Weaving

So, while I was out doing more 'arse dyno' testing yesterday, I used a regular 'circuit' of mine, it involves a long, fast, mild left hand bend, there are some undulations in the Tarmac but its not too bad.

Last time I 'tested' the bike here, it had clip ons and rear sets and a corbin seat which keeps you well planted on the bike and he bike itself was rock solid.

The bike currently has a dual seat, stock footrests and US bars. First thing to say here is that it takes quite a lot of 'hanging on' at speed compared to clip ons for sure. It makes for quite a fun and exciting ride to be honest, so I do like it like this.

But on the 'test circuit' the mild undulations that cause no issues with clip ons, set off quite a pronounced weave. It was not a proper 'tank slapper' thankfully, but it was enough to have me worried for a moment!

I was a little too focused on other things to study the speedo the time, but I was probably doing about 110mph, banked over mildly to the left, slightly up hill.

The bike has Lansdowne dampers, Falcon shocks, Roadrider tyres, cNw headsteady, isolastics set quite tight.

Any ideas? Is this to be expected with US bars etc? Or is there an issue here that needs further investigation?

On yes, just for clarification, all testing carried out on closed roads, on a private estate, under adult supervision...

Go and look for changes in the road surface first; i.e. get off the bike and have a look on foot :!:

Worn rear tyre can make this happen, check the centre.

Tyres are good Kommando, its the first thing I checked.

Road surface could have changed Bernhard, I never thought of that. I even if I do look closely at the surface on foot... I don't know what it used to look like !

Fast Eddie said:

Last time I 'tested' the bike here, it had clip ons and rear sets and a corbin seat which keeps you well planted on the bike and he bike itself was rock solid.

I was probably doing about 110mph, banked over mildly to the left, slightly up hill.

Any ideas? Is this to be expected with US bars etc?

Click to expand...

Yes, eating more pies and leaning further forward (as you would normally do with clip ons) will most likely cure this problem.

[video]https://youtu.be/z3OQTU-kE2s[/video]

L.A.B. said:

Fast Eddie said:

Last time I 'tested' the bike here, it had clip ons and rear sets and a corbin seat which keeps you well planted on the bike and he bike itself was rock solid.

I was probably doing about 110mph, banked over mildly to the left, slightly up hill.

Any ideas? Is this to be expected with US bars etc?

Click to expand...

Yes, eating more pies and leaning further forward (as you would normally do with clip ons) will most likely cure this problem.

[video]https://youtu.be/z3OQTU-kE2s[/video]

Click to expand...

Fantastic hypothesis Lab...

Now, where's my nearest pie shop...?

Makes sense that more weight on the front would help stabilize it and less (more upright riding position) would make it less so. If only the Roadholders could be adjusted for ride height, a drop at the steering head and perhaps some flatter bars would bring it back to a more stable atitude. Maybe some slightly longer rear shocks.....

Airhead BMWs have narrow low bars. At first they seem a bit scary and heavy at low speeds but you quickly get used to
that and at speed they are fine.
I could never get on with high bars. The American market bars arent that high or wide but once past 60 they hang you
out in the wind and the make the front end feel awful squirrelly.
Once rode a 1000cc Trident at the century mark with the stock US bars. Terrifying.
...add to this the fact that Im nine stone fully road ready, so LAB may well be entirely correct.

Onder said:

Airhead BMWs have narrow low bars. At first they seem a bit scary and heavy at low speeds but you quickly get used to
that and at speed they are fine.
I could never get on with high bars. The American market bars arent that high or wide but once past 60 they hang you
out in the wind and the make the front end feel awful squirrelly.
Once rode a 1000cc Trident at the century mark with the stock US bars. Terrifying.
...add to this the fact that Im nine stone fully road ready, so LAB may well be entirely correct.

Click to expand...

I was nine stone once... When I was about 12....

Yes, eating more pies and leaning further forward (as you would normally do with clip ons) will most likely cure this problem.

[video]https://youtu.be/z3OQTU-kE2s[/video][/quote]

I knew a little more weight would help, yea that's it. Good one L.A.B.

The wider/higher bars do hang you out in the wind stream significantly more than clip-ons do.
Plus the wider bars give you more physical leverage on the front end.
The buffeting from the wind stream may be making you unconsciously pump energy into the front end through the bars, resulting is a slight weaving motion.

Besides, ",,, just for clarification, all testing carried out on closed roads, on a private estate, under adult supervision..."

Now, where's the fun in that.

$50,000 question what size is your front wheel/tyre? and what pressure are you running. 110MPH on a sit up and beg Commando is at the edge of the flight envelope :lol:

Second question, how much do you weigh? I tried lashing my tool roll under the headlight and filling the tank to bring the C of G forward, no difference. Try this. Pop down the Sunday market tomorrow, buy a 56lb (25KG to you sir) bag of spuds and lash it onto the seat and try again. No one gets hurt if it all goes to hell in a handcart apart from you. My weaving problems disappeared when I had my son on the back but that's valuable self loading ballast and you probably won't get to 110 two up. Having seen the photos of your bike on here, maybe you will. We endeavour to lighten our bikes in the quest for speed but there is no point in having speed if you can't keep it together in a civilised manner. More ballast

Ok , here's a theory , feel free to pour scorn , ridicule , or generally take the piss. It appears that all naked bikes suffer from weaving , lying (laying?) down cures it . Science tells us of something called Vortex Shedding , which is an oscillating flow when a fluid such as air or water flows past an unstreamlined body ( a rider ). Repeating patterns of swirls cause an alternating low pressure area either side of the rider . Is it possible that these low pressure areas push the rider from side to side and this is transferred to the handlebars , making the bike weave ? A lighter rider is easier to push , go flat on the tank and it stops this vortex shedding lark . Now don't laugh , it's only a theory , I aint no scientist . Bob.

There's no doubt that putting more weight on the front wheel helps with 70s bikes. Modern naked bikes generally don't weave because they have stiffer frames, forks and wheels and better damping. The Commando chassis is quite flexible, even with the isos set tight. I tested this out on my own Commando and classic Beemer. Euro bars and rear sets were the best solution for me, along with good tyres (Avons, like you have) and up the pressures a bit. I found that the Beemer likes more pressure in the rear tyre too (36 psi on mine). There's no problem running higher pressures on the road because the tyres never get that hot like they do on track.

What you could do for a bit of fun is mount a Go Pro or similar to the rear and monitor what the swinging arm and tyre are doing on your 'test circuit'. Or get someone to follow you with a front mounted cam! Is the swinging arm bending and rebounding like a spring and is the tyre deforming? I bet that even with isos set tight, if you grab the rear wheel and pull hard side to side you will get some flex. At a particular speed, the flexing of the various components becomes harmonised and then you get your weave - with the upright riding position and forward mounted 'rests.

Also, if you insist on the US 'bars (!) you could experiment with upping the rear spring preload and reducing the front preload, along with higher tyre pressures, and see if that makes a difference. Can't remember your rebuild thread, but with all the work you put into it, I assume you have a good head steady?

Dave

My bike would weave at odd moments, usually letting off at over 70 mph. It would do it loaded up for touring or just out riding around. Keeping on the gas made it better, but has obvious limitations. A new rear tire helped a lot. Straightening the frame also helped, even with a worn rear tire.

The bike has never done anything really evil but that may be down to grabbing the seat cover with a sphincter and shifting the center if gravity...

Greg

I had a problem with my 750 a long time. Tyres looked good but the old Avon GP that was on the front from new ended up being the cause of the problem. In the early days i had the rear rim replaced because it had a flat spot in it near the weld and I had the front reworked but still the problem. I made a new oversize king pin for the swing arm as well. The tyre looked perfect but at 70 mph in long sweeper the bike went wild. Probably one of the reasons I stopped riding it after marriage and kids. Later when I got the bike out again about 10 years ago, I mentioned the problem to Bob Nesbit, [ he asked me if the steering shook if I took one hand of, which it did ] and he said it was the front tyre. He was right. I later fitted it to the rear and wore it out. Now I tend to replace the front tyre more frequently and shove it on the rear. Wish I could find a rear tyre that would last more the 3000 miles though.

Dereck

Bob Trigg once said in an interview, "Motorcycles are basically two casters; the trail of the front wheel and the rear wheel following the front. They can be MADE to wobble."

Like all of us who have ridden many miles for many years, I have experienced weave and the rare tankslapping wobble. But never with the frequency or severity of those riders in the Dunlop film. Many factors such as tire condition, road condition, suspension settings, chassis integrity and even wind direction can conribute. But those guys in the film look to have maximized everything for the sake of demonstration to get the shimmies that bad.

kerinorton said:

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

Click to expand...

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

cjandme said:

kerinorton said:

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

Click to expand...

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

Click to expand...

Absolutely!