'86 Vf500 Rear Tire 110/90 Vs 120/80

I read quite a few threads here (and elsewhere) about rear tires on a '86 VF500 but never found a good answer for this:

110/90-18 versus 120/80-18

I'm replacing front & rear with Sport Demons and they offer both sizes for the rear. If the performance is the same, I'd vote for the slightly wider tire for purely aesthetic reasons, but there's always a drawback. What's the drawback for the 120/80?

Here's my typical use: I haven't seen a trackday in 20 years but wouldn't mind doing one this summer. Most of my riding is around town, but I live in Denver and the mountains have thousands of miles of nice, twisty roads.

The bike has been in storage for the last 12 years, so I'm a little out of touch with the available options.

Thanks!

I had an 86 model in the 80's. Back then, the Dunlop K591 Sport Elite and Metzeler Comp-K were the cutting edge of bias ply sport tires. The 120 sized Dunlop K591 rubbed the swingarm, but they had an odd profile with the tread sticking out farther than the sidewall. I've heard of people running modern 120 sized rear tires without issues. What's funny is that the K591 is now a standard Harley Davidson touring tire! LOL!

I went out the wash the relic and saw it already has a 120/90... and it's the Dunlop K591. This is what I get for posting before double-checking.

I looked all around the swingarm and stuck my fingers back there and couldn't feel any contact.

Any idea what would make a difference between a /90 and an /80?

Acemon

I went out the wash the relic and saw it already has a 120/90... and it's the Dunlop K591. This is what I get for posting before double-checking.

I looked all around the swingarm and stuck my fingers back there and couldn't feel any contact.

Any idea what would make a difference between a /90 and an /80?

Maybe my buddy had 130's on instead of 120's. I do remember seeing the edge of oversized K591's rubbing, but I guess I don't remember the size. I was thinking the OEM Bridgestones were 110 rear and 100 front. I didn't go oversize on mine and I know my Dunlops didn't have much clearance.

If a 120 doesn't rub, I'll stick with it but I'll keep the front stock.

Acemon

I went out the wash the relic and saw it already has a 120/90... and it's the Dunlop K591. This is what I get for posting before double-checking.

I looked all around the swingarm and stuck my fingers back there and couldn't feel any contact.

Any idea what would make a difference between a /90 and an /80?

120 = nominal tire section width in millimeters

/80 = aspect ratio (tire height expressed as a percentage of the width)

So in this case, the height of the tire from the ground to the rim is 80% of 120mm

The other choice is 90% of 110

From that you can figure the total height and circumference to see how it will affect chassis attitude and final drive gearing.

120 x 0.80= 96mm tire height

times 2 (for the top half of the tire) = 192mm

convert to inches 192/25.4= 7.56 inches.

7.56 inches of tire plus 18 inches of rim = 25.56 inches overall height.

The other tire:

110 x 0.90 = 99mm times two = 198mm

198mm/25.4 = 7.83 inches plus 18 = 25.83 inches.

So the 110/90 is taller which will raise the rear of your bike and also the gearing.

110 is 0.27" taller

divide by two is 0.137 inches.

So using the 120 would lower the rear of the bike 0.137 inches or 3.5mm

Hardly enough to worry about.

The difference in gearing is 1.1%, again, hardly enough to notice.

Might raise your RPM on the highway by approximately 50-60 RPM, I dunno what your bike spins at 60 MPH, so I grabbed a number out of thin air.

Like Veefer800Canuck says, a 120/80 will theoretically lower your rear end slightly, but as its some wider, leaning down the height might be the same. A 120/90 will rise your rear end quit a bit, so can be issues whit stability if the steering is fast as stock, will also change the gearing in a negative way, bike will feel slower.

Now, as tires has a tolerance, the actual size difference between brand can be huge. For examble, a old 120/80-18 Metzeler ME99 is as big or small as a 110/90-18 Pirelli Sportdemon! And both are way bigger than a 120/80-18 Bt45 That on same rim, so a stock size 110/90 SportDemon might be just enough.

I understand how the sizing itself works, I was more curious how/if the slightly different tires would affect handling. I think the best answer is that I'm not coming anywhere close to the potential of bike and tires, so it's unlikely I would know if one is better or worse. Now, if it was someone like Eddie Lawson, he could tell me exactly what separates them and his advice would be to spend a lot more for stickier, low-mileage rubber.

Regardless, I appreciate the math and advice.

I bought the Sport Demons with the stock sizes and I'm sure they'll work just fine for my style, which often seems slow and awkward.

Acemon

I understand how the sizing itself works, I was more curious how/if the slightly different tires would affect handling. I think the best answer is that I'm not coming anywhere close to the potential of bike and tires, so it's unlikely I would know if one is better or worse. Now, if it was someone like Eddie Lawson, he could tell me exactly what separates them and his advice would be to spend a lot more for stickier, low-mileage rubber.

Regardless, I appreciate the math and advice.

I bought the Sport Demons with the stock sizes and I'm sure they'll work just fine for my style, which often seems slow and awkward.

Eddie would tell you to upgrade the suspension because the OEM stuff is mushy and isn't keeping the rubber on the road.

I put the Sport Demons on my 86 500F. Stock on the front and a 120/80 on the rear. Definitely like the tires. Handling doesn't seem to be an increase in improvement to me BUT it definitely isn't worse. If you want a wider tire on the rear, you will have to remove the chain guard or modify it to keep the tire from rubbing it. Go have some fun on the Sport Demons.

:goofy:

Just ordered the stock sized sport demons for the '86 500 I'm resurrecting. Man those things are tiny. After fitting the pr4 on my 6th gen, these look like rubber bands! Even though the 120s look a little better, I like keeping the stock size for this particular bike.

Eddie would say just ride the wheels off it when you fall off just go a slight bit easier and keep it on 2 wheels .If in doubt gas it !

Since the mushy suspension was mentioned, what are some suggestions? I've considered progressive springs, but are there emulators that would work with the semi-anti-dive?

Rear shock? Maybe rubber blocks between a coil or too, just like the ones used for sagging trucks? (laughing)

Acemon

Since the mushy suspension was mentioned, what are some suggestions? I've considered progressive springs, but are there emulators that would work with the semi-anti-dive?

Rear shock? Maybe rubber blocks between a coil or too, just like the ones used for sagging trucks? (laughing)

I suggest contacting Jamie Daugherty. http://www.daughertymotorsports.com/v4section.html