Tyre PSI?

[ThePowster]

So today my CB700 Nighthawk S was finally road worthy and I had my first proper ride.

On the way back from the MOT station she handled really bad, I could barely take a corner.

I checked the tyre pressures and I had both 20psi front and back.

I checked the tyre wall for pressure limits but I couldn't see any psi information on them, so I put 40psi in front and back.

Does anyone know what PSI is needed in the Nighthawk ?

Seemed to ride 10 times better with some air in them, tipped into bends so easy.

Thanks in advance.

[Pete in PA]

Im not a big believer in the sticker on the bike.  On my 92 750 I run 42 rear and 40 front.

[mollusc]

From a post on the old version of this forum:

Rule of Thumb:
Tires should not increase pressure more than about 10% from cold to full operating temp.
Put 33 in each tire cold, and then run them until they get to full temp. If the pressure is more than 33 + 3.3 (36.3), reduce the starting pressure by .5 lbs and do the test again. If the pressure is less than 36.3, try increasing the starting pressure by .5 lbs and test again. It takes a few tries, but when you get it right, it will work perfectly.
It does not seem to matter whether the outside temp is high or low, the difference when the starting temp is correct just seems to work. This works if you have larger or smaller tires than the recommended, and is even easier when the tires are the correct size.

[ThePowster]

Thanks for the input fellas 👍

[Larry Fine]

Put 33 in each tire cold, and then run them until they get to full temp. If the pressure is more than 33 + 3.3 (36.3), reduce the starting pressure by .5 lbs and do the test again. If the pressure is less than 36.3, try increasing the starting pressure by .5 lbs and test again.

Wouldn't it be the other way around? The lower the initial pressure, the greater the tire flex, heat, and pressure gain; the higher the initial pressure, the less the tire flex, heat, and pressure gain.

[Gene]

 My 650 runs 32-32, unless someone is on the back, in that case it's 32-40
Did you check for a tire presure label? my 85 650 has one on the right side of the swing arm.

[mollusc]

Put 33 in each tire cold, and then run them until they get to full temp. If the pressure is more than 33 + 3.3 (36.3), reduce the starting pressure by .5 lbs and do the test again. If the pressure is less than 36.3, try increasing the starting pressure by .5 lbs and test again.

Wouldn't it be the other way around? The lower the initial pressure, the greater the tire flex, heat, and pressure gain; the higher the initial pressure, the less the tire flex, heat, and pressure gain.

I'm not the original source for the quote -- I just pulled it from the archive for informational purposes.  Might be worth testing to see how things responded in a real-world situation.

I normally run 34 front/36 rear I think.

[ThePowster]

Maybe rider weight, tyre choice and if to pillion or not have an affect. I could press both my tyres in by hand which isn't good. Even at 32psi I could press the front tyre in a bit.

She's a heavy girl and my ZX14 has 42 front and rear so I went with 40 front and rear on the Nighthawk and what a positive difference it made to the handling.

I'll come down 2psi and try again until I find that sweet spot. With 20psi in the front the bike was so difficult to make a slow turn so getting the right psi will make a big difference to the handling.

[Gene]

No tire pressure sticker?

[mollusc]

Or you could read your User Manual/Service Manual.

[ThePowster]

Or you could read your User Manual/Service Manual.

Jeez.... I didn't have access to it at the time and I was away from home.

[mollusc]

Sorry, I thought this was just a general question, not a situational one.
What age are your tyres?

[ThePowster]

That's ok mate. I am not sure so much about the tyres, I couldn't believe they didn't have PSI guidance on them. The rubber seems to be a little cracked and perished in places. They're old and will need replacing at some point.

Still the 40 F and R seemed to do the trick, for now anyways. 

[mollusc]

If they're older than 5 years, they need replacing NOW.  They're not safe to ride on.  There should be a 4 digit code inside an oval on one side of each tyre.  That DOT code indicates the week (first two digits) and year (last two) of their manufacture.
Bike tyres need to flexible.  As they age, the elastomers evaporate, they become hard, and they lose their flex and grip.

[McG99]

This is a good video.  HOWEVER by no means take this as if it's ok to use "old" tires and expect them to be as safe as younger tires.  This does show that it isn't a Good Tire/Bad Tire timeline.  I've ridden on a 10+ and a 20+ year old tire(one front and one rear at the same time after purchasing my bike) and I can say that they felt sketchy. 
https://www.youtube.com/watch?v=WwbLt8vZH5w&t=2s