Weave at speed on '68 mk 1

Hello,
I have been reading this site with great interest for some time now, but due to my one finger typing i.e slow, have until now not joined in.
Does any body know how to stop my mk 1 750 weaving at 75 mph +. The bike has always done this at least as long as I have owned it. At this point I have to say that I am familiar with the breed having owned a 850 mk 2a for 12 years. This is fine at any speed.
The bike has had in the last three or four years-
New Isolastics, headstock bearings, swingarm bearings, rear Avon 100/90/ 19 Super venom, front Avon 90/90/19 Roadrider, Dave Taylor head steady, forks rebuilt by Maxton, Maxton shocks.
I rebuilt the whole bike four years ago and checked the frame for straightness and also the swing arm. The wheels are perfectly in line.The frame is not the original dangerous type but was replaced by Nortons in the late 60s. The yokes are the Atlas type and have been checked for straightness.
Below 75mph ish the bike steers and handles well, but at 75+ feels like the headstock bearings have been over tightened. Does the reduced trail of these yokes and 27* head angle cause this ??
Any help to solve this would be more than gratefully received.
Martyn.

Welcome! Where the swing arm spindle goes through the engine cradle wallows out/is loose from the onset. There is a cure, very simple bolt on. Called the Heinz Kegler mod, we have a member here that makes the kit, send a PM to user name "bill", AKA Windy. Install, ride enjoy. :mrgreen:

Thanks for that Concours but I should have mentioned I have a new rear cradle with the mk 3 type cotter pins. Also I converted the set up to roller bearings to get away from the oily mess that Norton fitted. Overkill maybe but as I'm a precision engineer, not very difficult.

Matchless said:

Thanks for that Concours but I should have mentioned I have a new rear cradle with the mk 3 type cotter pins. Also I converted the set up to roller bearings to get away from the oily mess that Norton fitted. Overkill maybe but as I'm a precision engineer, not very difficult.

Click to expand...

LOL, it sounded like you were very on top of it. So you've go NO perceivable lateral movement at the rear wheel now? Or just "some" from the iso's? I'll be standing by to see what shakes out.. so to speak :mrgreen: It makes me wonder, is the pizza cutter 19" rear part of the puzzle? Have the guys running 18" rears seen the weave disappear?

Dang it a Cdo should be just getting its second smooth sailing wind at 75. you seem to have covered all bases mechanically and upgrades. I've experienced stiff stem bearings and it shows up before hwy speeds with a tossing sideways sense to road texture and corrective pilot input. All 750's have 27' stem rake and I've inquired of road racers to land speeders and to a man they say never found a speed limit that upset at all going straight. My 750's once and a while become weird to hold a straight line after doing fine, which is my first clue a slow tire leak occurring. My only hint here is might try tire pressure balancing,ie: with front 2-3 lb lower than rear and see what that feels like. Oh yeah on pre-Peel Combat it had one season with ride intervals of hinge handling just going straight a legal-ish 55-65 mph or easing easy sweepers lolly gagging sightseeing - could fine nothing to account for it - [except open jacket flapping or chain saw blade sticking out one side aggravating it], till depressed in shed - just started tugging on stuff - till I noticed the primary chain flop was same freq. as the weavie wobbles - to also observe this chain flop was tugging on the clutch basket looseness which then tugged on drive chain which then tugged on swing arm which then tugged on front iso side to side pivoting on rear iso, which oscillated the forks towards tank slapper. Then re-did the sleeve bushes with a 3rd as spacer and got chain tensions right and that was the end of unstable sense till going around too fast on lumps w/o any rod link tammers. If still in frustrated desperation, only other add on solution besides and dang draggy stiff steering damper set to minimum - is a Bryan Tyree, hobot above iso or Comstock below iso type front link. Oh yeah, when my rear hub bearings begin to really get worn out there is a upseting wiggle on first easing off a stand still but once over runing speed its was stable as any un-tammed Commando, even after the axle broke on trip to leave smoke cloud of tire rubbing away on swingarm and fender, till flagged over to look behind then see tire about cut through. I have had hub cushions completely gone a bunch of times as only thing noticed was more drive train slack on power cuts and blip ups.

Tire pressure experimenting is an easy one to test per hobot.

Re: 18" wheels - I ran a Mk3 Commado frame with Norvil head steady and got the wevae at very high speed but only coming off the bank at Daytona. Otherwise, it was rock steady.

Are you using saddle bags or any additional weight in the back. Reason I ask is Commandos have submarginal unladen weight distribution (45.5% front / 54.5% rear). Can the weave be allevited, eliminated or prevented by getting more of your riding weight forward?

Using a tank bag or wind screen. I've read elsewhere that a wind screen can induce nasties at speed.

How certain are you about wheel alignment. Using a front wheel to rear wheel alignment check is ok if everything is straight in the frame department. As an example, if you have a twist in the swingarm you can still get the front and rear to appear parallel.

If not already done I would start by getting the bike on the center stand and pulling and shaking all major componenets to see if there's any significant looseness. Swing the handlebars gently to assure smoothness of the bearing and no sticking.

Ugh Dances flying off Daytona banks is whole 'nother orbit than road bike behavior. As too fr/rr weight I find the more I sit back on Trixie or pile up cargo like beer cases or camp gear for two like 2 wk ends ago heading for Texas, the more I can let go of handle bars w/o getting escalating fork oscillation and the fast I went the less unstable hands off. The more front is loaded or low air the more road and pilot and windscreen mess with my hands off stability. Just saying... I sense troubles any time the front is over powering the rear influence.

Can I add a few thoughts?

Weaves often start at the rear. Apart from making sure your tyres are pumped up firm, is the tyre concentric with the rim? Put it in gear with the bike on the centre stand when the engine is running and that will tell you very quickly if the rim or tyre is round or not. Side-to-side out of round is more critical here. Check the front wheel too.

I assume your rear shocks are in good nick. If there is little or no rebound damping, it won't help. Are your shocks the right length for a Commando and are your forks standard?

I agree with Dances that weight distribution is important. Try leaning forward and see if that helps. Hobot - I don't understand how it works for you, but if it does, great!

I would take the bike off the centre stand, hold it vertical and swing out of the rear wheel. With the isolastics set at, say, 0.008 thou clearance, you should not have any slop at the rear wheel. The rod end head steady will help here. I think it's better to check and set the isos with the bike held vertical off the centre stand. Your head steady's rod end bearings should be free to turn with the bike vertical and off the centre stand.

Having said all this, tyre problems often turn out to be the root cause. Many people, including me, have reported their bikes handled so much better with a set of matched fresh tyres, especially Avon Roadriders. I read that BMW twins used the weave back in the day until the factory commissioned Continental to make a rear tyre with a stronger carcass.

Get a larger-sized friend to help. Locking front tire with his knees try moving the handlebars sideways back n' forth. That's a cheapo quick front-end test. Take off the seat and have him face backwards lifting up on the rear frame rail to enable you to grab the rear wheel and move it back n' forth. Sloppiness should become apparent or not. Another idea is your shock mount points. Do they have rubber inserts to fit as these can cause sway. Also how old are these shocks ? Tire pressure is so important ,I err on over-inflated. Good luck.

Thanks for all the replies. Today I have increased the static sag on the forks to 30mm & taken it out for a test. Just the same!.
Previously I have tried 24 25 & 26 psi in the front tyre & 26 27 & 28 in the rear. Currently using Avons rec. of 29 & 32 psi. I've tried the isolastics tight, not so tight, slack, very slack. The wheel bearings are fine. I've also removed the wing mirror just in case. When I first aquired the bike it was fitted with an 18" wm4 rear rim with a 120 section tyre. It wasn't any better like that either.
During my test ride today I did notice something odd. When I slid forward so I was almost sat on the tank, the weave became worse. In fact it was rather scary. So I tried sliding backwards onto the pillion section of the seat & the weave virtually disappeared.
What to try next is the question.

Weave is not normal.

As "daveh" stated, I would have a look at the condition of the rear shocks.

Have a look at the bushing conditions, check the rear spring preload - maybe too much. Should have about 1/2" sag when you sit on the bike.

If you can get the shocks separate from the springs, have a look to see if they are functioning.

Do you have dampning in the front forks? Drain and re top off the forks if in doubt.

Try substitution ,put other brand shocks on if available as I have not heard of that type being used on other bikes. Also inspect tires to see if fully seated home on rim. Overinflate ,then deflate to specs. Maybe the rollers in swingarm allowing sideways driftage ?? Both wheels are seriously offset , wheelbuilders of beginner status need to be reminded of that.

The Trick is to coincide the weave with the corners at speed , takes a bit of practise . :lol: :roll:

Steering head bearings ? tapered roller ? spacer tube ?

One outfit found out of round rear tyre caused similar depredations .

My pet rave is ALIGNMENT of ISO's , i.e. shimmed central . Equal Gap Ea. Side At Front . Checked by leavering at rear and measureing both sides front , twice , rear @ ea. side .
Also the Pins / lugs checked for parrallel if / when dissasembled .

Tyre sizes aler rake from altered hights . Worn tyres obviously can muck up steering .Need to spin them with touch stick against ( dial Guage :shock: ) to check run out .

What year & series is the device ? Can be just to firm a grip if rider insecurely located . As in if youre comfortable & not slipping around , the old finger tip touch is sufficent .
Worming and squiggling on road irregularities from the rider are transfered into the steering if a death like grip is required to stay on . Set shocks on SOFT pre load for starters .

So I tried sliding backwards onto the pillion section of the seat & the weave virtually disappeared.

Click to expand...

Dang who thought eh. Just to rub racer's raw guess what this implies to why I know current bike philosophy - depending on front tire to turn are dangerous corner cripples to me.

Anywho you have covered all basics yet something seems weird at front, maybe fluid flushed out one side or both? I like 175 ml as much more can lock up action which tosses one opposite expected to road or pilot inputs. Fork stanchions must be rolled against each other to pick up full bow out of both combined. I'm stumped and await some new finding we're all over looking.

So like get on those scales and tell us all your weight . We can keep a secret.

Total mass and mass carried distribution should have very little to do with secure handling, until ya get a bit crazy and the flinging leaning rate makes the cargo shift and wiggle then can crash ya but that unrelated to just sticking like crazy hunkered down with a passenger to scare. Main issue on extra mass is can't change lean and fork angle as fast or that can upset the suspension over loading it but with smoother actions it essentially the same as unloaded.

Again tires keep coming to my mind so if ever riding enough to wear em out might experiment with widths. Larger rear would be my first try, then larger front too.

The tyres are fitted correctly & are running true. The shocks are spot on in every way as are the forks. It seems that more trail would cure this i.e 850 yokes, but I would like to keep the original yokes, headlight brackets etc. I have been told by more than one person that these earlier bikes could be rather twitchy on the steering at speed, but my 650ss has the same type of yokes & similar headstock angle & wheel size. It is rock steady at any speed. Yes I know it dosn't have isolastics, but as mentioned previously I have tried almost locking these up tight.
500yards down the road from me there is a pier. I'm sorely tempted to push the bloody thing off it at the moment!!
Does anybody out there have one of these earlier bikes. If so what does yours steer like at speed?

It would be WAY more fun to drive it off the pier and let go -jump off halfway through launch. We have been doing this for years with bicycles out on our Toronto islands Park and there is a plywood flip ramp built for this activity , points for style and drama scale of splash-landings. Parks admin. got wind of this last year and issued a health alert as landings were in swampwater and of course liability. Insurance clowns take the fun out of being alive. Does your frame use early open-adjustable style headbearings or sealed type ?

The heeadstock bearings are taper rollers which I have adjusted carefully. I have even tried running them on the slack side. Drove past the pier this evening . It looked tempting.

hit that pier really hard in a South Eastely direction, I'll try to catch it here in Virginia