008, 11:23 am Post subject: Radian Arm, rear shock question

Hello folks. I wanted to run this plan by you guys to see if there was any input.

I have a '79 650 SF. It has the stock 19 and 16 inch front and rear mags. I have some 19" and 18" spoke wheels coming from ebay.

I'm taking my inspiration from the bike in two of the attached pictures. Steven Snoep, the guy who did that bike's modifications, has been very helpful so far in explaining what he did. Essentially it is a '79, like mine, with a Radian arm and 12.8" Koni (now Ikon) rear shocks.

He has an extra set of the Ikons he will sell me for a fair price. He also thinks he has an extra set that are 1 inch longer.

I've read mixed reviews of the Radian arm and it's claimed "bolt-on" application. I know that I will at least have to make an axle adjustment and get a longer chain. I'm prepared to do that.

y question is about which shocks to choose. I've read that the shock mounts on the Radian arm are farther back than the stock, in essence lowering the rear if the same shocks were used. I'd rather raise the rear if anything.

I've read that the stock shocks are 12.75" (on an 81, anyhow - not sure for a 79) - if that is the case, and if it is true that the mounting of the shocks on the Radian are farther back than stock, then 12.8" shocks might net a lower rear end height than the stock shocks.

If it isn't clear - I'm not sure what to do. My goal is to have a better handling bike. Raised rear end preferable, if it resulted in better handling. That said, I don't want to throw off the geometry and end up with worse handling. Longer shock? Longer front forks? How does the larger rear wheel factor in?

Am I better off ditching the Radian arm idea and finding one from a TX750?

I've also attached a picture of my bike for reference.

Any help is appreciated.

xsleo

i'm not sure on the swing arm. as far as the rear wheels go, the hieght of the tire is about the same the 18 inch tire is a 110-90 is about 25 3/4 inches high. the 130-90-16 is about 26 1/4 inches high. the about 1/2 inch difference won't change the ride height enough to worry about. if you in crease the rear height, it decreases the rake and trail making the steering guicker. lowering the rear increases the rake and trail making the steering slower. most sport bikes have a steeper head angle to give them quicker steering. crusiers have a kicked out head angle to give them a more relaxed steering. the steeper head angles tend to be quicker but have more head shake and less straight line stabilty. thats why the have steering stabilizers. a shallower angle isn't as quick but less head shake and more straight line stabilty.

yamaman

Radian s/arm is longer than stock (so is TX 750); longer wheel base = slower handling though more stability.
As Leo said, sure you can lift the rear, but to get the same or even more effect, you could run your forks up through the triple trees. Then its adjustable, you can run them up or down till your happy with it and it will cost you $0. Dont run them through more than 1", take it in 1/4" or so increments. You might find you need a steering damper, depending on all the variables, when you find YOUR setting!
Put new bushes in your existing s/arm, new tapered roller bearings in the head stem, grab the koni's and you'll have a great hanling bike!

baniels

Thanks for the advice. I think it was 5twins who did the TX750 arm. I'd like to hear more about his experience with it (or anyone else who has used it)... Maybe he wil stumble upon this thread.

Ended up not increasing my bid on the Radian arm and was luckily outbid. Not sure that's the way I want to go anymore.

5twins · Full Member Joined: Aug 05, 2006 Posts: 1616

The TX750 arm is probably the simplest swingarm swap out there. It's identical to the 650 arm except for it's larger tube diameters and chain guard mount. It uses the same axle, bushings, pivot bolt, etc. as the 650 arm so you can source any of that stuff from MikesXS if you need it. The chain guard mounts like the early 650 ones with a pin and grommet at the front. I didn't try a 650 one, just using the 750 guard I had instead.

I think the 750 arm is a major improvement, or at least it was on my bike. It feels more stable at ALL speeds, not just going fast. I still even have the original plastic bushings in it because they weren't worn at all. The 750 arm replaced a stocker with brand new bronze bushings and even with it's original plastic bushings, it's better than that one was.

There's no difference in length or any of the other dimensions compared to the 650 arm. Shock mounts are in the same place as well. The only difference is the tube diameter (bigger by about 1/4") and that really helps stiffen it up. Here's the 2 side by side. As you can see, other than the tube diameters on the arms, they're identical.

baniels

Mr 5twins. Thank you for the details I was hoping you would provide.

Based on your other mention of the arm, I was surprised when I read it was longer. Glad to know it isn't. Seems to me its worth a try.

Thanks again.

Ben

dwyatt

I second 5twins suggestion. I put on a TX 750 arm and it was very easy and quick - 2 beers and you're done. Much stronger and only people that really know the XS will notice that you made the swap.

yamaman

Apologies for the bum steer baniels Embarassed

According to MMM from 650central, the best frame mod you can make (and it improved my bikes hard cornering) is to brace the swing arm mounting point on the frame like this:

yamaman

Heres a better pic:

650rider.com/index.php...1354&pos=3

baniels

No sweat. It lead to more information coming to the thread, so not really a bum steer. More of a countersteer Wink

yamaman wrote:

Apologies for the bum steer baniels Embarassed

According to MMM from 650central, the best frame mod you can make (and it improved my bikes hard cornering) is to brace the swing arm mounting point on the frame like this:

5twins · Full Member Joined: Aug 05, 2006 Posts: 1616

Yamaman, I have a question about your brace. This is a mod I'd like to try some time in the future. When you bolt the swingarm in an unbraced frame, the mounting points are a bit spread apart and pinch in tight when you torque the pivot bolt. This makes fitting and removing the arm pretty easy. My question is, when you welded the brace in, did you have a swingarm in place and torqued to spec? If so, do the mounts still spread slightly afterwards when you loosen the pivot bolt so the arm can be removed/installed easily? Or should the brace be welded in with the swingarm removed?

yamaman

thats a good question there 5twins, one I've been racking my brains to answer as I did it a fair while ago. I think I remember tacking in place with the swing arm secured in place, then removing to complete the welding. I then changed to a needle roller set up, new pivot bolt ect before final assembly. Then I freaked out trying to get it all back together thinking I'd stuffed up as it was a bit tight. But a bit of prodding and poking saw it home and when it was tensioned with the shocks still removed it swung just like it should. So it probably would be better to have it tensioned in the manner you wish the finished product to be, ie if your going to replace the bushes, do that first.

I guess you've seen the details on MMM's site, 5/8th tube relieved for a snug fit attached near the apex and welded on the frames spine also.
Do your mesurements with the bike all together to ensure chain clearance ect

dwyatt

I braced with the original swingarm still on, then replaced with a TX750 arm. It was a snug fit going on, but went OK. I didn't even think about the frame flexing enough for this to be an issue, glad I had not removed the original SA!

baniels

So, braced original versus unbraced TX750 arm. Which do you like the best?

dwyatt wrote:

I braced with the original swingarm still on, then replaced with a TX750 arm. It was a snug fit going on, but went OK. I didn't even think about the frame flexing enough for this to be an issue, glad I had not removed the original SA!

baniels

I was bidding on a TX750 arm last night. But it started to get out of controlso I stopped bidding. Ended at just under 125.

That seems a bit high.

yamaman

[quote="baniels"]So, braced original versus unbraced TX750 arm. Which do you like the best?

I think he means he braced the frame, not the swing arm, he now has the best of both worlds, braced the weakest part of a XS frame & a beefier TX swing arm

5twins · Full Member Joined: Aug 05, 2006 Posts: 1616

Yamaman, I couldn't find the brace details on 3M's site. Where are they located?

yamaman

appears not to be there anymore, I hadn't been on his site for a while, it was different then!
It didnt have any dimensions, just a pic of a length of 5/8th tube, around 1/3 scolloped off at the ends, with a similar amount taken from the middle to go around the spine of the frame. I just dressed it with a grinder until it fit.
I'm at work at present, I will measure and post more details for you tommorow.

5twins · Full Member Joined: Aug 05, 2006 Posts: 1616

I might have that picture saved from a while back. I saved the bottom "cradle" part of the frame from my recently acquired parts bike to use as a jig. I'd like to take a shot at bracing a stock swingarm and making one of these braces. Basis for the swingarm brace will be the original rear fender loop. With this frame jig, I can now fit things up and check for stuff like chain and brake torque arm clearances.

dwyatt

That's right, I braced the frame and ran the stock TX750 swingarm. I didn't compare braced 650 vs. plain 750 arms, just got a good deal on the 750 arm and thought it would be the cleanest change. I bet they are very similar as far as regidity.

baniels

Can somebody shed some light on this?

www.650motorcycles.com/MelLamb.html

He says he has an RD400 rear wheel using Avon Distanzia tires. All of my research has told me that the RD400's rims are 1.85x18 which, according to the guys at Avon, is way too narrow for a Distanzia.

They told me a 1.85 wheel would need 90/90 and the only Distanzia with 90/90 is a 21" diameter.

I am confused.

I'm just trying to figure out how the heck to run an 18" mag with a decent or stock width tire. Having trouble finding a way.

xsjohn

1.85 is even to narrow or a front as far as I am concerned........but we are stuck with that.......for a rear with any size at all on it you would wear the center off the tire before you got there......think the stock 16's are 2 1/2".......and I wish they were 3 and the front I wish was 2 1/4" at least instead of 1.85......what aboutthe sprocket on the RD400......willyou be stuck with a huge sprocket......

xsjohn

baniels

I'm not sure. I've seen at least a few people using this wheel, so I assume there is something workable. But I'm not sure about the sprocket.

sschering · Last edited by sschering on November 4, 2008, 8:17 pm; edited 1 time in total

I think my 77 XS-400 rims are wider than that.. I'll check when I get home.

Ohh DUH.. SR-500 ran a 2.15x18" rear rim with a 4.00x18 tire... I think my XS runs the same rim.

Good thread on Tire sizes here..
Says the 120-80-18 should fit on the 2.15" SR-500 rim

The Distanzia is available in 110-80-18 and 120-80-18

What Happened to 650rider.com?

Yamaha xs650