18" rear tire rim guru question

[stereomind]

I just took the 18" rear wheel off my 750 Seca (120 Dunlop) and replaced it with a Virago (or Maxim, dunno) 16" wheel (130 Kenda).

The main reason - wider rear tire. The narrow 120 on the 18" wheel was not very corner-friendly. The secondary reason - I brought home a 3 1/2" nail in the original tire

Overall, ride height changed less than 1".
I have to rev about 300RPM higher to go 65-70. But.... I have more take-off power and the bike can lean around the corners like a crotch rocket (well, almost).

[fredintoon]

Yeah, theoretical.

Hi Ernie,
like you imply, the numbers on a tire really mean that it's nearer to that size than to the next one up or the next one down.
I am familiar with the XS650 where the 130/90-16 rear and the 110/90-18 rear can be swapped over on most models as a fashion statement. (The 16" is usually a cast "artillery wheel" like on the XS11 while the 18" is a wire wheel.) Now those two tires have just about the same OD. In theory. Depending on the Mfr. Or how worn one is compared to the other.
With Mike's 120/90-18 vs 130/90-16 swapover and all of the variations between a tire's actual vs it's theoretical cross section measurements (strange, the bead diameter is always spot on, eh?) I can see that there may be an inch or a tad more difference between the ODs of those tires. But Mike claims a 15% revs reduction due to his bigger OD rear tire. My mathbox says that a tire 15% bigger than the 26.8" OD would be 30.8"OD.

[CarolynS]

STOP

You guys are making my head hurt!
Percents, and algebras, and equations-Oh, MY!!
Just kidding...
So much for being a bunch of 'dumb' bikers!!
You all scare me sometimes...

[DiverRay]

IF that's not enough for you, some of us play with neutrons and protons for a living!!

[CarolynS]

Cool!

I love fig neutrons!! They're delicious, especially with vanilla ice cream.

Kidding, I'm kidding....

[fredintoon]

"Neutron, the atomic superman"

Remember him? Black & white movies from South America dubbed into poor English on the "Chiller-Thriller" late night movie slot?
The protagonist was a mild mannered news reporter who rode about on a sidevalve motorcycle and metamorphosed into Neutron the Atomic Superman to thwart the bad guys. Hairy bare chest, hood & long gloves combo, white tights and his bum stuck out (not nekkid, he just had a fat arse.) As a nuclear designer it amused me that the movie writers presumably did not know that a neutron has no charge, making their hero a eunuch.

[Wildkat]

Here's a fun toy...

Tire Size Calculator

That may or may not be of use...



Ray... playing with neutrons and protons...

[egsols]

My mathbox says that a tire 15% bigger than the 26.8" OD would be 30.8"OD.

I agree that 15% may be a little optomistic. Based on my abacus I get about 6% which is only 40% of the 15% claimed being a 60% reduction in the original estimate.

Its like the golf study they recently did whereby they came to the conclusion that 95% of putts that don't reach the hole, don't go in!

[soccer4m]

final proof of rim size conversion

Here ye Here Ye, after reading the great replies from all, I got down to the nitty gritty and measured the overall diameter to the 1/10 inch with a tape measure. Lo and behold the rim meas. were 83.75 inches for the 18" and 72.1" for the 16" rim on my xj. The percent gain was 16 percent verifying the orig. estimate. Actually, I just wanted to know which rim the bike had on it . A pleasant surprise to find out the revs were way down on the highway compared to the 16 inch xj metzeler. So hope the pic link works, maybe others will want the easy conversion.....regards, Mike in Sun Diego

80 special, found and slowly restoring
82 xj1100, a smooth beast pics

[Paintman]

18 inch rim

Hey Soccer what did your revs drop to on Hiway i have a perfect 18 inch rim here with a brand new tire off a 750 i also have a 750 FD that i was going to use for my conversion > i wonder how combining both would work it may be just like having that 6th gear we always look for .

john

[soccer4m]

added note

Excuse me before I get persecuted,,,not the radius or diameter but the circumference, measurements were 83.75" versus 72.1" for a 1.16 ratio or 16 percent gain in size and travel for the 18 inch tire versus the 16" tire.......Mike, SD,

[soccer4m]

revs

Hi Paintman,,,since the speedometer isnt recalibrated I found that traveling at the same revs, I was going 9 mph faster at the same revs, when the speedo said 60 mph. My son drove along me and signaled at 60 mph his speed and mine said 51 mph. I guess there is also a 15 percent reduction in revs. Dont quote me but I think the revs were under 3500 at the actual 60mph on the xs 18 inch tire, and my xj, 16" tire is revving at 4200 at 60 mph. Noticibly much less engine revs, and the engine pulls it, no problem, but it is slower off the line. Once in second gear, the engine pulls smoothly. So, the easy 6th gear fix is here, just get the xj 18" rim, pop it on and drop the revs.....I am glad the xj rims fit without mods{info, thanks to Rob in England},,,,good luck,,,,Mike

[egsols]

Ah HAAA!

Lo and behold the rim meas. were 83.75 inches for the 18" and 72.1" for the 16" rim on my xj.

In my math lab, based on bridgestone's numbers I did calculate the 120/90-18" tire at 83.88 inches in circumference. That's pretty close to what you measured!!!! The calcs for the 16" Bridgestone 130/90-16 however came in 78.85 inches. So me is thinking that maybe the 16" you got is smaller than normal? What make is it and is it a 130/90-16?

[oldnortonrider]

Hey, soccer~~re: that "2.15" (x18) rim that you're running~~(?)would that mean that the center/middle line of the "2.15", (and including the tire itself), would be running .35" inch further to the left, than the center line of the mt2.50X16 (or 17") that originally was on it?

[oldnortonrider]

Re-computed ; left side of the rim is still in the same place; make that half of .35" further to the left.