Vibration from 2500 to 3000 rpm

I love everything about my Norton except the damn vibration from 2500 to 3000 rpm. The worst of it is that it occurs
in the 45 to 50 mph speed range, right where I find myself driving 95% of the time. ISOs are good and have tie-rod headsteady.
Primary belt bumped the drive ratio up so to get back to normal I dropped back to a 20 tooth (or was it a 21 tooth) front sprocket.
I'd hate to gear it up to run less rpm because then I'd end up in 3rd more often running through the layshaft. I suppose I could drop to a 19 front sprocket and let the engine spin a little faster. That may make using the bike as I do more comfortable, but won't do
anything about the shaking. Can anyone tell me if having the crankshaft balanced or the balancing factor changed would give me
what I am looking for. Of course if I had a little more disposable income I would try Jim's light weight pistons and rods, but that will have to wait.

GB

Have you done the basics though ?

Synched your carbs.
Shimmed your isos tighter /looser.
Replaced your iso rubbers .

You say the isos are "good", but if you are getting big vibes then it doesn't sound like it....

Wasn't there a report that replacement iso bits at one point were somewhat soft and mushy ??
Or was that hard and unyielding...

" soft and mushy ??
Or was that hard and unyielding... "

I thought THAT was the Goverment . :shock:

That is a a difficult range to smooth out. I would get rid of the tie rod head steady, but that just me.

To help fine tune what you have, take the tank off and while sitting on the bike, adjust the rod ends so the bearings swivel nicely and be neutral. Again, while siting on bike giving it as much of your weight as possible and off if the stands, that is to say, as if you were riding it.

I suggest this "while sitting on the bike" adjusting the head steady with any and all types of head steadies. I think that because the rod end units have no lateral movement, this method is a must for the low rpm range of smoothness

yep, I think from 2500-3000 rpm is the absolute worst shaking rpm range for a Commando, and you are not alone

the only thing you can do is downshift to keep the revs up but then suffer the busier motor noise at a time when you should be enjoying the ride

and that, is probably the single worst thing about owning a Commando, just think we could still be riding 750 Atlases with no isos and rattling our teeth out, oh well......

Maybe Heinz was right when he said the Norton 650ss was the best twin to come out of England, but they had to have a little faster quarter mile time to sell lots of them in the big USA market, so they punched it out and added isos

It's normal. I work at getting to 3000+ RPM and if I can't in 4th vs speed, I drop down a gear. But keep in mind that in 1970, that vibration wasn't much vibration!

1. bevel edge of rims of the big cushions 45+ to almost a peak instead of flat band.
2. Inflate tires to as high a PSI as still comfortable for the surfaces most traveled.
3. Go down to 19T sprocket for rpm to isolate and lube cam lifter surfing.
4. retard spark a tiny bit to soften the torque hit peaks
5. open iso gaps to at least factory spects
6. add counter balance spring if not already
7. compliantly triple helms joint links as far away from iso mounts as practical both vertically and horizontally.
8. Buy JMS light piston/rod kit.
9. Up crank BF 10%.
10. put heavy oil or sand in the frame tubes and lead shot in bar ends.
11. thicker bar grips and peg cushions and shoes and gloves and seat layer.

Thank you all for your responses. My ISO rubbers have been in there going on 9 years now, although I did check the front one a couple years ago and it seemed OK. Maybe its time for a new set of rubbers. I will also take Pvisserii's advice to adjust the tie-rod head steady for a neutral position, and Hobot's suggestion for MKIII type head steady spring. I will also take Matt's suggestion to make sure front and back ISO are evenly spaced.

GB

If you change the balance factor to about 65% by filling the hole on the crank with a threaded steel plug, your bike will shake when idling but smooth out as the revs get above 3000. You should not have to do this.

If you don't have an easily adjustable iso setup, get one, I like the Hemmings adjusters. The spring may help if you don't have one. I tightened mine up as tight as it will go. If you get new rubbers, make sure they are the old soft type unless you have the 850. I got mine from OB, but Fair Spares also has them, probably Walridge too.

That said, I find hardly any difference in any adjustments I make. Only thing the spring did was to drastically reduce the idle vibrations. I find no difference in my old original head steady and my home made rod link. However the old one could vibrate like crazy if it was pre-loaded. The tie rod can't be pre-loaded. You could try going to the production racer type head steady, people say it's better, ludwig has a good plan on making your own, let me know if you need it.

Dave
69S

Adding on to Steve's complete reply, rather than using the old Harley (I'm talking 1950s here) technique of filling the handlebars with lead, buy a pair of bar weights from Manic Salamander:

http://www.manicsalamander.com/products ... ights.aspx

The 7/8" bars on our Commandos are light and flexible, and resonate at 50 Hertz or so (3000 rpm). These weights do NOT dampen the vibration but do reduce the resonant frequency to a few Hertz. While I can still feel the tingle at 2500 rpm through my bootsoles, my hands are free of vibration and my mirrors clear. It's a good product. Get the heavy ones (Pounders). Works fine with bar-end mirrors. They add an inch or so to the bar length, but, to me, its was a small price to pay for vibration relief.

"Works fine with bar-end mirrors. "

Looking at the pics on the site, I can't see how they work with bar end mirrors. It looks like the bar weights use the same sort of "plug-in and tighten" mechanism that the mirrors do so how do the bar-end mirrors plug into the weights or visa versa? Am I missing something obvious?

With all standard stuff and 21 tooth cs sprocket, 45 mph is 2650 RPM in high gear. The manual states that if annoying vibration is felt at rpms under 3,000, the rider should shift to a lower gear. The vibration should disappear. 45mph in third equates to 3200 rpm with 21 tooth cs. This is still a nice low rpm for slow cruising and if one needs to suddenly accelerate to pass, you are ready to go.
With my own Mk3, once revs are over 3000 it is very smooth, as promised in the owners manual.

Glen

The main thing that bothers our nervous systems with vibration is the freq and the sharp peaks, so anything that dulls the peaky edges off the engine vibres is more pleasant. Do note that past Ms Peel and I had left the vibration building awareness behind to flat disappear from pilot sense of a bike under me by lower 2000 rpms [just like Frank told us his original test Cdos did] but for the road texture and wind gusts which were also dulled to point of almost complete transparency of minor road annoyance texture, ONLY LIFE AND DEATH tire road surface adhesion sense transmitted so clear and clean MS Peel may well be the smoothest most road holding motor cycle ever fielded which means there is no such a thing as the perfect best line accelerating through decreasing radius turns, Peel made them all fast decrease-rs. This going faster and faster is real risk to face if configuring like hobot's Ms Peel as can't go by sense of speed through bike feed back only wind sense which is dulled by full face helmet and the blurring of scenery changes too fast to focus on up close zooming up on. I've been able to save Peel and myself beyond expectation d/t the insane roadholding w/o getting upset one iota. I normal Cdo and likely the solid mounted racers can not remain stable tires in line enough to ride bike lengths on locked front tire but was daily hobby of Peel to try to stoppie everytime I entered my village square from hwy speed. I'd slide so far that other bikers had time to see me comming and run out to yell at me I've locked front tire - as if I didn't dam well know what I was doing on purpose routinely, if some what nerve wracking, using butt to keep tires in line w/o only front in traction. For normal cruise legal-ish I would trance out merged as one with Peel as extension of my own body. Catch up when ya can I'm missing company to sing the joys of isolastic wonders with world wide.

Finally got me a ride on 2008 full dresser Honda Goldwing, feels like a tractor at low idle and speed then smooths out the tractor sense for buzz of valve train and tire texture getting through. HOT DOGGIEDOODOO, I now know Ms Peel can easy match or top a big old clunky car like Goldwing!!! Proof in future with smartphone or such vibration montioring. Only cars as smooth a sense as Peel were LS400 Lexus and Jaguar XJS, with Lexus beating out the Jag for smooth and secure and fast. My 6.9 liter Mercedes always felt like just a very fast truck compared.

This low rpm lumpiness is pretty much the nature of the beast, in my opinion. I've never found them particularly pleasant around town but back in the day, I was rarely to be found holding a steady throttle opening. It was either flat-out or hard on the brakes and in those circumstances, the brief period of shaking was hardly noticed.

I suppose one shouldn't really run a Commando if one wishes to ride at side-valve speeds.

Mike,

I use CRG mirrors. The internal clamp piece is 7/8"; just chuck it and clamp the mirrors directly to the handlebars right at the end of the bars. You'll need 1/2" of bare bar to clamp. The weights then mount behind the mirrors using their internal clamping piece and provide some additional security to the mirrors. I'd provide some clear photos, but at the moment my Commando is on Mercer Island and my ass is in Austin (ass and bike will reunite in May). Found an old photo, however, where you can make out the arrangement.

You shouldn't feel ANY vibration in the frame or handlebars above 900 rpm. If there is, either you have Isolastic problems or something is hard-mounted/touching between the engine/transmission cradle and the frame. The engine/transmission cradle is supposed to be able to "orbit" within the Iso's in about a 3/8" elliptical pattern without any of that vibration being transmitted to the cycle parts.

On the prototypes, we did a lot of experimenting with Iso hardness and pre-load to get it right. They were still tweaking it after I left in 1968!

If the head steady you mention is solidly attached to both the engine and the bike's frame, it's essentially shorting out the effects of the Isolastics. It would give you a similar level of vibration to an Atlas, which was really bad. We developed the Isolastic concept at N-V to give the Commando rider a break from that discomfort. The early models didn't have a head steady. If N-V added one on later models, it would have been tuned to work with the Isos. I'm not sure any after-market steadies would have the right flexibility to work with the Isos, unless they were designed by someone very familiar with the Isolastic concept.

I'm in Frank's camp on expecting lack of vibration as the most unique and key feature of Commando's even more than their TS cover to me which I hardly ever see or reach to touch to make sure of what we don't want to be aware of.

Designers first began with a set amount of rubber cushion area/section for isolation on set in the upper 6000s. then cut it in half for isolation in the mid range rpms then half again to get below 2000 for a wrap and out the doors. Can do backwards math to find the area they started with. The physics of this implies any extra amount of rubber supporting the engine or be bumped by the engine will increase the isolation threshold upward and a big reason there are more top knot rod links in a just a few years than all the 3rd isolastics ever tolerated.

Unless you are racing around beveling the front cushions and added extras in the rear is way to go and how I did my factory part only Trixie, not feeling like I'm cheating just modifying and moving internal cushions. No way do I get really racy on Trixie Norton but I know for a fact Trixie can hang with the most elite trigger happy pilots out for joy rides after a nod to the old gal then a flippant attitude on asking which way they were heading - which more than anything reveals just how corner crippled them over powered balloon tire wallowers are, they wisely to slow down so much even my antique has to slow up not to run into their tail pipes and don't get left out far in the opens. No way can I ride Trixie around real turns as fast as elites but they do slow down enough I can catch up to grit teeth using my whole body to some degree to dampen down the impending Hinging I make dam sure I have reserves enough the Whole Way Around, before wet sumping rpm again, like Combats Gods intended.

BTW, if you run in lower gears, realize sleeve bushes are above oil level, their spinning under load pumps lube out fast and no way can lube re enter till in 4th so the spinning relation stops and oil splash can drool down and seep back through the very thin areas as deep as it can before slinging oil back out again. Done often or long enough the inter spaces fill with hard ceramic grit. Most all my clutch wobble events trace to sleeve bush wear. Your call to configure most comfy for enduring life in 4th or not.

frankdamp said:

You shouldn't feel ANY vibration in the frame or handlebars above 900 rpm. If there is, either you have Isolastic problems or something is hard-mounted/touching between the engine/transmission cradle and the frame. The engine/transmission cradle is supposed to be able to "orbit" within the Iso's in about a 3/8" elliptical pattern without any of that vibration being transmitted to the cycle parts.

On the prototypes, we did a lot of experimenting with Iso hardness and pre-load to get it right. They were still tweaking it after I left in 1968!

If the head steady you mention is solidly attached to both the engine and the bike's frame, it's essentially shorting out the effects of the Isolastics. It would give you a similar level of vibration to an Atlas, which was really bad. We developed the Isolastic concept at N-V to give the Commando rider a break from that discomfort. The early models didn't have a head steady. If N-V added one on later models, it would have been tuned to work with the Isos. I'm not sure any after-market steadies would have the right flexibility to work with the Isos, unless they were designed by someone very familiar with the Isolastic concept.

Click to expand...

I recall reading that the original hope was to remove almost all vibration at all rpms. A soft type rubber was used in order to quite vibes at the low end of the rpm range. This was great up to 3,000 rpms or so but above that the soft rubber was overpowered by the stronger high rpm vibrations, which came back big time by 5 k or so.
The compromise was to use a harder rubber, which remained relatively inflexible with the less powerful vibes at lower rpm, but really soaked things up above 3000rpm. The end result is that these bikes can be expected to shake a bit below 3,000 but should be butter smooth from there on up to the redline. This is exactly the way my 850 mk3 runs.
I'll have to do some digging to find the source, but it was some common publication on Commandos.

Glen

Here is a bit from the original "Norton Riders Manual for Commando 850 mk111 motorcycles"

Under "Driving"
"This motorcycle incorporates the revolutionary Norton Isolastic frame and engine mounting system to provide an exceptionally smooth ride. However, some vibration will still be experienced at lower rpm and can be eliminated immediately by a change of speed or gear"