Tire sizing question.

puftas

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Recently bought a '77 xs650, stock set-up for the most part, Beatiful bike. I'm in love... However:

Realized shortly after buying that tires were woefully underinflated. Fixed that quick, and then my problems start. A new noise, whining when decelerating under 20mph. Not tied to RPM's just wheel rotation, thought it might be cam chain.
Turns out the PO was running a 130/90 x 18 which when properly inflated rubs significantly on the Rear Brake Torque Arm. That's my noise, now the fix.

Can I just downsize the tire without changing the wheel? Specs for this bike's rear is 4.00 - H18, but I'm not sure what aspect ratio that old measurement has. Will a 130/80 work for me or do I need to get a 110/90, which is the best fit in modern measures.

If you've got any experience with this year (after 79 they ran spec 130's I think, obviously that doesn't work on mine), please let me know what your experience taught you.


Also, proper inflation totally whacked my rear drum calibration out of order. Drum is constantly ingaged, haven't had the time to check whether a simple adjustment will fix it. ALSO, around the same time my valves started knocking at startup. Can't imagine that's possibly related but I'm not an authority on the subject.

Ride on.
 
Hi puftas and welcome,
XS650 Specials run a 130/90-16 rear tire and some of those can lightly kiss the torque arm.
As a 130/90-18 has a 2" larger diameter it will rub on the stay even more because the stay runs in towards the tire as it approaches it's forward attachment point.
The 130/90-28 is too effin' fat and a 130/80-18 is the same effin' width, you need a narrower tire. Stay with the recommended 110/90-18.
Also, how old are the existing tires? Check the tire sidewall for the age stamp and if they are more than 5 years old get new ones because even if the existing tires look perfect they are getting age-hardened.
WHAT rear drum, what you say about that don't make sense?
Nor do I see any possible link between rear tire rub and valve clatter.
 
Hi puftas and welcome,
XS650 Specials run a 130/90-16 rear tire and some of those can lightly kiss the torque arm.
As a 130/90-18 has a 2" larger diameter it will rub on the stay even more because the stay runs in towards the tire as it approaches it's forward attachment point.
The 130/90-28 is too effin' fat and a 130/80-18 is the same effin' width, you need a narrower tire. Stay with the recommended 110/90-18.
Also, how old are the existing tires? Check the tire sidewall for the age stamp and if they are more than 5 years old get new ones because even if the existing tires look perfect they are getting age-hardened.
WHAT rear drum, what you say about that don't make sense?
Nor do I see any possible link between rear tire rub and valve clatter.
 
Thanks man. Simple advice to follow. Appreciate. New to the bike and trying to be a good owner. Yeah the brake stuff has me confused. I'll post again once I figure it out.
 
that way to ide tire is not only rubbing on the brake strut it's rubbing on the brake rod. The pressure on the rod is what is binding the brakes.
Leo
 
went with 120/80, fits great, no problems. We'll see if that's still the case next time I replace the chain.
 
I know this is an old thread but it's gonna be new to somebody else just like it was to me.
I'm adding to this because I was somewhat appalled at the incorrect info that I found when I searched this topic so now I'll humbly offer what I've just learned.
Stock tire size for my '77D is 3.25/19 front and 4.00/18. Those numbers represent inches so if you want to know the modern size equivalent just google 'inches to mm' and use one of the converters that you find. I did and the nearest proper sized front tire is 90. Many people say 100, and a 100 will fit, as will a 110, but they are oversize and will affect your handling. Rear tire is 4 inches = 101.6mm = 100 for tire size. Again, many people say 130, and a 130 will fit, but it's waaay oversized.

I purchased this bike last year (see avatar) with Kenda Challengers sized 110 in the front and 130 in the rear. Handling was clunky and leaning was dangerous. I could barely swerve around potholes. Even the most mild mid-speed turns felt squirrel-y. I read in another post that when a tire is put in a rim that too narrow it affects the contour of the tire as it hits the road and can mean less rubber on the ground.

I now have properly sized K70s and it's almost like a different bike. To each his/her own though...
Ride safe y'all
 
Hi stereobongos,
it's the slash that makes the difference. Inch-sized tires are square; that is, the tire cross-section height is equal to it's width.
So yes, a 90mm tire is the same width as a 3.25" tire but as the 3.25" tire is also 3.25" high and a 90/90 tire is only 81mm high your so-called equivalent
metric tire ain't, it has a smaller outside diameter.
You gotta fit a 100/90 front to get back to the design tire OD.
Same deal with a fitting a 110/90-18 rear to get the design rear tire OD instead of fitting a 100/90 to get the design width.
Agreed; 110/90 front and 130/90 rear is one step too far.
Also note that modern thought puts tires on wider rims than they reckoned to be OK back in the 1970s.
 
Hi stereobongos,
it's the slash that makes the difference. Inch-sized tires are square; that is, the tire cross-section height is equal to it's width.
So yes, a 90mm tire is the same width as a 3.25" tire but as the 3.25" tire is also 3.25" high and a 90/90 tire is only 81mm high your so-called equivalent
metric tire ain't, it has a smaller outside diameter.
You gotta fit a 100/90 front to get back to the design tire OD.
Same deal with a fitting a 110/90-18 rear to get the design rear tire OD instead of fitting a 100/90 to get the design width.
Agreed; 110/90 front and 130/90 rear is one step too far.
Also note that modern thought puts tires on wider rims than they reckoned to be OK back in the 1970s.

Hi fredintoon,
Thanks for this. I don't doubt that you know what you're talking about but I'm just trying to fully understand. You say a 3.25 is 'square', I get that. But how is a 90/90 not 'square' when that's what the numbers say? 90mm wide and 90mm high?
Sorry but I'm not the kind of person that just accepts what he's told. I need the full story to understand!

And what happens when the OD is slightly less than spec? The speedometer is inaccurate? Do you guys actually look at your speedometers? I don't but I'm not judging.

Your last sentence explains what I read on another thread describing how a wide tire on a narrow rim equals a smaller contact point on the ground because the tire becomes more rounded than it's designed to be. I lived it for a year and it sucked. I'd say rubber on the ground is most important to me but again, to each their own.
Again, thanks!
 
Hi stereobongo,
sorry about that. I should have explained it better.
The number after the slash ain't a measurement, it's the tire height expressed as a percentage of it's width.
So a 90/90 tire is 90mm wide and 90% of it's width (81mm) tall.
You can do the arithmetic on other sizes to see the differences. BTW /80 down to /60 ratio tires are also available.
What happens when you have a smaller diameter tire is kinda complicated.
On the front:-
The bike's stance will be a tad nose down which decreases the fork's effective rake angle which quickens the steering.
The front wheel will be turning quicker so your speedo will read fast.
On the rear:-
The bike's stance will be a tad nose up which increases the fork's effective rake angle which slows the steering.
The rear wheel will be turning quicker so your motor will be always revving higher.
Note that all these differences are quite minor and some of them cancel each other out.
BTW, like most Yamaha speedos mine has deteriorated over the years so while the odometer still reads OK the speedo is hopelessly slow.
And the tach has gotten a bit dodgy too.
What I do in town is simply keep up with traffic. What I do on the highway is follow a family sedan.
 
Speedometer error was common for Japanese bikes. A magazine road test that I recently read complained about the XS's speedometer error being more than average for Japanese bikes, and that was when they were new. Another thing that really bothered the testers was the squirrely side stand, leaning the bike far to the right to lower it.

Scott
 
The number after the slash ain't a measurement, it's the tire height expressed as a percentage of it's width.
So a 90/90 tire is 90mm wide and 90% of it's width (81mm) tall.

I actually read that elsewhere but temporarily forgot, thanks for the reminder, and for all the other info!
 
The Special drum brake stay is dimpled and may provide some additional clearance .....

SpecialTorqueArm.jpg


But I agree, fatter than stock tires are no good on these. Even that 120 you installed won't handle as nice as a 110. My bike came with a 120 rear. I used it up then switched to the more correct 110. There was a very noticeable difference, and for the better I might add.
 
Hi Guys
backalong I swapped my Heritage Special's cast artillery wheels for a pair of '76 aluminum rim wire wheels for looks sake.
Then I swapped them back for practicality. And to keep peace in the house. My dear wife bought me a new back tire, alas that it was a 130/90-16.
My experience with the ride difference between the 110/90/18 tire and the 130/90-16 tire (they have just about the same OD) on the same bike was that the fatter tire ran steadier in a straight line and the thinner tire cornered better.
BTW,
nothin' fatter than a 130 width tire will fit between an XS650's chain guard and it's brake torque stay. And some of the fatter 130s have trouble doing tnat.
going fatter than a 130 involves spacing the rear wheel sprocket out, installing a stepped transmission sprocket and modifying the rear frame.
 
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I'm not stating this as fact, just an idea. Seems that differences in tire size/brand would be more noticeable on a light bike vs a heavier bike.

Scott
 
Hi been reading your posts on tyre sizing bought mine this year please see avatar its a 1980650 sg special ive altered it to look more like the roadster. I'm in the uk england this is a us import ,imported back in the early eighties to the uk.
it currently has chinese tyres on at the moment they look ok but im not happy,I want avons or dunlops is there a tyre that fits thats a decent trusted make ?
 
Many "chinese" tires work well, very well indeed, I have put a lot of Shinko's on my bikes including high horsepower sport bikes and heavy cruisers with no issues. Many "traditional" brands have their tires made in China, especially the lower cost units.
 
thats very reassuring news ive got chengshin tyres on at the moment ! no doubt there's a lot being made out there on the cheap sold back to us at high cost but i'm a lover of a brand name on all my harleys i had avons and dunlops had the bridgestones on a vtx1800 bought it new got through two front tyres in 4 months rubber was good but too soft.actually didnt know how the rear performed as i had to have it replaced after a month had a rear nonrepairable flat.
 
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