Topic: Tyre pressusres?

[Jim Shea]

What is the correct tyre pressure for these?
I use Avon Roadrunner; front, 90 90 19 and rear 120 90 18.
I was told the trye pressure should be; 2.3 bar(33 lbs) in the front and 2.3 bar (36llbs) rear.
Cheers,
Jim.

[mcpuffett]

hi jim, i run dunlop tt100's on my f1 , and i put 28 psi in the front and 30 psi in the rear thats for solo riding ,and according to the haynes manual for sustained high speeds (over 110 mph)you should increase to 32 psi front and 34 psi rear  mick.

[csendker]

Use the Owner's manual instead of whatever is printed on the sidewall (except if the manual calls for apressure in excess of listed maximum).  I don't have a 750 manual, but the 550 calls for:

Cold, up to 200# load:
Front 26 psi (1.
Rear 28 psi (2.0)

Cold, up to 330# (max. capacity):
Front 28 psi (2.0)
Rear 34 psi (2.4)

Of course, that's for 30-year old tire manufacturing technology.

[bryanj]

Depends on your weight and driving speed but start at 28 front, 30 rear adjusting up or down 1psi at a time till you find what suits you

[dusterdude]

jim,what do the tyre have printed on it?

[Gordon]

The markings on the sidewalls are most likely irrelevant since they weren't intended to be used with tubes. 

[MetalHead550]

Hmmm.. I was thinking about that a while back.  I run Dunlop D404s and they call for like 42 psi on the sidewall.  They are of course tubed and I was wondering if that throws off the pressure reading.  Ive been filling them to 40 psi(which constantly went down until the tube streached to fit the tire).  Are they really at 40 psi, and reading the pressures you guys are going with, is that excessive?!

[stevej]

Jim, I would agree with Bryanj its got to be what suits "you" the rider of the bike, experiment at different preasures starting with the handbook preasure. My 550 seems best at 28 lb-front and 32lb rear with 325/19 and 400/18 rear Continental K22-44 tyres bearing in mind I now weigh 15 stone (4 more than I did when I bought the bike 20 yrs ago) 

[26.2]

From the CB750 manual
---------------------------------
Front Tire
28 lbs.   

Rear Tire (Load Weight / Tire Pressure)
200 lbs. / 32 lbs. Minumum
240 lbs. / 34 lbs.
280 lbs. / 36 lbs.
320 lbs. / 38 lbs.
360 lbs. / 40 lbs. Maximum

[BobbyR]

I have noticed that the modern tires have much more flexible sidewalls and tread pattern than the original. I  run my pressures about 5 PSI over the label on the fender and manual. If i run them lower the bike begins to feel sloppy. As mentioned before, you adjust to feel and keep them there.

[Jim Shea]

I was most concerned about tyre safety issues rather than performance handling; but I have noticed that the bike seems skittish when going over white lines at low speeds, it seems to slide/skip a bit!! I thought that the pressures might be too high?
I think the pressure(33 front 36 rear) was recommended by the tyre guy when I had a new tyre fitted recently.
My Haynes manual says 28psi front and rear!!
I think I will try 30 front and 34 rear??

[BobbyR]

Jim,i think it will be trial and error till she feels solid under you. They never dreamed of these tire componds when they built these things. I use a digital guage to tweak the pressures.

[Bob Wessner]

but I have noticed that the bike seems skittish when going over white lines at low speeds

If they are painted white lines, you need to be cautious, particularly when wet. Some areas have taken to using adhesive stripes and some have texture. These are a bit more friendly, but hard to identify from a distance so I treat them all the same, possible bad news. 

[Jim Shea]

I always approach lines with caution, especially when wet. The situations that I can remember was when these lines were dry and they really made the bike skip, I was probably doing less than 10 mph. I will lower the pressure and see what happens.

[Gordon]

I always approach lines with caution, especially when wet. The situations that I can remember was when these lines were dry and they really made the bike skip, I was probably doing less than 10 mph. I will lower the pressure and see what happens.

I've noticed the same thing here on many of the adhesive strip lines.  Seems like when they renew the lines they just put a new one on top of the old one, so they can protrude quite a bit from the road surface. 

[Terry in Australia]

I think it's time to tuck the owners manuals back under our seats guys, and ask the people who sell the tires, (tyres) compounds have changed drastically since the 1970's, anyone else who was riding these bikes back then will agree I'm sure?

My OEM Bridgestones on my shiny new yellow CB750F1 were just horrible slippery things, a pair of Dunlop K81's purchased in 1978 would still have tread on them now as the compounds were so hard, Avon roadrunners were good, but for CB750's Continentals were by far the best!

But I digress, modern tires are meant to be run at higher pressures than are recommended in our ancient manuals, Jimmy, 33/36 sounds about spot-on, that's close to what I run in all my old dinosaurs, and the point someone made about tubes negating the inflation instructions is not quite correct, as there aren't too many modern bikes now running 18/19 inch "Mag" wheels, which of course are the only candidates for tubeless tires in those sizes?

Well, BMW do make a special rim that allows the use of tubeless tires with spoked wheels, but not in our sizes. Cheers, Terry.

[Gordon]

and the point someone made about tubes negating the inflation instructions is not quite correct, as there aren't too many modern bikes now running 18/19 inch "Mag" wheels, which of course are the only candidates for tubeless tires in those sizes?

Well, BMW do make a special rim that allows the use of tubeless tires with spoked wheels, but not in our sizes. Cheers, Terry.

Then why do all the modern tires, regardless of size, have the word "tubeless" written on the sidewall righ alongside the recommended tire pressure? 

[Terry in Australia]

and the point someone made about tubes negating the inflation instructions is not quite correct, as there aren't too many modern bikes now running 18/19 inch "Mag" wheels, which of course are the only candidates for tubeless tires in those sizes?

Well, BMW do make a special rim that allows the use of tubeless tires with spoked wheels, but not in our sizes. Cheers, Terry.

Then why do all the modern tires, regardless of size, have the word "tubeless" written on the sidewall righ alongside the recommended tire pressure? 

To tell you that they're compatable with tubeless applications Gordy, some tires (mostly dirt bike tires) aren't. Cheers, Terry.

[Gordon]

OK, so here's another point, then.  The speed rating on the tire sidewall is based on a tubeless application, and is not considered reliable when using a tube.  Why would the recommended pressure be different?

Isn't this fun?   

[Terry in Australia]

OK, so here's another point, then.  The speed rating on the tire sidewall is based on a tubeless application, and is not considered reliable when using a tube.  Why would the recommended pressure be different?

Isn't this fun?   

Good question Gordy, I worked at several gas stations that sold and fitted car tires for many years, and once raised a similar question with one of the tire reps as to whether the pressures for tubed and tubeless car tires should be different, and his answer was a simple "no".

We only used tubes when a tire had been punctured and "plugs" were banned due to them sometimes "letting go" when tires wore down to unserviceable limits or got overly hot. We've had various ad campaigns over the years about saving fuel by increasing tire pressures to up to 10 pounds over the manufacturers recommended ratings for older cars, and I've noticed that recommended car tire pressures have increased in recent years, but I'[ve seen nothing about how all this will affect "tubed" tires?

I think the much improved compound and construction of modern bike tires certainly lends itself to increased air pressures, and with the newest "revolution" for tire inflation, Nitrogen, becoming more common, (albeit at a cost) I can see manufacturers recommendations going up again. Cheers, Terry.

[Bob Wessner]

I have no idea as to the answer, but I wonder if using tubes in a tubeless tire won't result in more frictional heat being generated? 

[csendker]

I would think a significant impact would be the actual installation.  All of the ratings will assume a perfect installation & fit.  How even is the tire mounted on the rim?  How true is the bead?  If the tube is not evenly installed and has even a slight wrinkle, this will impact the pressure exerted on the tire.  A tubless will not have this problem, and would enjoy even pressure all around.  That is with a 'perfect' install.  Slightly off-center will change the forces exerted on the tire, making it ever-so-slightly out of true.

And this all assumes your spoked rim is close to perfect.

As long as you're somewhere in-between the manual's recommendations and the maximum allowed for the tire, I suspect it's drivers preference rather than splitting the technical hairs.

[ofreen]

Years ago, I settled on Metzeler ME33s for the front and ME88s for the back on the 750, so I don't have recent experience with other brands of tires with tubes.  I run 38 psi in the front and 40 in the back with the Metzelers.  Anything much less and the tires feel squirmy.  I have heard the Metzelers like a little more pressure than some other brands, though.  I agree with Terry that you can't base pressures for today's tire designs on those of yesterday's.

It is said that tread life for tires running tubes will be less than those without tubes, everything else being equal.  This is attributed to more heat in the tire running the tube.  I am inclined to think this is true.  A friend of mine gets a few more miles out of ME88s on his old Goldwing run tubeless than I do on the 750 running the same tire with tubes.

I read a few years ago that tire pressures will vary 10 percent between hot and cold with a properly inflated tire.  I did an experiment on the 750 awhile back on a hot day, and that is just about what I got. 

[Jim Shea]

With regard to the safety aspect of a tyre, will slight over or under inflation result in a tyre blowing, which one is more dangerous? This is what I am most concerned about?

[Gordon]

With regard to the safety aspect of a tyre, will slight over or under inflation result in a tyre blowing, which one is more dangerous? This is what I am most concerned about?

Under-inflation causes excessive flexing and wear of the sidewalls which could lead to failure, so I guess if you had to, slightly over would be better.