Which size? 17 inch or 18 inch tyres

[gavinnz]

Which size? 17 inch or 18 inch tyres.
The machine is in the planning stages only and is a classic racer based around a 1950's Moto Guzzi 500 ohv flat single engine.

Assuming these things.....

The frame and chassis geometry would be built to suit the tyre size.
In either 17 or 18, the tyres would have the same narrow width (90 or 100 front, 110 rear), construction and profile.
The same wire spoke wheel construction.
That the tyres are easy to get and they meet all regulations for the class.

Ideas I have had... wrong or right??


17 inch tyres.....

Less overall bike weight in spokes, rim, tube and tyre. All unsprung.
Lower overall centre of gravity.
Less and lower frontal area.
Shorter wheelbase.
Lower seat height.
Engine can move forward in the chassis.
Less total mass that is rotating at a smaller diameter....
Wheels would be easier to accelerate and decelerate... meaning more braking power can go into stopping forward momentum and more engine power will get to accelerate the bike forward.
Easier and faster turn into corners.
Being a flat single I have room above the engine to bring the engine up to achieve needed cornering clearance and bring the tank down for low overall build height.


18 inch tyres.

More cornering clearance.
Longer tyre footprint.
More high speed stability.


Seems a no brainer.... so what have I missed???
Regards
Gavin

[Kickaha]

What age bracket class is it being built for?, what wheels do the rules for that class allow?

[blackdog]

Longer tyre footprint.

you would be measuring the difference with feeler gauges......

More cornering clearance.

1/2" upright is only 1/4" at 45 degrees..........

[gavinnz]

What age bracket class is it being built for?, what wheels do the rules for that class allow?

"Assuming these things.....
....and they meet all regulations for the class."

Regards
Gavin

[gavinnz]

you would be measuring the difference with feeler gauges......



1/2" upright is only 1/4" at 45 degrees..........

I agree.
Gavin

[quickbuck]

Go with the 17's.
I think you missed one: Tyre Availability.....

[blackdog]

Go with the 17's.
I think you missed one: Tyre Availability.....

was thinkn' that, and price too. like car tyres more common=cheaper ie/ 17"s less than 16"s these days

[george formby]

Go with the 17's.
I think you missed one: Tyre Availability.....

My TDM has an 18" front which I am perfectly happy with but my tyre choices are very limited. Sports Touring or Adventure tyres, thats it, nothing soft. Damnit.
I must admit the 110/18/80 gives a quick turn in without flopping.

[quickbuck]

I must admit the 110/18/80 gives a quick turn in without flopping.

HECK, that is one very big tyre (Yes, yes, I knew what you ment).

I would actually say the "Flopping" issue you have had in the past would actually be the shape of your rear tyre.... Could be wrong (often am) and it could well be a different sensation to the Flopping I experience....
Reminds me. Time to book the bike in for my Metzler M3 Front (now the rear is scrubbed).

[scracha]

My TDM has an 18" front which I am perfectly happy with but my tyre choices are very limited. Sports Touring or Adventure tyres, thats it, nothing soft. Damnit.
I must admit the 110/18/80 gives a quick turn in without flopping.

Super soft sticky tyre = BT-012SS. The best I've used in the wet but wears out real fast. Metzler Z6 is best of both worlds on them but currently trying out a BT020 as much cheaper. Is your 98 a mark 2? I'm sure mine had a 120/70? The tediums tend to flop when the rear tyre is fooked or some dick has dropped the front forks too much.

[quickbuck]

The tediums tend to flop when the rear tyre is fooked ......

Heck,
You mean I am right more than once today????

[scracha]

Heck,
You mean I am right more than once today????

You'll make up for it tomorrow. Hell, you'd be as well just staying in bed.

classic racer based around a 1950's Moto Guzzi 500 ohv flat single engine.

We have all missed the main point of this thread. The guy is obviously a masochistic pervert.

[quickbuck]

You'll make up for it tomorrow. Hell, you'd be as well just staying in bed.

Bugger it, I'm taking the rest of the week off then.

I was actually right on half a dozen occasions today..... Made some very good educated guesses that turned out to be true.

Yup,
Tomorrow will be real sh1t...... Not good when I fix helicopter engines....

[Metastable]

Go 17, you have much more tire choices. All you need to do for the rear is put on a common front tire such as a 110/80-17 or .... a 120 that is pretty much what any sportbike takes.

Just make sure you install it in reverse tread pattern. The tread pattern should look like
>>>>>BIKE<<<< You could even just install the same thing on the front with regular rotation. i.e. I'd either go 110 on both or 110 front and 120 rear.

They handle pretty well apparently. It is what the guys do here in the CBR125R racing up, because they are mandated to use spec Pirelli tires.... and Pirelli doesn't make tires for that bike.
Read about it here... you can also see what it looks like:
http://www.passionperformance.ca/mot...articles/3832/

The 120 rear might take a bit more abuse.

[RDjase]

For pre 72 the regs are 18inch minimum and 2.5 max rim width.

MNZ Rule 25-3-2

I got that out of the rule book, cant seem to get the website to work tho. just comes up blank pages

Try these links

http://www.mnz.co.nz/competitionrules.aspx

http://www.motorcyclingnz.co.nz/down...st_Classic.pdf

http://www.motorcyclingnz.co.nz/down...ng_Classic.pdf

I run Pirelli Sport demons on my LC and they work well, good in the wet too. 100/90/18 front and 110/80/18rear. on 1.85 rims

http://www.nwa.co.nz/product_detail.php?P_ID=3150

I got some used Avon AM 22 and 23 to try. At $900 a pair I will probably stick with the demons at $320 a pair tho

Bridgestone BT45 are another option