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Thread: Tire pressure again

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    Yoda tahoe healey's Avatar
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    Tire pressure again

    Attending a tech session at the Solvang meet the Moss speaker and a gentleman I don't remember told the group that the tire pressure should be about 25 to 26 PSI. I've always run at 30 PSI. They gave some real good reasons. My particular problem is the 6500 foot elevation I live at and most of the rides my club go on are less than 500 feet. At 25 PSI the car runs well and smoother at lower altitudes.
    What is the consensus of the forum?
    Last edited by Editor_Reid; 10-28-2018 at 04:43 PM.

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    Yoda steveg's Avatar
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    Re: tire pressure again

    That was Tom Colby, a racing Sprite builder of long experience.

    IIRC, he actually said the pressure should be 20 - 22 psi, with 25-26 as an intermediate point for the skeptics until they get used to the idea of lower pressure.
    Steve Gerow
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    Yoda tahoe healey's Avatar
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    Re: Tire pressure again

    Sometimes I just hear what I want to hear. I would be hard pressed to go from 30 to the low 20's. On the other hand, I dropped the pressure to 25 for up here and that should drop it to 22ish at lower altitude. I may know this weekend as my club is going to the Delta (Sacramento).

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    Yoda John Turney's Avatar
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    Re: Tire pressure again

    I recall the 20 - 22 psi was for Sprites.
    John, BN4

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    Re: Tire pressure again

    I recall the recommended pressure being 26lbs. I read that with higher pressures the shocks do not function properly. The workshop manual calls for 20lbs front, 23lbs rear and 26lbs full load in rear. It also calls to increase pressure by 6lbs for speeds over 85 mph. this is found on page 8 of general data in issue 6 of the BMC 100/6-3000 workshop manual

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    Luke Skywalker RAC68's Avatar
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    Re: Tire pressure again

    Hi All,

    Keep in mind that the low tire pressure presented in the original Healey manuals was for Cross Ply Tires of the day a far cry from todays lower profile radials belted radials. As you may note, I said lower profile since my original equipped Dunlop Road Speed RS5s had a higher side wall and came on 4" wide 60 spoke wheels. This combination called a tire pressure of (if I remember correctly) Front 21lbs, Rear 25lbs.

    So, when considering the tire pressure you will be using, the first consideration is what wheel and tire you have mounted on your car. Todays tires are stiffer, wider, with less height and are mounted on wider wheels with more spokes. With this in mind, does it make sense to use as little air pressure as recommended for original wheels and tires or up the pressure to what is closer to that more commonly found on today's cars (i.e. Front 24-26 lbs., Rear 29-32 lbs.)? Also, keep in mind that with lower sidewall height, pre BJ8 Phase 2 models have little ground clearance to begin with and reducing tire pressure only diminishes this scarce commodity even further with increased tire flex. Additionally, tire flex also diminishes the tread stiffening function of a modern tires belt and, therefore, the tire's road adherence.

    So, is it a good thing to lower the tire pressure to pre-modern tire levels? What is the real benefit derived? Since I can't answer these questions, I have no recommendations.

    Just my thoughts,
    Ray(64BJ8P1)

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    Luke Skywalker vette's Avatar
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    Re: Tire pressure again

    So if you add 6 lbs to 23 you are pretty close to 30 . So set them at 30 and keep the speed above 85. ��
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    Yoda Michael Oritt's Avatar
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    Re: Tire pressure again

    30 psi x 4 on 180R-15 Michelin XAS's work just fine for me.

    Best--Michael Oritt
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    Re: Tire pressure again

    I've bought, I think, 5 sets of tires for my Mustang. When I first got the car I ran at Ford's recommendation, 35psi but found that the tires wore excessively in the middle of the tread. Over the years, I've lowered the pressure until I got more uniform wear across the tread--I'm assuming more even tread wear means more rubber on the road--I now run at 29psi front and 28psi rear. Handling is pretty well balanced (for a relatively high HP car with a light rear-end that wants to come around if I'm not careful). I think the TPMS fires if they get below 25. Healeys are lighter, I've settled at 28psi all around on both my BN2 and BJ8.

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    Luke Skywalker
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    Re: Tire pressure again

    23lbs rear and 26lbs full load in rear. It also calls to increase pressure by 6lbs for speeds over 85 mph.

    That's it then, that is why I inflate mine to 30 all round must be good for the 70 speed limit



    Bob

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    Moderator Editor_Reid's Avatar
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    Re: Tire pressure again

    I run 30 PSI in all four Avon CR6ZZ tires on my BN2. I can't defend the practice scientifically, but I don't mind a hard ride and figure that even with a little leakage they're still good. And oh yeah, 35 PSI in the spare, just to allow for more leakage since I seem to remember to check the pressure in the spare about once every five years.
    Reid Trummel
    Editor, HEALEY MARQUE magazine



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    Re: Tire pressure again

    The tech indicated that too much pressure made less contact with the road making braking less effective, poorer turning and a harder ride. I am interested because I've always had a problem with broken spokes (about 2 a year).

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    Re: Tire pressure again

    Quote Originally Posted by tahoe healey View Post
    Sometimes I just hear what I want to hear. I would be hard pressed to go from 30 to the low 20's. On the other hand, I dropped the pressure to 25 for up here and that should drop it to 22ish at lower altitude. I may know this weekend as my club is going to the Delta (Sacramento).
    Um, I'm missing something here. Ambient air pressure won't affect the pressure in tires (but temperature will). 25psi at 7,000' altitude and 59degF ('Standard Temperature') will be 25psi at MSL (mean sea level) at 59degF. Since the Delta is likely to be warmer than Tahoe, your tire pressure will increase this weekend.

    https://en.wikipedia.org/wiki/Ideal_gas_law

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    Re: Tire pressure again

    Quote Originally Posted by Editor_Reid View Post
    I run 30 PSI in all four Avon CR6ZZ tires on my BN2. I can't defend the practice scientifically, but I don't mind a hard ride and figure that even with a little leakage they're still good. And oh yeah, 35 PSI in the spare, just to allow for more leakage since I seem to remember to check the pressure in the spare about once every five years.
    Ditto....
    Dougie
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    Jedi Knight
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    Re: Tire pressure again

    My A-H reprint manual advises "5.90-15 tubeless or 5.90-15 road speed (optional alternative)" crossply tyres; with BN4 recommended pressures as 20 front; 23 rear; 26 rear full full load; and increase by 6 psi for sustained speeds above 85 mph (all on 15x4J wheels). For Mk I, II and III: 5.90-15 road speed (with tubes); 20 front; 26 rear; increase by 5 psi above 110 mph; 4Jx15 wheels.
    Section O.5 references "Radial ply tyres (Dunlop SP41)", but does not make any comment on pressures other than sticking with the recommend pressures (presumably, those for the cross-ply).
    https://racedunlop.com/40sand50s.php gives 30 psi as the maximum absolute pressure for a 5.90-15 road speed tyre.
    https://www.terrysjag.com/3.4%20L%20...%20Service.pdf has a copy of a Jaguar service bulletin dated 1960 advising an increase in tyre pressures for their 2.4 (weight 3175 pounds vs Healey 2550 lbs per Wikipedia) to 28 psi frt and 24 rear for all uses, including motorway; 25 and 21 psi respectively for town use or poor road surfaces where speeds are limited.
    The Oponeo web-site suggests increasing pressures by 4 to 9 psi when switching from crossply to radials. They advise "cars, in which cross-ply and radial tyres were used interchangeably, the latter always were given higher air pressure. Just to partially compensate for the much greater flexibility of their sides."

    I'm thinking that radial tires are more structurally stable at the tread (due to the belt) and thus do not generate as much heat as the crossply does, such that it is not necessary to increase pressures for high speeds to avoid the heat build up. I think I'm running 28 and 32.

    Have a look at http://www.eng-tips.com/faqs.cfm?fid=1148

    I found my old calculations of the above (with some assumptions that I'd really like to recheck such as when the car is on the road next summer and the weigh scales are operating) and I'd come up with 28.5 psi. I'd appreciate if some other folks would get their spreadsheets fired up and share their results. Doug
    1959 pristine 948 cc Bug-Eye; DCOE, 5 speed; bought 1971
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    Yoda tahoe healey's Avatar
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    Re: Tire pressure again

    With less molecules in the air at higher altitude the pressure in the tires will be higher. Your cosmetic containers will burst their contents out when opened. Carburetors have to be adjusted up here for the thinner air. You may be talking about ambient temperature.

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    Re: Tire pressure again

    Quote Originally Posted by Bob_Spidell View Post
    Um, I'm missing something here. Ambient air pressure won't affect the pressure in tires (but temperature will). 25psi at 7,000' altitude and 59degF ('Standard Temperature') will be 25psi at MSL (mean sea level) at 59degF. Since the Delta is likely to be warmer than Tahoe, your tire pressure will increase this weekend.

    https://en.wikipedia.org/wiki/Ideal_gas_law
    Ambient air pressure will certainly affect internal tire pressure. It is no different from submerging a balloon in water which causes it to compress and increase pressure in the balloon. Maybe I misunderstand you but Boyle's law is not just a good idea....it's the law.
    Jon Robbins
    1956BN-2 (do it all yourself, you'll be glad you did)

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    Yoda
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    Re: Tire pressure again

    By your logic Tahoe's tire pressures will INCREASE due to the massively higher ambient pressure at about 150' MSL (the Delta) vs 7,000' (Lake Tahoe). The 200-or-so millibar pressure change will not significantly compress your tires (at sea level, water is 754 times denser than air).

    https://www.windows2universe.org/ear..._altitude.html

    http://www.theweatherprediction.com/habyhints/216/

    I made my statement with the presumption the volume of the tires won't change significantly (they won't); tire sidewalls are considerably more rigid than a balloon's, and air is significantly less dense--754 times or so--than water. In this case--with a fixed volume container--the Ideal Gas Law applies.

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    Jedi Warrior roscoe's Avatar
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    Re: Tire pressure again

    I took what you said quite literally and ambient pressure will affect tire pressure although that effect will be small. And yes, as long as tires are flexible the internal pressure will rise as external pressure rises but it may be a small change for the given altitude. I'm not sure I agree with the scale of the temperature versus pressure change with altitude. I kind of like the description that they sort of cancel each other out given by these folks.
    https://www.tirerack.com/tires/tiret...&affiliate=HM5
    Jon Robbins
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    Re: Tire pressure again

    Comments:
    -- I spent a lot of time online looking for some authoritative recommendation for pressures in the mid-20s - unable to find anything.

    -- Brooklands' "Austin Healey 3000" collection of tests: Road & Track's test of the Mk III had Dunlop radials run at 20psi F/25psi R. Almost all the tests have the 5.90 x 15s.

    -- running lower pressures, one would need to be more vigilant about checking tire pressures.

    -- a "science project" would be to mount one of the USB pressure/temp monitor systems and do some freeway runs and check the temps at different pressures.
    Steve Gerow
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