Tires vs rim sizes.....

[salty_monk]

Came across something today... My Husky TE610 runs a 21" front, 18" Rear. Currently running a 90/90/21 & a 120/90/18. D606 so pretty aggressive knobs (on it when I bought it).

I recently bought another set of wheel from a TE630 which are the same but just black in colour as far as I'm aware. They came with some 90/10 tires on (Scorpion Trail) and the sizing was 90/90/21 & 150/70/18.

The rims are 1.6F & 2.15R. (both sets).

Got me to thing that rear was really oversized as I know a 150 is too big on our 2.5" GS rear wheels. So I went & looked up on some tyre charts... to find that the charts say both are technically too wide for the rim sizes. Recommended 80/90/21 & 110/90/18.

So... I went to look in the manual for the 610. Karoo's in a 90/90/21 & 140/80/18 as stock.

All seems bizarre to me... what would you run on these wheels in my situation? The Husky forums seem to suggest a 130/80/18 and a 90/90/21.

Also anyone have a good explanation of the difference running an 80 profile vs a 90 at the same width (yes I know the sidewall is circa 10mm taller but what does that do to the feel/contact patch or whatever?)

I like the current set for off-road. Going to run a bit more road friendly set on the other set of wheels....

 

[zed1015]

Running a tyre wider than specified results in sidewall flex and poorer handling especially on road tyres.
However running a lower aspect ratio ( lower sidewall) would go someway to reducing that effect.
Knobbly tyres usually have a more substantial carcass anyway so going from 120-90 to 150-70 probably makes little or no difference in that application.

 

[Gorminrider]

My personal experience is limited but he lower profile of one of my other bikes definitely feels different cornering than the almost-round tires of my old gs's...I note more air pressure is wanted for the lower profiles even where rim sizes and bike gross weights are nearly the same....

A lower profile with the same width will likely give more space around the tire in the swingarm but a larger width on the original profile -even 10mm can make the space notably smaller. Likewise in the fenders where it's already close to the tire with the OEM tires...scraping noises as mud or gravel gets pulled in a narrow space.

A different width might be the worst per what zed is saying above- i guess you are effectively forcing the tire into a rounder or flatter profile against its will.......so there's a list of things to be careful of. particularly on road bikes...I'd be thinking carefully just increasing 10mm on any one dimension of replacements. On a dirt bike I guess there'd be different considerations than a street bike but it'd be a good idea to look for them before buying.

 

[salty_monk]

Running a tyre wider than specified results in sidewall flex and poorer handling especially on road tyres.
However running a lower aspect ratio ( lower sidewall) would go someway to reducing that effect.
Knobbly tyres usually have a more substantial carcass anyway so going from 120-90 to 150-70 probably makes little or no difference in that application.

Yeah this makes sense. The 150/70 is the same height as the 120/90 give or take a couple of mm.
I guess I'm going to stick with a 120-130 going forward in general but I won't be scared of running that 150 for a bit (unless it handles like sh*te when I ride it). It's on its last legs from an age perspective anyway so I won't be riding on them for long.....

 

[bwringer]

On vintage bikeswith 90 series tires, I know stuffing 80 series tires on a too-narrow wheels results in very unpleasant handling from the hamburger shaped cross-sections rather than the nice smooth and round profile of the correct sizes.

And I know firsthand that stuffing a 110/90-19 on the front of a GS (most models) rather than the correct 100/90-19 makes an astonishing difference for the worse. The rear is less sensitive.

120/90 to 150/70 seems like one hell of a leap to me, but maybe it works on that bike? Usually going up or down one size is acceptable, but larger will rub on the chain and/or swingarm, smaller will get squirrely.

But if it works, it works.

 

[salty_monk]

Yeah, funnily enough the profile doesn't look too bad. I have a 150 Shinko 705 mounted on a BMW GS800 (4" rim) in my garage to compare with side by side & it doesn't appear too pinched.

I just think it's really weird that they would use a 140/80 on a 2.15 rim as stock. There is some discussion on the forum if it was an "that's all they could get" sort of decision & they always intended for it to be a smaller size...

 

[salty_monk]

Well they aren't as bad as I had expected but they do feel a bit like riding on a worn tyre. Whether that's because the tyre is a little square or because the pinch profiles them that way I don't know but I definitely prefer the handling of the 120 knobby to this 150 90/10....