Pressure to size ratio? - Alfa Romeo Bulletin Board & Forums
Reply
 
LinkBack Thread Tools Display Modes
post #1 of 19 (permalink) Old 06-17-2006, 11:40 AM Thread Starter
Senior Member
Gold Subscriber
 
ecohen2's Avatar
 
Join Date: Jan 2006
Location: Arlington, VA
Posts: 549
Send a message via Skype™ to ecohen2
Pressure to size ratio?

Is there a rule of thumb for adjusting tire pressure as you increase in tire size?

The previous owner of my car put 195/60 r14 on my car. I have upped the pressure by 2 lbs but it still seems low.

Thanks,
Ed

1986 Bertone
ecohen2 is offline  
Sponsored Links
Advertisement
 
post #2 of 19 (permalink) Old 06-17-2006, 11:48 AM
1966-2013
Platinum Subscriber
 
Tifosi's Avatar
 
Join Date: Mar 2006
Location: Northern NY, USA
Posts: 13,741
The only pressure specs I believe are the ones molded into the tire sidewall by the manufacturer or supplied by them as the pressure the tire was tested at and had it's performance numbers derived form.

Taller profile tires would tend to need more pressure in them to help prevent serious deformation in turns (though nothing exceeding the spec on the sidewall of course) while lower profiles can get away with a bit less (but generally never below what the car has on it's data label)

The ideal would be to go to the tire manufacturers site if they have one and see what pressure they derived all thier data for that tire at.

Pretty safe bet that if the manufacturer says something like 'these specs derived at 34psi', then there's a fair chance that you'll see that same performance at that pressure too (barring a # or 2 one way or the other to dial on the suspension of course)




Darren
'84 manufacture ~ '85 MY Spider Graduate

To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.


To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.

To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.

as hosted by

To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.

To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.
&
To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.
Tifosi is offline  
post #3 of 19 (permalink) Old 06-17-2006, 11:57 AM
Registered User
 
Join Date: May 2006
Location: Houston, TX
Posts: 37
Garage
Send a message via Skype™ to mlprater
Monitor tread wear

Good question. I don't know the answer.

But perhaps you could monitor tread wear at the middle, and at the sides, and find out how the tire is wearing over time.

Matt

Houston, TX
1983 Spider Veloce
mlprater is offline  
Sponsored Links
Advertisement
 
post #4 of 19 (permalink) Old 06-17-2006, 12:02 PM
1966-2013
Platinum Subscriber
 
Tifosi's Avatar
 
Join Date: Mar 2006
Location: Northern NY, USA
Posts: 13,741
mlprater makes a good and viable point too, though it's a bit difficult to keep track of wear over long term street use vs the amount you can readily see in track use along with keeping tire pressures set religiously so as to not skew the results over a few months.

And as an add-on to my previous, the pressure # I was talking about. (btw, numbers are for a Yokohama ES100)



Attached Images
 

Darren
'84 manufacture ~ '85 MY Spider Graduate

To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.


To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.

To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.

as hosted by

To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.

To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.
&
To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.
Tifosi is offline  
post #5 of 19 (permalink) Old 06-17-2006, 12:28 PM
Registered User
 
Join Date: Jan 2004
Location: 1613 E. Clover St. Casa Grande, Arizona 85122
Posts: 1,085
Afas have the recommended tire pressure on the door post, on the glovebox door, or in the manual, for the recommended tires sizes on that chassis. This is for spirited high speed driving. I generall stick to this recommendation.

Check with the tire manufacturer, or a large tire dealer, for the air pressure needed for a given load on that make and model of tire.

The tire pressure shown in the catalogs is for the amount of air pressure needed for the maximum load that tire is rated for.

Spiders and GTVs are generally light for the size tire spec'd from the factory and so (if I remember correctly) they recommend 24# front and 26# rear.

George Willet


To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.
520-374-2220: please do not use PM, email me direct, saves us both time.

THESE are the good old days!
There are no easy answers to complex problems.

Last edited by George Willet; 06-17-2006 at 12:37 PM.
George Willet is offline  
post #6 of 19 (permalink) Old 06-17-2006, 12:41 PM Thread Starter
Senior Member
Gold Subscriber
 
ecohen2's Avatar
 
Join Date: Jan 2006
Location: Arlington, VA
Posts: 549
Send a message via Skype™ to ecohen2
As I understood tire pressure, the Max is not the recommended for a car, its the maximum that the tire can withstand. What is written in the glove box is never near the max, but is suppose to be the balance between the weight of the car and the strength of the sidewall.

Does that make sense?

Ed

1986 Bertone
ecohen2 is offline  
post #7 of 19 (permalink) Old 06-17-2006, 02:50 PM
1966-2013
Platinum Subscriber
 
Tifosi's Avatar
 
Join Date: Mar 2006
Location: Northern NY, USA
Posts: 13,741
Makes perfect sense, just like it did on the Firestone equipped Ford Explorers with their exceptionally dialed in tire pressures on thier stickers and in thier manuals a few years back

There's really nothing 'wrong' with the recommended pressure specs that come with Alfas, other than myself personally and by my own personal preference feel that it's much too low and creates a great amount of wallow and sidewall rollover in the corners.

Great if doing a 700 mile road trip in the lap of luxury, pathetic if you want any serious handling. (I'm not overly content with a car as small as a spider feeling like one of those great wallowing hogs known as the Caddilac ElDorado )

Either way, the higher I go with pressure, the happier I am with the handling, up to a point. (I never do quite reach the stamped on tire max cold pressure, which BTW, that number also represents)

All aside, it still boils down to what your personal preferences are for (a) handling, (b) responsiveness, and (c) ride comfort.

The rules still apply no matter how you shuffle the pressures around the number on the sticker.

Higher front pressure makes oversteer, lower makes understeer and higher rear pressure makes understeer while lower makes oversteer.

Anything else relevant to actual pressure is just a matter of how much wear you get on the center or outer edges of the tread (over or under inflated), whether the sidewalls rend themselves or belts break (again, under and over inflated) and how cushy or firm the ride is.

As in I'm not trying to be difficult or start an argument, but want to make the point that there are other options besides being the slave to a sticker that was relevant to tires made when the cars were but not neccisarily of use with modern tires that have gone through several generations of manufacturing evolution since the last spider came off the boat 13-14 odd years ago.




Darren
'84 manufacture ~ '85 MY Spider Graduate

To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.


To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.

To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.

as hosted by

To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.

To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.
&
To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.

Last edited by Tifosi; 06-17-2006 at 02:56 PM.
Tifosi is offline  
post #8 of 19 (permalink) Old 06-17-2006, 08:15 PM
Registered User
 
Join Date: Jan 2004
Location: 1613 E. Clover St. Casa Grande, Arizona 85122
Posts: 1,085
Tifiosi, if I am not mistaken, this thread is not about you, but about the questions that Ed was asking, and how we all are trying to give him factual input.

George Willet


To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.
520-374-2220: please do not use PM, email me direct, saves us both time.

THESE are the good old days!
There are no easy answers to complex problems.
George Willet is offline  
post #9 of 19 (permalink) Old 06-17-2006, 09:07 PM
Registered User
 
Alfapaulic's Avatar
 
Join Date: Jun 2003
Location: Wisconsin
Posts: 377
factual input

George, The fact is the recomended pressures from the car do not reflect current tire technology and the tire recomendations do not reflect old Alfa technology. So we have to experiment and find the proper pressure for our individual ride/handelling desires, that's all.

Paul
76 Spider
71 Spider & many parts
67 BSA
Alfapaulic is offline  
post #10 of 19 (permalink) Old 06-18-2006, 07:49 AM
1966-2013
Platinum Subscriber
 
Tifosi's Avatar
 
Join Date: Mar 2006
Location: Northern NY, USA
Posts: 13,741
You're right George, I should have bowed out a couple posts ago.

How could I possibly believe, or *gasp* worse yet, suggest to others that there might be something other than the all knowing sticker which is the be the sole and only source of information in regard to tire inflation on a spider, regardless of tire size used, manufacturing changes or any other nuances that obviously couldn't make a difference anyway as the sticker knows all and was designed to be used eternally for all instances and implimentations.........


I'll leave the thread now.


The sticker has spoken.




Darren
'84 manufacture ~ '85 MY Spider Graduate

To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.


To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.

To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.

as hosted by

To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.

To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.
&
To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.
Tifosi is offline  
post #11 of 19 (permalink) Old 06-18-2006, 05:23 PM
Registered User
 
genericwood's Avatar
 
Join Date: Jul 2003
Posts: 2,789
I'll make this really easy. The best pressure is almost always something in-between the Alfa owner's manual recommendation and the tire manufacturer's maximum load pressure. It varies some with the type of Alfa and the type/size of tire, but 32 pounds is not a bad starting point for most street driven Alfas on radials.

Erik
genericwood is online now  
post #12 of 19 (permalink) Old 06-19-2006, 02:34 PM Thread Starter
Senior Member
Gold Subscriber
 
ecohen2's Avatar
 
Join Date: Jan 2006
Location: Arlington, VA
Posts: 549
Send a message via Skype™ to ecohen2
It occured to me that I could take the current pressure and increase it by the percent change in the sidewall. Funny enough, that comes out to about 32 pounds.

Ed

1986 Bertone
ecohen2 is offline  
post #13 of 19 (permalink) Old 06-20-2006, 06:24 AM
Registered User
 
GV27's Avatar
 
Join Date: Dec 2004
Location: Lakewood, Colorado
Posts: 1,749
Garage
Send a message via AIM to GV27
I'll wear out my sidewalls before the tread if I run sticker pressures. I usually go in the high 30's.

Chris

1990 Spider Veloce
GV27 is offline  
post #14 of 19 (permalink) Old 06-20-2006, 07:01 AM
Supporting Vendor
 
JoeCab's Avatar
 
Join Date: Dec 2002
Location: Colorado
Posts: 4,325
I think what we're all trying to say is that you generally want to run the lowest tire pressure that will prevent excessive sidewall flexing.

Put another way, the needed tire pressure is inversely proportional to the stiffness of the tire's sidewall.

So, a max peformance tire with really stiff sidewalls (for example, the Falken Azenis RT-615) might perform and feel the best at 28 psi, while an all season street tire with soft sidewalls in the same size might need 35 psi to feel like it's not rolling over excessively.

Joe
JoeCab is offline  
post #15 of 19 (permalink) Old 06-21-2006, 01:04 AM
 
Join Date: May 2005
Posts: 25
I would like to very respectfully disagree with just about everybody except George Willet, whom I believe hit the nail exactly on the head.

According to the Washington State Patrol pursuit driving school (EVOC) the proper tire pressure should provide uniform downward pressure across the entire width of the contact patch.

Too little pressure, and the bottom of the tire becomes concave and the edges of the tires bear an excessive proportion of the weight, causing excessive edge wear and REDUCED TRACTION.

Too much pressure, and the bottom of the tire becomes convex and the center of the tire bears an excessive proportion of the weight, causing excessive center wear and REDUCED TRACTION.

Extra pressure does make the car easier to turn, particularly at low speeds, since it's easier to pivot the wheel when the weight is borne on a 'point' (the center of the contact patch instead of across the entire patch). This is however at the cost of reduced traction.

Extra pressure will of course also stiffen the sidewall a little, but I don't think it's worth wrecking your contact patch. (If you really need the sidewall stiffness, get lower profile tires -shorter sidewall - azenis rt615's!)

The proper pressure is a function of the weight borne by the tire.
Police cars in Washington have so much extra 'stuff' installed (roll cage, weapon racks, computers and radios in the trunk, pushbar on the front) that they have to deviate from the manufacturers spec based upon the increased weight. The pressure increase is a calculated amount based upon the borne weight.

A fully plumped out Crown Vic weighs about 5000 pounds (I'm told); more than a Ford Explorer! The stipulated tire pressure for our 'laden' Crown Vic's is somewhere in the upper 30's
My employer doesn't even allow me to mess with my tire pressure. They think it's that imortant. For liability purposes, I have to take my car to my agencies' 'tire shop', which keeps pressures to spec.

George basically said the above, but much more concisely.

Having written all that, I would observe that the spider got heavier as the different series' evolved.

I don't know how scientific alfa's pressure spec's are, but the specified pressures should increase from S2-->S4.

My S3 veloce stipulates 24 psi front and 26 psi rear.
I'm curious to know what the S2 and S4 cars stipulate.

I have Azenis Rt615's. Sticky.
I run the fronts about 2 psi over (26) in order to make it easier to turn the steering wheel at low speeds. Rears are 28-29 psi, to balance the front.
I accept the fact that I am trading traction for ease of steering. I have rotator cuff issues.

I won't even start about variations between tire pressure gauges...

DaveB
Seattle
(donning nomex suit)
Cord is offline  
Reply

Quick Reply
Message:
Options

Register Now



In order to be able to post messages on the Alfa Romeo Bulletin Board & Forums forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
Please enter a valid email address for yourself.

Email Address:
OR

Log-in










Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page
Display Modes
Linear Mode Linear Mode



Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

 
For the best viewing experience please update your browser to Google Chrome