Tyres and Tyre Pressures.... sooo many questions...

Ok... I know how important it is to keep these correct.

But as a general query, does the tyre pressure go down quicker if you use the bike a lot or if its left standing?

secondly, why put on a bigger tyre to the wheel? is it just aesthetics? I know it affects handling but for the better or worse?

thirdly, say a bike has a bigger tyre on it (eg rear cbr6 FL should be 130-80-17, my tyres a 150-80-17) what should I do about the pressures?

I dont know what it should be but Im assuming a standard 33/36 pressure split front/rear. If Ive got an oversize tyre, should I put slightly more air in to accomodate the size? I found a 36psi at the rear was the reason for the wierd handling... putting 38 in has stopped it...

Also, measurements - get the 130-80-17 is 130 width of tyre and 17 is the diameter of the tyre. But whats the 80? Ive been told is a percentage which relates to the height but what is it exactly?

The first number is the width of the tyre, the second is the depth or the profile and the third is the diameter of the tyre. The letter at the end is the speed rating that the tyre can be safely used at.

In general terms, people put "fatter" tyres on the rear than what is standard purely for asthetic reasons.

Tyre pressures are an ongoing issue. I would check what the manual says in the book and then check what the manufacturer of the tyre says.

The tyre pressures for the SV in the manual are different to what Bridgestone say the tyre pressures are. I go for what Brigestone say

Originally Posted by lynw

But whats the 80? Ive been told is a percentage which relates to the height but what is it exactly?

It's the aspect ratio, i.e. height/width*100.

FWIW, My Hornet has 120-70-17 on the front and 180-55-17 on the rear.

The recommended pressures (Bridgestone BT56 tyres) are 36psi front, 42psi rear. Your CBR will not be much different, so try these pressures

thanks.... but whats the aspect ratio for? why have it?

have michelins at the moment so will try higher pressures....

Originally Posted by lynw

thanks.... but whats the aspect ratio for? why have it?

Because you can buy different aspect ratio tyres that will fit the rim. The higher the ratio the deeper the tyre. This will affect the ride and handling of your bike.

Ask Yamaha about the R6 front tyre

Originally Posted by lynw

have michelins at the moment so will try higher pressures....

Yeah - start with 36/42psi and fiddle about a couple of psi either side.

Originally Posted by lynw

thanks.... but whats the aspect ratio for? why have it?

have michelins at the moment so will try higher pressures....

A higher aspect ratio or sidewall height, will give a somewhat smoother ride, but will seem "wallowy" compared to say lower number. Higher numbers usually lean toward touring type tires, and lower numbers toward sport oriented riding. It is generally not a good idea to change these numbers too much from standard, as it can have a negative effect on handling, if not done correctly. I've heard of people who race SV's using a 120/70 front and 150/70 rear instead of a 120/60. front and 160/60 rear. I am told the standard 160 on SV's is too wide, and a 150 will improve handling. (this could explain why the rear tire chicken strip doesn't go away ) I personally am not one to trust all I hear tho......

[jambo]

120/70 R17:
120mm wide
to fit 17 Inch rim diameter
height is 70% of width of the profile so 120 * 0.7 = 84mm above the rim at it's highest point.

Wider, lower profile tyres give a greater amount of rubber on the floor so when lent oer you have a bigger contact patch and more outright grip, but are slower steering because of this. A narrower, higher profile tyre vives less outright grip but a more nimble handling bike.