Topic: 5 PSI Front tire drop over night. Issue?

[Honda3]

Yesterday afternoon I aired the tires. 40 in the 40 40 in the back - (1972 Honda CB 750 btw).

Did about 20 miles on the bike yesterday when it was 80 degrees out.

Parked the bike last night. Temps dropped to 59 degrees.

This morning I checked the air pressure in the front tire. It went from 40 to 35. So I added more air. Is this an issue?

Also im getting new carbs boots for my bike. Do i need to go with Genuine Honda or will aftermarket suffice? 4 into 1 has OEM front and back boots for $146 but they ALSO have a repro set for $77.

Thoughts?

[beemerbum]

You can't go wrong with OEM Honda

[1976cb750f836]

Cold air does change things! Keep an eye on them, or add a tad of sealant, tire sealer, not fix a flat. Just a small amount, if it needs more than an ounce, it needs attention.
Just my 2cents worth?

[Jerry Rxman Griffin aka MuthaF'er]

If the front AND back didn't decrease equivalently then I'd definitely watch for a leak.

[Don R]

 I don't know about the 4-1's but for a while some replacement brands were getting soft from fuel soaking in. That may be resolved by now. 

[Bodi]

That sounds like a reasonable difference between cold and hot, did you air up after a high speed ride or aggressive twisty run?
Tire pressure should be adjusted cold. If you're losing 5psi in cold pressure overnight something is leaking. The soapy water test should show what. Spray it in the valvestem first, replace the valve core if you see bubbles and check again. If not, wet down the bead and around all the spokes and stem at the rim. Bubbles from anywhere there means the tube is leaking.
Take it seriously if you do have leaks. Small tube leaks never get smaller, only larger... and depending on the cause the tube may suddenly develop a major leak and fast deflation: this is no fun while riding. Check for foreign objects in the tire, I've had a leak from what looked like a steel wire strand through the tire - just a tiny metal speckle barely visible from outside but about 1/2" poking though the tube.

[Honda3]

That sounds like a reasonable difference between cold and hot, did you air up after a high speed ride or aggressive twisty run?
Tire pressure should be adjusted cold. If you're losing 5psi in cold pressure overnight something is leaking. The soapy water test should show what. Spray it in the valvestem first, replace the valve core if you see bubbles and check again. If not, wet down the bead and around all the spokes and stem at the rim. Bubbles from anywhere there means the tube is leaking.
Take it seriously if you do have leaks. Small tube leaks never get smaller, only larger... and depending on the cause the tube may suddenly develop a major leak and fast deflation: this is no fun while riding. Check for foreign objects in the tire, I've had a leak from what looked like a steel wire strand through the tire - just a tiny metal speckle barely visible from outside but about 1/2" poking though the tube.

Thanks for this.

I just aired up both tires to 40psi while cold. We will see what happens today and overnight. I def dont want my tire to go flat all of a sudden.

I take this stuff super seriously so Im going to watch it. Just in case, ive ordered an inner tube.

[bryanj]

Thats quite a high pressure unless you are two up and kitted

[Honda3]

Thats quite a high pressure unless you are two up and kitted

Nope just me but all indications say its ok to have it at 40

[low-side]

That pressure is probably safe for the tires, but unless you're a pretty big guy it's probably going to handle squirrelly and get shortened tire life.  I run 38 / 40 on my '78 Wing, but it's 600 lbs and I weigh 250 lbs in all my gear.  As for the 5 psi loss, I'd keep an eye on it.  Sounds a bit much for a 20 degree drop in ambient temperature.

[bryanj]

Its Ok to go even higher but for a normal size person with no fairing or luggage 30f 32r is more "normal"

[PeWe]

My CB750 get the pressure recommended by Honda sticker for sustained high speeds.
Front 2.1 BAR (30 PSI)
Rear 2.4 BAR. (34 PSI)

Normal speeds are 2 PSI less so the pressure can be lower.

Higher pressure worse handling, very bumpy ride. This with stock tire dimensions or 100/90-19, 120/90-18.

[Honda3]

just checked this morning.

Last night i put the front and rear tires at 40psi

Last night temps when from 18c to 10 c that is around 74F to 50F. Gauge read 39 on both. Wooo hooo! No leak!

[evinrude7]

been reading lots of comments lately on tire pressure in various posts.   32/40 being the upper end of tire pressure on the model guide in the photo i posted below.  i find that anything south of 32/40 seems soft.  i've even run my front tire around 36.  serviced my front forks 2 years ago with 10W30.  no leaks.  rears are the original shocks with the preset all the way out, meaning highest if that makes sense. 

[PeWe]

Honda placed s sticker on rear fender, read that one and try.

[1976cb750f836]

I like the lower side of that sticker, 28-30 in both. With more bike seems bouncey and slippery on wet roads, and in grass!

[andy750]

Honda placed s sticker on rear fender, read that one and try.

But that was for 1970s tire technology how about for modern tires? Is recommended psi not a function of tires and weight of bike (and rider) so that if tire technology changes then so should psi?

[PeWe]

I have tried higher pressure and that made the bike's handling no good and very uncomfortable ride as well.  Too hard at front "white fingers", too high pressure rear ended up in bumpy handling and loss of grip much easier.
The tires are the old sizes with stock rim sizes.
I doubt that my rear tire used today differ from my Metzeler ME99 I preferred in the 80's. That worked really well.

Do not mix up the tires max pressure with what fits the bike.

This is easy to test. Inflate the tires to different pressures and have a ride or two