tyre size question

[mainline]

could someone please enlighten me with regards to the metric equivalent of a 4.00 - 18 tyre for the rear of the YZ? I've been to a couple of websites that give you the conversion tables, but i'm not sure how the "aspect ratio" comes in it. Is this the equivalent of the profile in a car tyre?

Lastly, how would a 120-90-18 tyre compare to my old 4.00-18 version.

Cheers

[LWC82PE]

ok. a 4.00 is a 100 aspact ratio tyre. for a metric equivalant you would need to look at a 100/100 but you would have to check first before buying that the knobs dont stick out too far.

a 120/90 i would say is too big and is more close to a 4.60 which is a low profile

a 100 aspect ratio tyre is measured at the carcase
a 90 aspect ratio tyre is measured at the knobs

thats just the info i have got from some tyre books

a 100 ratio tyre means the tyres height is 100% of its width
a 90  ratio tyre means its a low profile and the height is 90% of its width
a 80 ratio tyre means its low profile and the height is 80 % of its width

[mainline]

Cool, thanks for that. I don't want to have to widen the swingarm to fit a tyre in 

[geraldo]

tire sizes are so variable though , a michelin S12 120/90-18 is no bigger than a 100/100-18 & fit nicely on 125's & 200's

[pbear62]

Hey LWC, The 'height to width' formula is how it's been explained to me in the past but I still have my doubts - on a car tyre size 235/45/17 for example - there is no way that the height of the sidewall is 45% of the width?? Buggered if I know!

[cyclegod]

Hey LWC, The 'height to width' formula is how it's been explained to me in the past but I still have my doubts - on a car tyre size 235/45/17 for example - there is no way that the height of the sidewall is 45% of the width?? Buggered if I know!

235mm = width of tyre
45mm = profile height from bead
17 = Diameter of rim (always in inches)

[Nathan S]

Hey LWC, The 'height to width' formula is how it's been explained to me in the past but I still have my doubts - on a car tyre size 235/45/17 for example - there is no way that the height of the sidewall is 45% of the width?? Buggered if I know!

It's supposed to be the width of the tread, rather than the width of the tyre carcass, which might be the difference.

The other thing is that tyre manufacturers often 'fudge' things... A World Rally Car tyre is supposedly 205/65x15, but if you put one next to a 'normal' 205/65x15 rally tyre, the WRC tyre is more like a 235/65...

The same happens with bike tyres. It seems that a Pirelli or Michelin 110/100x18 is about the same size as a Dunlop 100/100x18.

I'm still trying to find a supply of decent quality 4.10x18s (or 100/90x18s) for my pre-78 125s. The 100/100s are too big and heavy, and the 90/100x18 (3.50x18), but they look goofy on the semi-LTR bikes.

[geraldo]

TM Bill & I have successfully run michelin S12's 120/90-18 on TM125's & RM125S's , size is just right , not too big , don't be put off by the size , fit good , well constructed tire , nice light carcass

[mainline]

this one again, just bought a tyre for the front wheel which originally was a 3.00-21, the new one is a Dunlop 90/100 21.

I don't know if if because I've never seen it with a tyre on it, but it seems bulky. Do the sizes above for old and new match up?

[Doc]

80/100 is the size I run on all my 21" fronts Mainline.

Nathan, a nice 4.10x18 is the IRC Volcanduro VE-33 I use them on all my 125's (whenever I can afford new skins ) reasonably cheap and very oldskool to look at and they still use the old sizing on the tyre which is a bonus for any 'proper' type resto if original tyres are not available. Not too big also..last thing you want is to 'overtyre' a 125. Bill and I ran the IRC's at Coff's and they handled the loose really well

[Nathan S]

Cool, thanks Doc - that's exactly what I've been hoping to find.

[Quicksilver]

Where do ya get em Doc. What do they cost?

[mainline]

Thanks Doc, should make for an interesting conversation when I call the bike shop on monday to arrange a changeover. You wouldn't think such a small difference would matter, but as soon as the wheel was on the bike it just didn't look right.

[LWC82PE]

For a good wearing, period looking pattern cheaper tyre in 4.10 X 18 look at the brand 'Kings' i cant  remember the exact model but its a full on knobby with 4&5 traditional knobby pattern. its even DOT approved for use on roads even though it looks like a full on motorcross tyre. it is a low pofile tyre though. if you need a higher tyre then use a 4.00 or 100/100. ive got a 3.50 x 18 kings and 4.10 kings and there is stuff all difference. they are the same size! Kings are ok quality and a lot better then cheng shin (yuck!). i got a Kings on one of my bikes and ive had good reports from a few guys ive sold them to who ride modern 4 strokes, who dont want to pay mega bucks for or need michellins, metzelers, pirellis, trelleborgs etc

[Doc]

Quicky, I got mine up here at the local Yamaha shop but any reasonable shop should be able to order them. Bill may remember the price better but from memory around $70 but we did get trade price at the time..they do still retail fairly cheaply tho. I'll be talking to the guy from the shop later today so I'll let you know for sure what the price was here and where he got em' from 

I know what you mean about such a smallish size difference making it look odd Mainline. I went the other way not long ago with a 19inch TS125 front..it was only a 70/90, I figured it would be smaller but still 'okay'.. Wrong! It looked totally pox when fitted as it had no sidewall height and made the rear tyre look enormous!  Might be okay on Chinese cheapies or something if anyone wants it. Brand new, $20 takes it