Why You Cant use the Frame Spine for Wheel Alignment.

The rear wheel has to be squared to the swing arm pivot - not parallel to the spine tube. This is because the spine tube is not square to the swingarm pivot, or at least you cant assume or don't know if it is.
If the wheel/swingarm angle is not 90 Deg, then as the swingarm moves through it's arc the wheel centre line moves transversely (across) in relation to the frame centre line.
To visualise this get four pens, pencils or whatever. Fix two together in a T-shape at right angles to each other. Fix the other two in a T-shape at say 120 Deg for visualisation. The top part of the T is the swingarm pivot. Lay the two T's on a table next to each other with the 'wheels' parallel to each other. As you rotate the pivots you'll see the different paths the wheels take. This is why Commando's get out of shape, especially on bumpy roads, the relationship between rear wheel to frame centre line and rear wheel to front wheel is constantly changing.
There is nothing fundamentally wrong with the isolastic principal. Peter Williams F750 Monocoque went more than ok around the Isle of Man.

There are some mixed messages here ?

Give or take a few thou, the spine tube bloomin well ought to be square to the swingarm pivot ?!
Assuming it is, at rest, is the only (?) way to set the wheel alignments....

And the monocoque JPN Commando wasn't an isolastic armchair ride, the engine was solid mounted ?!!
The early red-framed JPN Commando racer did some fast laps in the IoM though, faster than any Manx had ever done...

QUOTE :
" Isolastics for Race Bikes
by lcrken » Sun Dec 12, 2010 12:53 pm

We've seen a lot of posts here about isolastic bikes vs. solid mount bikes, with plenty of comments on how terrible the isolastics are for handling. My experiences on the race track with Commandos are that they handle just fine, when properly set up. In defense of them, I thought I'd point out a couple things that seem to have been ignored in these discussions.

All the factory race bikes with Commando engines that I know about had isolastic mounts, including the John Player F750 bikes. That includes the early tube frame racers, the monocoque bikes, and the final tube space frames. They were quite successful racers for the time, given how underpowered they were. Between them they set fast lap times in races, including the IOM, set some lap records, and won plenty of races. They were generally regarded as much better handling than their competitors, but at a serious horsepower disadvantage.

Peter Williams claimed that the monocoque Norton was the best handling bike he'd ever raced. Pretty strong praise for an isolastic bike from a reliable source.

Just to be really clear, I'm not trying to say that a Commando racer handles any better than a good solid mount frame, be it featherbed, Seeley, Rickman, or whatever. But I think they can be made to handle just as well, although they definitely have a different feel to them. The primary benefit I see for a solid mount frame is their weight advantage. Most Commando framed racers are at a significant weight disadvantage to a bike like Kenny Cummings Seeley replica.

Ken

lcrken
VIP MEMBER
Posts: 1090
Joined: Sun Mar 15, 2009 8:08 pm
Location: Southern California "

isolastics-for-race-bikes-t8212.html

THIS is the ONLY Non - Iso Works / Factory Race Bike , as far as COMMANDO powerplats are concerned .

Is the front wheel centered on the frame tube (spine)? As in between the fork tubes?

Russ

rvich said:

Is the front wheel centered on the frame tube (spine)? As in between the fork tubes?
Russ

Click to expand...

It should be.
Its tough to get a bike to even steer a straight line if the wheels are offset from a bikes centreline...

Were all the monocoque and space frame bikes isolastically equipped then ??
Musta been asleep in that thread, that doesn't ring a bell, at all...

Were there any rubber components in all those isolastic mounts ?
Its possible to look like an isolastic mount, wihout being an isolastic mount.
Not saying that was so, jist asking....

Al-otment said:

The rear wheel has to be squared to the swing arm pivot - not parallel to the spine tube. This is because the spine tube is not square to the swingarm pivot, or at least you cant assume or don't know if it is.
If the wheel/swingarm angle is not 90 Deg, then as the swingarm moves through it's arc the wheel centre line moves transversely (across) in relation to the frame centre line.

Click to expand...

If the swingarm spindle is not perpendicular with the 'spine tube' then the only explanation I can think of is that your cradle isn't lining up with the frame. It's crooked. The cradle is offset in the frame but it should line up fore and aft with the frame.

Yoose guys can speculate till the cows come home but I've done the experiments and so have major factories and both agree both wheels do not have to be in line on straight ahead rolling for bike to behave just fine in turning this way or that. Oh there may a tad easier to toss one way or another but it will be rather subtle to detect and so minor its a total non issue. The Only Thing that really matters most is the chain sprockets pretty close and if that means the rear slightly skewed main thing noticed will be a bit more tire wear. A good many past well respected handling BMW's came with off set tires and even mixed bias with radial tires. Have someone skew your rear tire a bit [not enough to screw with chains too much of course] see if ya can tell which way they skewed it or if they even did skew it and get back to us please. The down and dirty easy method on side of road to center rear is just feel for equal space between tire and swing arms. Try it then verify with strings straight edges or lasers and get back to us please. Hell I'm a princess on a pea when it comes to handling quirks and I can't tell a thing wrong with broken axle at illegal speeds with it rubbing to smoke on swing arm its so tilled both horizontal and vertical. Taking off till 5-6 mph feels like tail waging the head then smooths right up fine as can be. Obviously ideal is both tires in line with the CoG of bike but how often is that actually how it works out yet they still go around well as can be expected. [for corner cripples]

There's huge insight about each tire following a different line in turns and the differing aims of their force vectors acting though frame, but I'm done with that study and solved it by an isolastic Commando of all things.

The monocoque frames definitely had isolastics with the rubbers fitted. One reason was for rider comfort and, as Matt points out, the monocoque handled so well that Peter Williams could confidently drift the bike through corners, on tarmac.

The front wheel on any bike needs to be centred to the frame centre line. On a Commando it is a line through the midway point between the rear iso mounts and the steering axis. On any other bike it is typically the middle point between the swingarm mounts. On a Commando, you can forget the frame spine for alignment as it is distorted after all the welding, as well as the rest of the frame. Put a straight edge on the top of the spine to see for yourself.

How would you centre the front and rear wheel on a featherbed, Ducati or a current sports bike with a beam frame? A rhetorical question.

When the swingarm spindle is out of square to the steering axis it is because the isolastic frame mounting holes aren't square.

There's no speculation going on here, I've done the work on my bike. She steers dead straight and sticks to the chosen line through corners. The handling is transformed. I can't think of one single engineering reason why an engineer would design a road/street motorcycle, or bicycle for that matter, with the wheels offset i.e out of line, to the frame centre line. Maybe someone could explain the advantages.

While I was at it I made sure the drive sprockets were in line. Initially they weren't. I bought this bike 26 years ago when she had 17,000 miles on the clock. I've never crashed the bike and I have no reason to believe the previous owner did either. The frame was as it came out of the factory. Only other frame mod is the fitting of an isolastic head steady, which also wasn't square when I bought it......

Hope this helps to enable other riders to get their Commando's handling as they should, Si.

Al-otment said:

When the swingarm spindle is out of square to the steering axis it is because the isolastic frame mounting holes aren't square.

Click to expand...

No. The iso mounts are square in the frame. When you say, "When the swingarm spindle is out of square to the steering axis..", do you mean offset left to right? You must have noticed that the engine and, more to the point, the cradle are offset to the left. Your isolastic headsteady and the front and rear iso's show the offset. The front of the swingarm mounts to the offset cradle. It is made up by the asymmetrical swingarm. The axle plates at the end of the swingarm are offset to the right, they line up with the upper shock pickup points on the frame. The shocks are vertical looking from the back.

I've heard a few explanations as to why they wanted to offset the engine/transmission. The one I like is that they needed the clearance for the chain/rear wheel. I've had my assembled bike on a frame table and everything adds up. The wheels are in line with the backbone of the frame.

I have to agree with Bob, the cradle is offset as well as the engine/head steady, I'm not sure if the swing arm is symmetrical or not, or the wheel/hub is offset in the swing arm. (Disc is different from drum) But if the iso bolts are perpendicular with the spine, the swing arm ends are perpendicular also, I don't see any problem with using the spine as the center line of the wheels. Ludwig was adamant about that, plus he was particular about the squareness of the front forks too, and I trust ludwig's ideas.

I'm with Steve on this too, the little that most undamaged Nortons are out of alignment will hardly make anyone notice.

When I rebuilt my hubs, they both ended up centered in the wheel which ended up centering on the spine. They were that way from the factory, not that everything from the factory was right.

BTW I first drove my Commando home without the nuts holding the rear hub on the drum (bolt up hub) and I hardly noticed it (compared to a CB450 of course).

Dave

hobot said:

The down and dirty easy method on side of road to center rear is just feel for equal space between tire and swing arms. Try it then verify with strings straight edges or lasers and get back to us please.

Click to expand...

A piece of (straight) angle iron, 2X4 or even a handy curb will tell you if the front/rear wheels are in alignment if you have the same size F/R tire.

Yea the spine should be on the centerline , for sighting it up . a few bits of straight wood or aluminium can do wonders for visual alignment .
even if the Engine / Trans ARNT central on the same centerline , THEYRE PARRALLEL , problem comes if theyre Not , bent frame or unevenly shimmed ISOs , rear to front . Front shouldnt be hard over one side .
a feeler guage should have the same resistance / freedom , on Either Side at one End . And for appropriate guage for the other . or somethings missaligned , or incorrectly shimmed . Not neccesarilly Same No Ea Side , you understand . :mrgreen:
Or a Spirit Level . Good enough to get the Ford Falcon Alignment ( front end ) in from scratch , half way round the country , even tyre wear .

a plank ea side of the rear wheel , the front wheel should be in the middle ( diferant tyre sizes ) on cycles .
standing real still and eyeballing down the spine ( you need a elbow down ) should see spine parrallel .

Isos set up from scratch , should be checked for centrality :? so , the front ones shimms give equal slack ea side .
If the suckers been pranged that can be hard , and the frame wont neccesarily stay realigned . :x

Digressing back to 4 wheels , measure front & rear track overall ( outside faces ) and use the Dia of wheel x the 1/8 in ? Toe In , multiplyed by wheelbase .
throw a block of wood ( useually a inch or two ) against the rear tyres ( not on the BULGE ) & on flat ground its good for doeing the toe in , and the Camber .
Along with the spirit level - sight fr tyre edge to rear tyre edge , both should have the same narrow angle Neg Camber . Pre war beam axle stuff had ' datum points ' cast on the axle , for your spirit level .

Adjustable spirit level / angle - squares , are usefull . You can get it so a top alignment shop says it better done than at the factory . even though you dont know exactly what the angles are . :lol:
Theyre the SAME , with on the Chrysler the 1/4 degree bias . which a good draftsmans eye can pic , can use a pencil to 1/10 mm too .

The MORAL of this STORY , is a Good Eye , a Steady Hand , the odd string line , spirit level , builders square , and a few lenngths of matched alluminum offcuts from the aluminum joinarys scrap bin ,

can get it within a few thou . If you leave the disruptions till the jobs finished . The odd clamp comes in usefull . Or matched BRICKS to hold the darn sticks on the rear tyre . Matched builders off cuts for packers . :lol: :wink:

http://www.nortonownersclub.org/support ... el-offsets

Check out the last post. That's my bike.

Forget about offsets. It is logical to use the steering head axis as the datum, the centre line is then calculated from this and every thing else follows on.

Square means square in two planes to the steering head axis. Some of you seem to be getting confused between inline and symmetrical. I've never stated a Commando is or should be symmetrical.

I agree the spine should be on the centre line - but it's not. There was a big difference between an engineering drawing and what came out of the factory door in England in the 1970's.

I've talked to enough Commando riders and read enough threads on forums, as well as my own experience, to conclude that there is definitely a problem with handling. Isolastics typically get the blame but this is not the case, especially when you consider how Peter Williams isolastic mounted bikes handled. The problem is wheel alignment to the frame centre line and perpendicularity of the swingarm axis to steering head axis, end of.

Like I said, how would you align the wheels to the centre line on a featherbed? This is now not a rhetorical question.