The International CBX Owners Association

I have over 2,000 miles on mine now and plan to make a couple trips to Oregon this summer for a real mileage test. So far the front looks untouched but the rear is beginning to show minimal wear. I used a 120 for the rear and it handles very well (sticky). No rain groove wiggle either. One road in the mountains had some large areas that the hiway dept had scraped for repaving? with deep grooves and the bike really wobbled thru those areas though. The front end went every way the surface directed it. I may add some more PSI to the front, 36 maybe? I weigh about 240 anyway and have pro springs. The bike is very stable at speed and I've just added a fork brace but the 100 or so miles I put on it afterwards, frankly, I couldn't tell the difference even in triple digits. It may (be) smoother) but I don't feel it. Maybe next time out thru some twisties will tell me more. All in all I think the 501s are OK?

PS Remember the old K81s? First street tire to win the Isle of Man as I recall.

I tried a set of the GT501s, after the 591s were discontinued, on my '82 CBX (progressive suspension on both ends, superbrace). The front cupped worse than any tire I had previously run. I replaced the 501s with a set of dual compound Bridgestone Spitfires. I've a little over 3000 miles on the Spitfires and have found them to be the best overall tire I've run on my CBX.

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I had the worst results as far as mileage is concerned with Dunlops on my bikes. On two of them, the OE fronts (!) were wearing faster than the rears. Tried to get some words of wisdom from Dunlop reps during a big rally and got the cold shoulder. Will never buy another one.

Well, the results are in. Just about 5K on the rear and its bald. The front seems to be beginning to cup as well??WTF??? Not going to try another 501 so I'm looking in the catalogs...Venom X maybe, Avon, Macadam? We'll see. Got a question though, for those old enough to remember the Dunlop K81s. It was H rated and as I recall lasted about 8-9K on the rear of my bored out '70 750 which had just a touch less HP than my X. Is there inherently something better about a V rated tire vs an H rated tire, as long as you don't exceed 130 or so for any length of time? I've heard that its (old technology) but so is the X! It won the Isle of Man in the old days as I recall and always stuck real well in the corners for me. So, whaddyathink? Would it last on the X? I guess I'd have to put a tube in it though, as I remember, it was not a tubeless tire. Any problems with a tube on the Comstars? One more question, will a 120/70, 80 or 90 fit the rear of a stock 79? If so, what is the actual difference? Thanks, I need a tire real soon!!!

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Hold your horses, Mark! The rated tire pressure as shown on the rubber "max load PSI cold" is the maximum safe pressure the tire manufacturer recommends. It is not the optimum riding pressure for your bike.
42 PSI is a tad too high, I would say. Remember pressure increases when tires heat up. Seriously, I would not go over 36PSI.

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Mark, if the max tire pressure works for you, that's fine. But I still would argue and say that it is not the optimum. Sure, the tire manufacturers consider heating and pressure increase. That's why they put the number on. It's a liability thing. They don't want any moron out there inflating the tires to 100 PSI and then spilling.
And as far as wear is concerned: I dare to say, your tires wear in the center more than a tire inflated with optimum tire pressure, which again, I would put anywhere between 33 and 36 PSI in the rear .

Hey while the subject is tires where is the best place to buy metzlers?? I have a set of 880`s on my bmw and need a new rear...got close to 10K out of one on a K1200lt which is probably pretty good....front good for another 10K. I will mount and balance myself. I have no experience with the new dunlops...only the old ones which were good tires. Usually if it grips you are lucky to get 5 K out of them. On a hayabusa around 2-3 K was about it for a pilot sport.

EMS wrote:
42 PSI is a tad too high, I would say. Remember pressure increases when tires heat up. Seriously, I would not go over 36PSI.

Both the owners manual and the factory shop manual for my '79 call for 28 pounds front, and 40 pounds rear.

I think the harder you run a tire, the less it flexes, so the less it heats up - No??

Don

Mark,

I too believe you may be over-inflating the front 501. Afforded the opportunity - I spent some time discussing the CBX/501 application with the Dunlop folks at the last WSB event here. Though they couldn't give a spot-on figure - their recommendation was to run the 501's at about 32/36 cold for starters. After playing around a bit - I ended up running my CBX 501's a bit higher at a cold pressure setting of 34 PSI front and 40 PSI rear. This seems to work for me (though I certainly don't explore tire adhesion limits much on the CBX). If you are running 42 PSI front - you may want to try a lower pressure.

From my go-fast daze (many moons ago) - I seem to remember using an "8% rule" for setting tire pressure at the track. No expert here for sure - my recollection was that if the tire pressure increased more than 8% from dead cold (measured quickly after practice) - the initial pressure settings were too low. Excessive sidewall flex was causing higher temps and associated pressure change (the tires also wore more quickly). Conversely - setting the initial tire pressure too high resulted in less than the 8% "optimum" increase folks were shooting for back then - etc etc .....

Tons of variables - I doubt this or any "rule" is applicable for general street riding using today's tires (I am sure someone here can set me straight on this). But I find it best to rely on the specific tire and/or motorcycle Manufacturer's recommendations for a baseline (you may have to corner a technician to get anything beyond published specs - liability issues I suppose).

All the best,

John

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I agree, Mark. the higher the tire pressure the smaller the contact patch(!!), the smaller the rolling resistance. The bigger factor influencing horsepower required to sustain a given speed (and fuel economy), however, is the wind resistance. While rolling resistance remains constant with all parameters set, wind resistance increases with speed on an exponentional basis (square). As a result, doubling your speed requires quadrupling your horsepower. Ergo, a small fairing will do wonders to your fuel economy. Probably more than any tire pressure.
It would also be interesting to hear from a tire guy how the development in tires over the last 25 years has influenced pressure ratings and maybe making Honda's recommendation in the manual obsolete. While modern extreme low profile car tires are being inflated to significantly higher pressures than standard tires some years ago (I run 44PSI in a 315/35), some other designs allow lowering the pressure for comfort, traction and stability (remember the Ford Explorer/Firestone story).
I am sure, when you drag-race your bike, you don't use 42 PSI !?
As tire wear is still something most of us consider, the search for the best compromise is an issue. All points considered, I run 34PSI front and 36PSI rear in my Bridgestones on my 79 and in my Metzelers on my 81.

More on tire pressure setting:

I did take a look back at some logs to refresh my memory regarding the practice of setting race tire pressures using measurements taken between cold and hot (which I referenced earlier). In the 70's - the club racers here abouts actually used a "10% Rule" (somehow - I ended up using 8% - we really did not know much about tires back then).

Recently - I ran across this post from from Steve Brubaker (President, Race Tire Service Inc. - Dunlop's Eastern US distributor for racing tires). While we have covered most of these basics in our previous posts - it is an interesting read.

I wanted to post it here to help debunk the track tire pressure technique I mentioned previously - and to prevent folks from getting caught-up in any of the cold/hot pressure nonsense we practiced decades ago.
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TIRE PRESSURE:

All tire pressures are checked cold!

Street use:
OEM tires, run what the manual says
Replacement tires, check with the tire manufacturer for their
recommended tire pressure.

For Dunlop's check www.dunloptire.com/cycle/fitment.html

Racing use:
Check with the tire manufacture's track side rep for recommended pressures for that track.

The pressures are checked cold and then left alone for your ride.There is no other magic or special pressure you can do to get more out of your tires.

Sometimes riders like to check their tires hot and try to make the hot pressure 10% more than the cold pressure. This is simply stupid! In fact you are probably changing the bikes setup and stability for the worse.

The idea that the hot and cold pressures are related in such a simple manner, and then to change the pressure based on one factor (hot temp/cold temp) is very short sighted.

Factually the things that make a tire go up in pressure when hot,
are...
1) How fast you corner/ , lap times
2) Bike+rider weight
3) Bike horsepower and how the rider uses it
4) The newness or oldness of the tire (tread thick or thin)
5) Humidity of the air in the tire

Now frankly the tire temperatures on the street are not even close to those on the track. So hot pressures are not an issue in street riding.

As a rule, The hot pressure is simply the hot pressure. It does not mean change anything. It means its the hot pressure and has no other significance than that.

If you go faster the pressure will be more. The increased pressure does not mean "raise the pressure", because this will make the bike handle different and probably result in less overall grip.

After you check your cold tire temp, put the gauge in the bottom drawer of you toolbox and close it!

Spend more time on your riding ability and none on checking hot temperatures.
--
Steve Brubaker, President, Race Tire Service Inc.
Dunlop Motorcycle Roadrace Tire Distributor Eastern US
Order 800-772-TIRE, Tech Info 615-641-2234, Fax 615-641-8959
mailto:stevebru@dunlopracing.com, www.dunlopracing.com/

Posted on the FZ1 Owners Group (11/02).
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Back to the CBX and street tire settings:

Though supportive - the Dunlop race tire technicians I spoke with last year could only provide general information regarding street tire applications (and little for the venerable CBX) - no surprise really - they are heavily focused on track needs. I do have a call into Dunlop (Buffalo NY) to get their engineer's recommendations specific to the CBX and will be sure to post their "official" response - it should be interesting.

Cheers,

John
San Jose, California