Weaving

Fullauto said:

cjandme said:

kerinorton said:

... Now I tend to replace the front tyre more frequently and shove it on the rear.......Dereck

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To me, this is another good reason to keep the 19" wheels on both ends.

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Absolutely!

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sorry,... I don't see your logic here. what does rim size have to do with anything? Just the rim size alone doesn't take into account the aspect ratio of the tires, nor the engineering of the tire's shape... You can buy a tire to fit your rear 18" rim that is the same diameter as a tire that is on your front 19" rim... How is matching up the tire sizes some magical cure for weave??? I would bet that many of those bikes in that old video had the same diameter rims front and back and they wobbled like crazy....

There are certainly aspects of weave that can be traced to their cause, like worn steering head bearings, worn swingarm bushings, swingarm pivot play, loose isolastics, tire tread design, grooved pavement, etc, but honestly many people have said the "same tire size" comment, yet there are many bikes that handle great and don't have this particular specification. How do you explain that if matching tire size is so critical??

I'm not interested in an answer that says, "because that's what I've found in all my years of riding", I interested in WHY you think this from an analytical engineering perspective... thanks

I changed my bike from both 19" wheels to 19" front and 18" back. Each tire's different aspect ratio makes them pretty close to the same diameter, but not exactly like they were previously. My bike handles way better in regard to weave... which I have been struggling to cure for a few decades... which has taken form as upgrading isolastics, renewing swingarm bushes, kegler modification, change of tire brand, replacing wheels completely, one piece rear axle... etc. I still feel it could be better because I compare it to my honda 750F which I burried the speedo needle @135mph with a person on the back and it didn't weave even a hair's worth...

.......all those engine mods and you insist on riding with district nurse handlebars on it!

I think the original logic for 19" rims at both ends had nothing to do with putting the rear tire on the front or vice versa. I think it had to do with (lack of) tire carcass stability and integrity, where only 19"ers could be relied on not to disintegrate at +100mph speeds.


19" wheels on AMA flat track bikes may have come from the idea of swapping back and forth to keep racing less expensive.

If the rear isolastics have gone soft, they could be the problem. In the early testing we'd get weaving quite often. It wasn't until one of the mechanics took the rear isolastics assembly out that we identified the problem. On the prototypes, the spacer tube between the two isos was a simple steel tube. We kept tightening the nuts on the bar holding the isos in the frame, not realising that the cross-tube was getting "bell-mouth" shape at the ends. A revised design put end-plates on the tube to stop the distortion and the weaving went away, as did the need to keep tightening the iso installation.

If most "simple" fixes don't solve the issue for you, I'd suggest taking the aft iso assembly out to see if there's any problem with it. I don't think any production bikes went out with the original plain tube.

Just a comment on the 19" for swapping tars. I have always done this but I have noticed that the front can get some areas of more wear
due I think to hard braking which affects front more than rear unless you are given to burnout starting.
And recently have begun to dislike the chore of changing the front as well as the back. I may not mind so much now that I have gotten
the Madass shock hub which truly is quick disconnect.

"Even the very stable Laverda could be made to weave a bit"... :mrgreen:

I own one, and a few years ago I had to change the rear shocks, same type, same settings...but the first time I hit 100mph with them, the bike sarted to weave quite a bit which it had never done before. I couldn't understand why until I realized the new shocks were about 1,5cm shorter than the old ones, I stopped on the motorway and simply set them one or two clicks stiffer...and the weave was gone forever. Since then, on all my bikes, I tend to run rear suspension stiffer than I used to and it does make a difference. I also have clipons on all my bikes except the narrow flat bars on my BMW 75/5. Incidentally, this is the only bike of mine I ever experienced tank slap with...but at slow speed on very bad Belgian cobblestone roads...

Due to a normally worn front TT100 tyre, my bike started a steering head wobble when I was doing 30 mph through Blenheim. It was fine at faster speeds and never gave anymore problems. I fitted a new tyre to the front when i got home 1500 miles later and haven't had the problem since. That tyre then got chucked on the rear to be used up.
Dereck

00norton00 . the good thing about having the same tyre front and rear is you can do what we did. you can fit a tyre to the rear which has been misbehaving on the front and still get good wear out of it. You need to when you live miles from anywhere.

I had weave issues running a 3.60 TT100 on mine. The bike had just been rebuilt from the ground up.
I replaced it with a 4.10 and the problem disappeared.

kerinorton said:

Due to a normally worn front TT100 tyre, my bike started a steering head wobble when I was doing 30 mph through Blenheim. It was fine at faster speeds and never gave anymore problems. I fitted a new tyre to the front when i got home 1500 miles later and haven't had the problem since. That tyre then got chucked on the rear to be used up.
Dereck

00norton00 . the good thing about having the same tyre front and rear is you can do what we did. you can fit a tyre to the rear which has been misbehaving on the front and still get good wear out of it. You need to when you live miles from anywhere.

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well....... yes..... but don't you know that solo motocycles counter steer :?:

You could try standing slightly on the footrests at that particular spot to lower the C of g, but if you have cowhorn bars on how do you hang on at 110 :?:

Bernhard said:

You could try standing slightly on the footrests at that particular spot to lower the C of g, but if you have cowhorn bars on how do you hang on at 110 :?:

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Not only 'cowhorn bars' but also a stock seat... so 'hanging on' is the right description... and possibly the root cause of the problem...!

Dommie Nator said:

.......all those engine mods and you insist on riding with district nurse handlebars on it!

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I've got it so I can change from clip ons n rear sets, to stock, quite quickly.

Guess I'd better get the clip ons back on it !!

Have you read the article 'The World's Straightest Commando' (which is on Access Norton)? When you've done so - and probably been alarmed by it - you might like to contact Simon Ratcliff of PARALLEL ENGINEERING in Wolverhampton, who will put you straight, in more ways than one. Money well spent in my opinion.

frankdamp said:

If the rear isolastics have gone soft, they could be the problem. In the early testing we'd get weaving quite often. It wasn't until one of the mechanics took the rear isolastics assembly out that we identified the problem. On the prototypes, the spacer tube between the two isos was a simple steel tube. We kept tightening the nuts on the bar holding the isos in the frame, not realising that the cross-tube was getting "bell-mouth" shape at the ends. A revised design put end-plates on the tube to stop the distortion and the weaving went away, as did the need to keep tightening the iso installation.

If most "simple" fixes don't solve the issue for you, I'd suggest taking the aft iso assembly out to see if there's any problem with it. I don't think any production bikes went out with the original plain tube.

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I've always been able to tame the weave by taking weight OFF the front by moving my ass back on the seat.

That said, and seeming to confirm what Frank Damp says above - I went for a ride Sunday after doing some work on the bike - new tires and I welded up some new brackets to help the exhaust roses. The brackets tie the head pipes to the engine at the lower front isolastic's steel bracket. As part of that fabrication project I loosened the front main isolastic bolt but then . . . . . after I got my new brackets all aligned . . . . I forgot to tighten that isolastic bolt back up.

Well, the bike weaved at high speed and I thought to myself "must be the new Avon AM 18s, but then, I argued with myself - AM 18s never caused weaving before?" Further, the bike shook too; and I thought "maybe my new brackets . . . .?"

Anyway, when I got home, I noticed that the nylock nut on the main isolastic bolt was about to fall off.

Re "worlds straightest Commando" with hindsight, I should have done this when I rebuilt the bike. But stripping it again to do all of this is not my preference at this stage!

Re isolastic adjustments: this is on the list to check. They were set tight, and the bike has not done that many miles since the rebuild. The frame was powder coated though...

Pull the rear wheel side to side, bike on centre stand. Does it move, if so is it the isolastics or swinging arm. If it pivots at the swinging arm it needs fixing.

Steves said:

Pull the rear wheel side to side, bike on centre stand. Does it move, if so is it the isolastics or swinging arm. If it pivots at the swinging arm it needs fixing.

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I'll do that Steve and carefully check the clearances on the ISOs. I've forgot what I set mine to, but it was about half of the stock setting IF I recall correctly, it lets a bit more vibes through, but not excessive, and tightens the handling.

I think my swinging arm pivot is OK, it's only done a couple thousand since being renewed by Norman White, inc having the cotter pin mod.

I'll report back findings to all in a couple of weeks when I get chance to check properly.

The cotter pins should keep the swing arm behaving itself Fast Eddie. I installed a set of Kegler clamps from Holland Norton which sorted my weave out, (even with TT100's ). Handling not as solid as my old Ducati 900SS, but the Norton is an addictive pleasure, don't need an excuse to ride it.

If your feeling what I suspect your feeling, the only way to fix it is going back to a prone riding position.

Ever followed a big truck down the highway and noticed what the air does.

It does the same thing behind your upright body at high speed. Jim

Here's a bit of a weird one.
Last trip home from Twisp, WA, on the Commando my legs got a bit cramped. Moved my feet back onto the passenger pegs at about 75 MPH and a slow gentle weave set in. Moved back to the rider pegs and it was back to arrow straight down the road. I had maybe 45 lbs in the throw over bags on this trip.

I tried this a few times, same result every time.
Never have had any weave from the bike at any speed or any handling issues either solo or two up except this solo weave when the feet are shifted back, which also tips the upper body forward a little.

The end result is- I don't ride with my feet on the passenger pegs even if cramped up a little on the rider pegs. Better to stop and go for a walk.

Glen