Tire pressure, how low can I go?

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Tom Hicks
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Tire pressure, how low can I go?

Post by Tom Hicks » Sun May 19, 2019 7:58 am

My speedster is lightweight and has a hard ride, bumps in the road can effect handling at higher speeds. The tires are Universal with balance beads and I had them filled to 45 lbs. I have been lowering the tire pressure and have found she rides much more smoothly at 20 lbs. Can I go any lower with the tire pressure?
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Re: Tire pressure, how low can I go?

Post by Dallas Landers » Sun May 19, 2019 8:14 am

I would guess that the lower pressure would let the tire slip on the rim and cause the valve stem to start leening which will wear the stem or pull it inside the rim. As to how low I cant answer. Good question Tom.

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Re: Tire pressure, how low can I go?

Post by Rich Eagle » Sun May 19, 2019 9:13 am

21" are more forgiving than 30". You are asking for trouble at High speeds as Dallas explained.
How smooth is the wheel where the tire fits on it?
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Re: Tire pressure, how low can I go?

Post by MWalker » Sun May 19, 2019 9:25 am

Since you have 21" tires (as shown in your pic), you don't need so much pressure. I run 35 in mine, and it rides great. I switched from years of running clinchers to a set of 21" McLarens a few months ago, and I was quite surprised at the major difference in how well they ride with the lower pressure. The problems Dallas mentions are what you need to observe with clinchers, not balloon tires.

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Re: Tire pressure, how low can I go?

Post by Steve Jelf » Sun May 19, 2019 9:32 am

I think 20 psi is too low and is likely to cause damage. I agree with Mike on 35 psi for balloon tires.
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Re: Tire pressure, how low can I go?

Post by Adam » Sun May 19, 2019 9:32 am

A6A8F27F-2C07-49A3-AB81-E8656408718C.jpeg


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Re: Tire pressure, how low can I go?

Post by Tom Hicks » Sun May 19, 2019 9:44 am

Rich Eagle wrote:
Sun May 19, 2019 9:13 am
21" are more forgiving than 30". You are asking for trouble at High speeds as Dallas explained.
How smooth is the wheel where the tire fits on it?
Rich
Smooth. The rims were new a few months ago.
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Re: Tire pressure, how low can I go?

Post by Ruxstel24 » Sun May 19, 2019 9:51 am

While balloon tires run at lower pressure, I also believe damage to the tube/tire slipping on the rim, will occur with under inflation. It also adds heat.
Stick with the chart Adam posted and you'll be fine.


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Re: Tire pressure, how low can I go?

Post by Kevin Pharis » Sun May 19, 2019 2:51 pm

On a light speedster, I have found that bout 25-30 psi burns thru the center tread section pretty quick. Down around 10-15 psi will even up the wear... but also provides inevitable complications such as tire slippage and slushy ride. Not worth the trade off!! I usually try for 20 psi (it’s not unusual to be down around 15... oops), but spend a bit of time switching the wheels from right to left side to keep the valve stems from getting too bent out of shape.

If you zoom in on this pic close enough, you’ll notice the valve stems are laying flat against the rims... :shock:
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Re: Tire pressure, how low can I go?

Post by DanTreace » Sun May 19, 2019 4:11 pm

Kevin

Good insight. IMO, have run 30-32psi in the '27 touring, it carries passengers most time. Tire wear hasn't been an issue, nor bent-over rubber stem.



Yours.....oh my.... :o

Am just wondering if those big disc brakes on that nice speedster are having most of the impact of pulling your valve stems flat!
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Re: Tire pressure, how low can I go?

Post by AndyClary » Sun May 19, 2019 6:22 pm

Dan. If you notice the direction the stem leans, it is from the pull, not the braking. Kevin's speedster runs "ok". ;)

Andy


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Re: Tire pressure, how low can I go?

Post by Dennis Prince » Sun May 19, 2019 7:59 pm

I have run 55 psi in my 21 inch tires for almost 50 years and never had a problem with tire wear and I found I could make a bigger difference in ride by lubing the springs often. My model A drives way better at 55 psi than 35 psi and the T's roll a lot better at the higher pressure. I run 525x21 tires on two of my T's and run 40 psi. Just my 2 cents.


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Re: Tire pressure, how low can I go?

Post by D Stroud » Sun May 19, 2019 8:31 pm

If I'm not mistaken(and I may well be) the stems will be pulled the same direction by braking OR pulling. Dave
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Re: Tire pressure, how low can I go?

Post by D Stroud » Sun May 19, 2019 8:33 pm

Nope, never mind. Finally got my brain to wrap around this. :? Dave
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Re: Tire pressure, how low can I go?

Post by Wreckrod » Sun May 19, 2019 8:38 pm

Lol that valve stem lean means that Kevin just isn’t braking hard enough before the corners. A more aggressive speedster driver will pass him in the turns! (Just picking Kevin)


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Re: Tire pressure, how low can I go?

Post by Kevin Pharis » Sun May 19, 2019 9:05 pm

One may not have noticed the front brakes in the pic... but I do have issues with the front stems too :D


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Re: Tire pressure, how low can I go?

Post by SurfCityGene » Sun May 19, 2019 11:20 pm

No expert on your tire pressures but to me it seems like 20 is really too low and the slip verifies it. Better a slightly bumpy ride than a tire problem. For the Montana 500 we run the Max pressure!! IMHOP

Hey, can you tell us more about those front disc brakes. Looks like the same on the back?

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Re: Tire pressure, how low can I go?

Post by AndyClary » Sun May 19, 2019 11:38 pm

As to the original question, I run 35 pounds on my 26 coupe (fairly heavy) and 30 on my speedster.

Andy

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Re: Tire pressure, how low can I go?

Post by tinman080 » Mon May 20, 2019 6:02 am

My 2 cents worth....lol Any transverse spring vehicle is not inherently stable under fast or emergency maneuvers. An underinflated tire will contribute greatly to a high center- of- gravity, stiffly- suspended vehicle's tendency to roll over. These wheels are not safety beaded rims, they are smooth inside with no raised ridge to retain a underinflated tire on the rim. In a straight line go like the devil, but IF an unforeseen situation arises underinflated tires could end up with a very bad result.
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Re: Tire pressure, how low can I go?

Post by D Stroud » Mon May 20, 2019 6:14 am

I don't have any experience with tires on a speedster or other light car, but if my tires were slipping enough to bend the stems, I would for sure add some more pressure, regardless of the ride quality. JMHO Dave
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Re: Tire pressure, how low can I go?

Post by MWalker » Mon May 20, 2019 10:10 am

That chart posted by Adam D. is informative. Thanks, Adam. I assume it's from some of Ford's literature? Based upon the chart, it looks like I can reduce the pressure in my tires just a bit. I'll probably run them at 30 now. I'd rather err on the side of too high than too low. And as I said in my post above, even at 35 psi they ride 10 times better than what I've been accustomed to. One thing is for sure -- if you are seeing your valve stems lean, your pressure is too low for your car and your driving habits.


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Re: Tire pressure, how low can I go?

Post by Adam » Mon May 20, 2019 11:10 pm

Chart is from the Ford Manual.


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Re: Tire pressure, how low can I go?

Post by Alan Long » Tue May 21, 2019 8:27 am

Reading the official Ford Data on Tyre pressure I rounded the pressure up from 27 to 30 PSI on my 26 T however minor signs
of under inflation wear tell me that 32 or 35psi maybe the go. Forget how low you can run tyres at but how high which is a worry when you understand how crappy the new stuff is!!!
Alan


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Re: Tire pressure, how low can I go?

Post by Poppie » Wed May 22, 2019 6:22 am

Alan,
I have 30x3.1/2 oversize Universal tyres on a 1913 Crossley 5 seat touring and I run 90/100 psi to stop the tyres turning on the rims and cutting off the valve stems. I run hard tyres and lubricated springs and have had no problems with ride and or tyres.
Warm Regards...Neil.


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Re: Tire pressure, how low can I go?

Post by fliverfan » Wed May 22, 2019 10:27 am

If the ride and handling are still objectionable at 28 lbs, take a leaf out of the springs.


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Re: Tire pressure, how low can I go?

Post by Poppie » Wed May 22, 2019 5:59 pm

NO, Just lubricate them.

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