Britbike forum Forums British Motorcycle specific Brands Bsa bulletin board Tire pressures

I have a new set of the original type Dunlop K70's on my bike. The factory manual says tire pressure should be 18/20 psi front/rear. Seems some what low. What are other people using with these tires?

30 PSI or more its better on corners.

18/20 psi. Any more and my teeth rattle.

I'd say nothing less than 28psi.

H town
18-20 is far too low, 28-32 should be correct. If your teeth rattle you have suspension problems and you are using the sidewall of the tire as suspension

18/20 was in 1960's pounds with the current rate of inflation it would be 28/30.
bad pun, I know.
I'll get my coat.

Yeah, 28-30ish is about right. I bump my rear up to 36psi when laden with camping gear.

18-20 is far too low for modern compounds.

Beezageezauk

Thanks guys. 28/32 is what I use in my Bonneville, about the same weight. I shall give that a try.

Rebuilt front end and new rear Shocks, so no suspension problems here.



The Firebird handles just fine for me. I'm not heavy. The tires have been on it for years; still have better than 75% tread remaining.

Made in Japan



Been there, done that. Personal preference I guess.

I run a '58 B31 and a few Sw/Arm A10's and normally ride long distances carrying camping gear. These models don't have tyre security bolts and I've found over the years that I've had to gradually increase tyre pressures, particularly in the rear, to prevent tyre creep and valves being pulled out of the tubes.

I now run at these increased pressures and leave the knurled nuts off the valves. This way any tyre creep can be spotted before a problem occurs and I've had no sign of it for 4 years now. Prior to this I got pretty good at replacing inner tubes at the side of the road.

Beezageezauk.

Old post, I know, but just for reference; the BSA workshop manual specifies 18psi front and 20psi rear, add one pound pressure for every 14lbs. of weight over 140lbs. It's been a long time since I've weighed 140lbs. 29/31psi is about right for me.

My teeth used to rattle too Gary E.

Went to the dentist and he sorted the problem out for me.

but that only applies if you are using an original 1966 tyre with the same 1966 compounded rubber that the factory fitted.

Dunlop made motorcycle tyres in a dozen different countries and each country used a different compound.
Down here in God's gift to motorcycling we got really hard compounds.
The tyre just about stood up without any air in it.

However almost any Dunlop bught now days will be made with a compound so soft back in 1966 it would have been classed as a racing tyre.
Things have changed

My B44 Victor Enduro and Shooting Star and B25 Barracuda instruction manual has this caveat: "If the machine is to be driven at sustained speeds exceeding 90 mph, the pressures for both front and rear should be increased by 5 psi."

That will be why the new K70 doesn't last much more than 3000 miles. Some of the French made dunlops last twice that long. And my England stamped Avon road riders are not even seeing half their life span at 4000 on the rear.

If I remember correctly when I was a instructor for MSF we told student to run tire pressure at what tire is rated for or within 10% of it. Not sure if that would apply to these older bikes. But that's how I run mine.

K70's are typically sensitive to tire pressure. The little pads "squirm", instead of rolling over the road, if under inflated. This causes "cupping" and shortens the tires useful life. In a modern tire/compound the tire pressure supports the motorcycle.

If you look back at owner's manuals over the past 50 years you will find recommended pressures as low as 15 pounds (Vincent Black Shadow). In those days the carcass of the tire did a lot, if not all, of the work.

As a rule of thumb the tire should not increase in pressure more than 2 pounds when being used. If you set your tire at 30 pounds and after a decent ride the pressure increases to 33 pounds, or more, the tire is under inflated. The tire is flexing. Flexing creates heat which increases tire pressure. Flexing will also decrease fuel mileage.

Required tire pressure varies with load. A modern K70 is rated for 617 pounds @ a max. pressure of 36 pounds. If you and your "main squeeze" are full figured and choose to carry a bit of kit on the back you will be pushing the 600 pound limit. Failing to adjust the tire pressure from solo pressures for the new load the least you can expect rapid tire wear. There are other important considerations as well. Not all deflated tire incidents end up with you fixing the tube on the side of the road.

As far as running the tire at what it is rated at, when riding solo, the ride would be quite stiff. Not particularly unsafe, but uncomfortable especially on the kidneys. For sure the contact patch on the road would be smaller than if the tire was properly inflated. At 10% of the rating, or 32 pounds for a K70, you would be in the ball park. Again take pressures cold and then after a ride when the tire has had a chance to warm up. If the pressure hasn't raised a bit, and the ride is still a bit stiff, you could lower the pressure a bit. You want the tire to flex a bit so you get a decent size contact patch without the tire flexing so much as to create a lot of heat.

If I was riding two up a lot I would set the pressure for two up riding and leave it there and not lower it for when I rode solo. All you have to do is forget raise the pressure for two up riding once. This is especially important for a K70, because very little riding with an under inflated K70 will start it cupping. Once cupping starts it cannot be reversed by raising the pressure after the event.

Since we're resurrecting zombie threads,(*) the following more recent one beat the subject of tire pressure into unconsciousness, saving having to do it again here:

http://www.britbike.com/forums/ubbthreads.php?ubb=showflat&Number=623829#Post623829

(*)Doing so is best avoided unless there is "essential" new information on a topic since it adds unnecessary clutter and possible confusion.

I usually ride 28 front - 30 rear in winter. I work on the basis that the road and outside temps are colder, my contact patch is greater and I ain't putting down the power

In the summer I will run 30 front, 32 rear, and if carrying camping gear I go up to 32F 35R of the ride is too hard I adjust the suspension.

When I eventually removed my K70 off the rear, it's unladen un fitted shape was convex.

When I had a 3:50-19 and 4:00-18 on my 250 Canam, it was rated in 1980 to run 28F 30R, that was for a 1980 off road motorcycle in military use. It went around corners like it was on rails.

Thanks for posting that reference.
It was thread when I was busy learning how much I did not know anout mowers and thus not reading this forum.

And for the fun of it
BSA recommends 12 psi front and 14 psi rear for the M20.
I run 18psi front & 20 psi rear with a speed master & K81
Any more than that and the bike tankslaps , bounces all over the road and skipps on the girders under breaking.

Just a general comment about something that struck me after re-reading these two threads, that applies to most pressure recommendations that are made; it's interesting how few of them are for odd numbers. It seems that 90% (er, I mean 89%) of them are for pressures that are even numbers.

Not odd at all, MMan--it all goes to make a very even life.
Just think about it--if the psi s are odd they would fall over each other.
(well--Brit bikes beget Brit humor!)

So I had a look this morning.
The Japaneese K81 I bought for the 2001 BSA international that I never got to is stamped , max pressure 41 psi.
The K 81 I just fitted is also made in Japan and also stamped 41 psi

But---Trevor--everything is upside down in Oz!