Ah, so THIS is why the handling was twitchy!

Status
Not open for further replies.
MexicoMike

MexicoMike

Joined
Jan 31, 2010
Messages
3,031
Country flag
Here's a pic of the front/rear wheel that was on my 850 Commando before I just recently swapped back to oem wheels/tires.

The front wheel is a 100x18 The rear is a 120x18 . The tire next to the rear wheel is a new Avon 4.10x19.

Until I decided to install oem size, I had never had both wheels off the bike and had never realized the dia difference. But I thought the handling was twitchy compared to my old 750 and it needed a steering damper, which I never had/needed on my 750.

The downside of the new 4.10s is that the bike's centerstand will not get the rear wheel off the ground without a piece of 3/4" plywood under it! ;)

Another of the "you never know what you're 'gonna get" when you buy a used bike...

Ah, so THIS is why the handling was twitchy!
 
MexicoMike said:
The downside of the new 4.10s is that the bike's centerstand will not get the rear wheel off the ground without a piece of 3/4" plywood under it! ;)

Another of the "you never know what you're 'gonna get" when you buy a used bike...
Click to expand...

Did they shorten the centerstand?
 
I assume so though I really don't know. I can't remember what a proper one should look like on the bottom. This one is well rounded, as if a grinder was taken to it so I believe it was shortened. We're going to weld some material on it to make it actually work.
 
Sorry if I was unclear...I bought new 19" rims and tires to replace the 18" rims'tires that were on the bike when I bought it in '06. It now has oem type 19" wheels and the 4.10x19 tires front/rear.
 
Got it, thanks Mike. Have you installed the wheels/tires yet? How do you like 'em?
 
Yes the 19" wheels/tires are on and the improvement is substantial - it's much more stable and the steering damper, which I used to need set fairly stiff is fine at minimum setting. I don't think the bike needs it at all now but I'll probably leave it on anyway. The rear tire looks REALLY skinny compared to that 120x18 but now I can actually align the wheels! With the 120 I couldn't without fouling the chainguard with the tire...the poor alignment was probably another reason for the twitchy handling and need for the damper. The smaller dia front tire automatically made "turn-in" much easier but to the point where it it was too easy.

I was just looking at the parts book and the centerstand on the bike looks pretty much like the pic. So it doesn't appear to have been shortened. In fact, there is a 1/4" piece of rod welded to the point at which the stand sits on the ground. So, if anything it looks like it had bee lengthened before. Makes NO sense to me at all then why the stock size rear wheel would not be held clear of the ground. I'll have to look more carefully at this...
 
Hi Mike,
The center stand on my stock wheeled and tired 74 barely lifts the rear wheel. It drags enough to make lubing the chain difficult. Fortunately (or unfortunately) the sidewalk outside my shop is unlevel enough to find a spot on which I can elevate the rear wheel. I haven't gotten around to check yet, but I suppose the mounting bolts could be loose or the bolt holes have become elongated. Come to think of it, it does kick way back when I put it up on the stand, so much so that I (5' 9") have to dismount in order to get it off the stand. If I find that it is not loose maybe an alteration of the stop would do the trick.
GB
 
Yet another myth . I have heard stories of bikes wich didn't handle and all sorts of reasons why they do (or did) it but I can say for sure that 18 '' rims and or tyres do nothing to improve or ruin the handling of a Commando . Yes they may spoil your groundclearance and yes you can easily overtyre them . I had a wm3 18'' on my 850 without handeling problems and on my 750 I have at the moment 110/80-18 rear and 90/90-18 front and it handles.As you can see these are fairly skinny tyres and groundclearance is limited to the primary chaincase grounding (I have no exhausts or sidestand in the way...)btw my 900ss of 1992 vintage has got its big rear wheel thrown away and a smaller 750 rear wheel fitted . Guess why ....Even the factories make that mistake because we all want a BIG rear tyre because it looks cool (god i hate that word) . I think you have to look for the handeling problem a bit further
 
swooshdave said:
How much ground clearance at the tires will a stock bike have on the centerstand?
Click to expand...

That seems to vary with year and stand design. My brother's '72 lifts the rear tire about 1/2 inch, on my '74 the tires touch the ground. I can move the rear wheel if I yank on it, but typically I'll put a slab of 3/4 plywood under the stand when I need to adjust the chain or clean the rear rim.
 
"Yet another myth ."

If you put a smaller diameter front wheel on a bike that was designed to have both wheels the same diameter, you change the "turn-in" rate of the bike. The quicker turn-in translates to less straight-line stability. That's just the physics of the set-up. In this case, the front wheel dia was, (by PO) nearly 2" in diameter smaller than the stock wheel, while the rear wheel was essentially the same diameter as stock. The handling change going back to the stock size wheels (same on F/R) was easily noticeable and, as I said, the need for the steering damper is gone. It had nothing to do with 18" wheels per se, it was the fact that for some reason, the PO decided to put a smaller diameter wheel (tire) up front. I assume he could have fitted a tire that would have been the same dia as the rear. It appears the bike was tracked since virtually every bolt on the machine has a hole for safety wire. If so, perhaps he wanted the quicker turn-in for handling purposes at whatever track(s) he ran on.

But the bike handles much better on the street with 2 wheels of the same dia than it did with the two different dia wheels.
 
maylar said:
swooshdave said:
How much ground clearance at the tires will a stock bike have on the centerstand?
Click to expand...

That seems to vary with year and stand design. My brother's '72 lifts the rear tire about 1/2 inch, on my '74 the tires touch the ground. I can move the rear wheel if I yank on it, but typically I'll put a slab of 3/4 plywood under the stand when I need to adjust the chain or clean the rear rim.
Click to expand...

It also will depend on the condition of the rubbers in the isolastics. Worn and tired isolastics will allow the frame to sag on the iso mounts rather than support it. Of course the pre-70 models don't have this issue as the centerstand is frame mounted rather than engine cradle mounted.
 
MexicoMike said:
The front wheel is a 100x18 The rear is a 120x18 . The tire next to the rear wheel is a new Avon 4.10x19.
Click to expand...

MM, That picture is a great example of the importance of aspect ratio, the /number on a tire. Different effective tire diameters seem to be ok if the layout of the bike is set up for it so that the geometry is what you want. I remember reading 20 years ago about the problems with steering that comes with different tread widths though.
 
I looked up Tony Foale's front end geometry calculator and remembered that trail is a function of rake, tire radius, and offset. So, if you use a smaller front tire, all other things being equal, you've steepened the rake and reduced the radius of the tire. Both will reduce the trail and increase the twitchiness. It's under freeware.
http://www.tonyfoale.com/
BTW, the article I read about different tire widths affecting steering was referring to large differences in width, probably not marginal amounts.
 
quote / In this case, the front wheel dia was, (by PO) nearly 2" in diameter smaller than the stock wheel, while the rear wheel was essentially the same diameter as stock.

So your rear wheel was greatly overtyred.Logic isn't it? But you would be surprised how many people make that mistake As said before I used a 18 rear with standard 19 front without handeling problems
If you jack up the rear by using balloons then your geometrie is all over the place
I used that 18 '' because in those days 19-110 were difficult to come by in Flanders
 
I have just stuck a 19" on the front because of the crap handling with 18" wheels. 110 x 90 x 18" on the rear & 100 x 90 x 18" on the front. Now gone for the same on the back & 100 x 90 x 19" on the front. Yet to try it.
 
Just to clarify the matter of why the rear tyre rubs on the ground or not when the bike is up on the centre stand, is because there were 4 different centre stands for Commandos. The very first series (pre '71) had the stand attached to the frame. The next on the '71 p/n 062192 attached to the gearbox cradle, superseded by the '72 750 p/n 063051 (the '71 frame was shite) then the more robust '73 850 version p/n 064031 which has a bigger insert hole 9/16' which also carried over to the Mk3. Don't ask me how I know this.........

Mick
 
Status
Not open for further replies.
Back
Top