what size tyre can/should i fit?

[Maffoo]

I have an early 80's (1981 i think) CB250N with a 100/90-19 tyre on the front & a 18-3.50 tyre on the back
the back tyre looks waaaay too skinny, & the ass of the bike feels really unstable at 100kms

the rear rim is a 18x1.85 ....

what is the biggest tyre i can put on the back? is there an optimum size i should get? & should the front & rear be the same or should the back be wider than the front?

any help would be appreciated ... thanks

matty

[yod]

http://motoprofi.com/motospecspictures/honda/cb_250_n-1981.html

say stock rear should be 4.10

[Maffoo]

ive tried looking on trademe at the tyres there & they all seem to have a sizinf system more like whats on the front tyre ... IE 150/70-18R or similar

how do the sizings relate?

is 4.10-18 in inches... & 150/70-18 in mm?

[Grub]

We have a lot of people asking these questions and the best advice I have ever heard was from a racer who knew and understood the very complicated geomertry designed into each bike. His advice ... fit whatever is the factory spec sized tyre and no other (unless you really know your shit)

Think about it, Honda spend a higher pecentage of their revenus on R&D than any other manufacturer. They have supercomputers, CAD modelling and all the other trick stuff to design and evaluate bike design, setups and specs. They also own Suzuka Circuit and they employ hundreds of test and development riders. All of these resources came up with the spec of tyre for your bike.

Putting ... "the biggest tyre" ... on the back may just turn it into a pig at the very least and kill you at the worst. Screw it up and you can stop your bike from tipping in, you can foul the swingarm you can mess up the rake and trail ... etc etc

[Maffoo]

which is why i asked for 'optimum' size :)

i cant find anything using the old '4.10' sizing.. so i need to know what that tranlates into in new (metric) sizing

im looking at something like this
http://www.trademe.co.nz/Trade-Me-Motors/Motorbikes/Parts-accessories/Wheels-tyres-brakes/auction-137756201.htm

[yod]

ive tried looking on trademe at the tyres there & they all seem to have a sizinf system more like whats on the front tyre ... IE 150/70-18R or similar

how do the sizings relate?

is 4.10-18 in inches... & 150/70-18 in mm?

4.10 would probably be something like a 110/120 and given its an older machine I would think it would probably be an 80 profile....might pay to ask a shop tho

[Grub]

i cant find anything using the old '4.10' sizing.. so i need to know what that tranlates into in new (metric) sizing

im looking at something like this
http://www.trademe.co.nz/Trade-Me-Motors/Motorbikes/Parts-accessories/Wheels-tyres-brakes/auction-137756201.htm

That auction tyre may be way different in sizing to what is supposed to be on the bike. It's fair to guess that someone has just put any old thing they could get on it. You really need to find out what the original spec is for the bike and then go and see what is currently available from a knowledgeable shop dude.

Lets de-code the metric tyre sizing for 150/70-18
- 150 = 150mm and is the width of the tyre across the tread
- /70 is what is commonly called "a 70 series tyre". That means that the height of the tyre sidewall is 70% of its width. The tyres you see on boy-racer cars can I think be as low as 45%.
- 18 is the wheel size in inches.

Dumb isn't it, you have both metric and imperial in the same specification. So the correct series tyre can be really important as that is where the contact point of suspension geometry and road is defined.

I just looked up the CB250N for '80 and '81 and the tyres are the same for both years
1980/81 = Front 3.60-19 Rear 4.10-18

The last of the CB250N's was in 1986 and the same tyre is specified. I was hoping that they'd changed to a metric reference, I have no idea what the imperial measurement means. The bike shops all have the tyre manufacturers books with most bikes listed.
1981 =

[xwhatsit]

I like this site: http://www.conti-online.com/generator/www/de/en/continental/motorcycle/themes/faq/hidden/conversation_en.html

It works out with my front tyre perfectly, 3.00-18 becomes 90/90-18 (which is recommended by Honda). Your 4.10-18 would be 110/90-18.

[Maffoo]

Grub - i do know what the markings on a tire mean :) i am assuming that the 3.50 & 4.10 sizings are in inches

& in that case, that tyre in the link i posted would be perfect, no?

i might grab it .....



cheers for your help guys

hi xerxes :wave:

[pritch]

I can't link to Trademe from work. Somebody thinks the staff would waste time on the interweb thingie... Surely not :innocent:

It follows that I can't see where you are thinking of buying that tyre.

Tyres should be sticky to a degree and they lose their "stick" at about 10 to 15% per year. After three years (from memory) you are supposed to take them back to the factory to have their sticky checked. Nice idea, but a bit difficult hereabout.

Basically you want tyres to be as new as possible. Saving money on tyres is a bit like hiring a cheap lawyer, if you can manage it at all it's likely work out reeeally expensive in the long run.

[vifferman]

the ass of the bike feels really unstable at 100kms

Mr Matty Maffoo,
If the donkey of the bike feels really unstable at 100kms, it's more likely to be the suspension wots doing it than the tyres.

[xwhatsit]

It's a brand-new BT45, likely to be more than sticky enough for the old Wet Dream :D

Mr Matty Maffoo,
If the donkey of the bike feels really unstable at 100kms, it's more likely to be the suspension wots doing it than the tyres.
Hey, good point. Heavy bikes with piddly little swing-arms. Also used the infamous FVQ shocks -- pieces of shit. Hell, Magua snapped one a while back. Not uncommon to hear. Very little damping, useless springs, doesn't help the silly little swing-arm.

Also, chuck it up on the centre-stand and try pulling the swing-arm side-to-side. If there's any movement at all, the bearings are shagged and you need some new ones. Pain in the arse to do, but that will definitely help handling along.

Late 70s/early 80s Hondas -- don't ask me why I love 'em :laugh:

[Maffoo]

i scored a michelin M39 (i think) for a good price ($150 fitted) 110/90/18 ... nice tyre... & at least its not a ghung fung or dong wang or wtf the old one is called

ill try that swingarm shake idea too ... bike feels way better with the fatter tyre on it though :D

[Rosie]

I like this site: http://www.conti-online.com/generator/www/de/en/continental/motorcycle/themes/faq/hidden/conversation_en.html


A potentially useful link, thanks.
Dual purpose tyres come in both metric and imperial sizes, which makes for confusing tyre shopping :blink:

[vifferman]

Late 70s/early 80s Hondas -- don't ask me why I love 'em :laugh:
But.... I must!
Mr da Phat: why do you love late 70s / early 80s Hondas?
Is it because you didn't grow up with them? :spudwhat:

I did.
So now while the sight of a beautifully restored CB750 or similar may give me funny feelings of yearning for what were probably the best years of my life, I have no desire to return to the crapness of handling, braking, performance and reliability that was the 1970s motorbike.
My first Honda was reliable(ish) and easy to maintain, but the engine was asthmatic and uninspiring (apart from that first ride), the camchain was an evil thing that spent the few brief years of its life busily eating the inside of the engine, the fuel required regular dosing with Redex to stop the zorst rotting... need I go on?

I much prefer my 21st Century Honda, thanks.

[xwhatsit]

I much prefer my 21st Century Honda, thanks.

They're feckin' boring, though.

Not big on the Comstars myself, or the porky Wet Dreams, but you get a motorcycle like the 400 Four produced and that's enough to redeem the sins of three decades, not just one.

You had a CB500RS, you might recall the crude thrills and laughs that gave you :laugh: I'm sorry, but after doing some marginal kilometres on a GSXR600, I'd rather buy a car than be that bored again.

[MSTRS]

They're feckin' boring, though.

Not big on the Comstars myself, or the porky Wet Dreams, but you get a motorcycle like the 400 Four produced and that's enough to redeem the sins of three decades, not just one.

You had a CB500RS, you might recall the crude thrills and laughs that gave you :laugh: I'm sorry, but after doing some marginal kilometres on a GSXR600, I'd rather buy a car than be that bored again.

You need to do more than sit on it...:doh:

[xwhatsit]

You need to do more than sit on it...:doh:

Hahaha. No, but seriously, it needs the windscreen and fairings tossed out. And one of the cylinders cut off, so it vibrates and spits and farts. And a flexible twin-shock swing arm and forks that twist and untwist through bends. Then you'd have a fun motorcycle.

[Maffoo]

well two trips down the expressway & i like the new tyre !
still moves a bit but feels a lot more secure on the road

tried that swingarm trick xerxez, & its solid as a rock, no movement at all :D

[xwhatsit]

tried that swingarm trick xerxez, & its solid as a rock, no movement at all :D

Geeeeeewd, another thing to check is your steering head bearings. If they're a bit loose and shagged out they'll feel like that too.

Probably just the piss-poor rear shocks.

Glad to hear the tyre sorted most of it out. Win!

[FROSTY]

mafoo-90 90/19 front. 110/80/18 rear.
They have skinny tyres-thats life.
Wobles if all is tight a-bearing etc is shagged shocks-they wallowed when back shocks were worn out

[DEATH_INC.]

On the old car tyres the measurements were 1 to 1; IE 4.10 X 18 means 4.1 inches wide times 4.1 inches high on an 18" rim. 3.50 X 18 means 3.5 inches wide times 3.5 inches high etc....
4.10 X 18 is converted to: 4.1 inches = 104mm.... so you end up with 104 / 100 (remember this is a percentage of the height) X 18. No such tyre but if you go to a 110/90X18 you end up with 110mm/99mm X18. pretty close. 110/80X18 is 110mm/88mm X 18, not quite tall enough. Rule of thumb in car stuff is to try to stay within 2.5% of original height.
There ya go.....:eek: