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Old 26th June 2017, 13:45   #1
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Default Rationale behind the 'recommended' Tyre Pressure?

I was reading the Tiago TD thread, when the tyre pressure sticker struck me.

Its running around 30psi in the back and 33 psi in the front for 175/65 R14 (higher variant)

My Indica Turbo came with stickers saying 28 psi front 24 psi rear

I remember thinking aloud in an earlier thread (Queries On The Indica Dicor's Footwear) why the indica Dicor ( which is essentially the same car with a common rail engine) was recommended 36 psi front and 28 in the rear

all these for essentially very cars with similar wheels, tyres, weight , power etc.

Why are these numbers so different? What is the rationale behind them?
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Old 26th June 2017, 19:34   #2
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Default re: Rationale behind the 'recommended' Tyre Pressure?

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Originally Posted by greenhorn View Post
...for essentially very cars with similar wheels, tyres, weight , power etc.

Why are these numbers so different? What is the rationale behind them?
I'll try and make this as brief as possible as this can be a fairly long topic.

There are several tyre technical advisory bodies worldwide that determine tyre technicalities including pressure and their decisions are based on various factors which include tyre construction, profile, curvature, compound, etc and most importantly actual user experience. These tyre technical advisory bodies also discuss between themselves to share experience.

And your observation is right - about 10 years ago most tyre pressures were around the 24 -28 psi. This has now universally gone upto around 30 - 34 psi. And that is why you see the difference between the earlier Indica and the Dicor. This decision to increase the tyre pressure was based on user experiences by these bodies.

Primarily there are 3 entities involved in the entire exercise - (1) Worldwide Tyre Technical organisations - (example in Europe it is ETRTO) (2) Tyre manufacturers and (3) the vehicle manufacturer. Tyre manufacturers and vehicle manufacturers are usually members of the technical organisations.

Last edited by AMG Power : 26th June 2017 at 19:39.
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Old 26th June 2017, 20:02   #3
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Default re: Rationale behind the 'recommended' Tyre Pressure?

Quote:
Originally Posted by greenhorn View Post
My Indica Turbo came with stickers saying 28 psi front 24 psi rear

I remember thinking aloud in an earlier thread why the indica Dicor ( which is essentially the same car with a common rail engine) was recommended 36 psi front and 28 in the rear
One change we understand is the difference in Engine (along with few kg mass difference) and what we are unaware is the change in other parts / systems including the suspension (stiffness and damping), body rigidity (a determinant factor for overall vehicle rigidity), change in overall vehicle mass, market feedback (customer voice or quality concern) for the earlier generation vehicle.

Tire pressure contributes a lot for Ride & Handling characteristics of the vehicle and it is chosen based on so many other parts (or system) characteristics. And one of the other reason is "FE"
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Old 26th June 2017, 21:27   #4
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Default re: Rationale behind the 'recommended' Tyre Pressure?

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Originally Posted by Mr.Boss View Post
And one of the other reason is "FE"
This is the real reason, along with pollution norms. Higher pressures mean more tyre wear but you get less rolling resistance and hence better FE and emissions figures.

As a low mileage user, my tyres get discarded because of age before wear. Within reasonable limits, I generally believe in going for higher pressures as long as the ride remains tolerable.
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Old 26th June 2017, 21:48   #5
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Default re: Rationale behind the 'recommended' Tyre Pressure?

I no longer run my tyres at the recommended pressure as i was getting the "underinflated" type of wear. These days I run at 30psi, and was thinking of going higher, so with modern tyres and current thinking, it should be fine I guess ?
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Old 26th June 2017, 22:10   #6
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Default re: Rationale behind the 'recommended' Tyre Pressure?

It is a general trend nowadays. Recommend tire pressures have increased in all cars quite significantly over the past decade. I believe this has more to do with ever increasing economy and emission related requirements. Pumping in an extra 5psi is a simple and sure shot way to increase fuel efficiency and make the car feel lighter on it's feet and more responsive. Plus, it also helpful in standardized emission tests. It can make a car using an older engine scrape through newer emission norms during homologation and extend the life cycle of the powertrain.

Basically, the game is like this:
Std pressure: 30psi (let's assume)
Economy setting: 34 to 36psi
Comfort setting:26 to 28psi

Last edited by Shreyans_Jain : 26th June 2017 at 22:13.
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Old 27th June 2017, 10:34   #7
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Default Re: Rationale behind the 'recommended' Tyre Pressure?

I remember reading on another Team-BHP thread that the type of tyre and its size, wheel size, speed capability of the car, its weight (kerb + laden), weight distribution, suspension hardware + tuning, target customer preferences (ride quality, handling, FE) etc. are also contributing factors to the recommended tyre pressure.

But yeah, today's cars definitely have economy-oriented PSI recommendations. There's been an increase across the board. Hard to believe that cars like the Zen had a 24 PSI rating in the 90s.

Quote:
Originally Posted by AMG Power View Post
This has now universally gone upto around 30 - 34 psi
Why do some cars have absurd tyre pressures? IIRC, the Passat had like a 42 PSI sticker (or in the whereabouts).

Quote:
Originally Posted by greenhorn View Post
These days I run at 30psi
Quote:
Originally Posted by Shreyans_Jain View Post
Std pressure: 30psi
Whenever in doubt, fill 30 PSI! Have followed that rule for many years on some cars (including my old Jeep).
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Old 27th June 2017, 11:09   #8
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Default Re: Rationale behind the 'recommended' Tyre Pressure?

I'm sure its also got to do something with the higher average speeds vehicles do these days. Also with lower profile tyres becoming more common, lower pressure will lead to rim damage faster.
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Old 27th June 2017, 12:00   #9
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Default Re: Rationale behind the 'recommended' Tyre Pressure?

Quote:
Originally Posted by greenhorn View Post
I no longer run my tyres at the recommended pressure as i was getting the "underinflated" type of wear. These days I run at 30psi, and was thinking of going higher, so with modern tyres and current thinking, it should be fine I guess ?
The tyre pressure you use depends on your driving cycle - amount of short or long runs in the city / your driving style - do you avoid potholes or crash through them, are you an anticipatory driver or do you drive on the brakes etc / your average speeds / amount of long distance drives / the loads you carry / the type of roads you drive on / the way you maintain your car and a lot more.

Generally you can increase your usual tyre pressure by 10% on long distance / high speed drives. Going above that will lead to loss of traction and hazardous driving conditions but you are the best judge and if you are experienced enough you would be able to take that judgement call.

The main reason for the general increase in tyre pressure is NOT FE as mentioned but the amount of accidents and blow outs that were happening.The most scandalous was the one on Ford recommended around 24psi for its Explorer which led to a lot of accidents. This led to the general increase in tyre pressure.

Quote:
Originally Posted by GTO View Post
Why do some cars have absurd tyre pressures? IIRC, the Passat had like a 42 PSI sticker (or in the whereabouts).
The Passat came with 42 and even more weird was the Vento with 39 !! These are tyre pressures relevant for high speed runs on the autobahns of Germany and as is typical of VW it didn't bother to tweak these for any other market and its applicable to all markets where these cars were sold. Just as every other component of their cars aren't really modified for any other market - at the most you have a rough road package - that's it!!

Low profile tyres on high performance cars are generally recommended with higher tyre pressures.
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Old 28th June 2017, 12:07   #10
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Default Re: Rationale behind the 'recommended' Tyre Pressure?

Quote:
Originally Posted by GTO View Post

Why do some cars have absurd tyre pressures? IIRC, the Passat had like a 42 PSI sticker (or in the whereabouts).
Even the Mercs have very high recommended pressures. I know for a fact that the car's suspension was designed to work best with tyres at 42-44 PSI on the Merc. The W212 model had a lot of tyre related issues when it was launched in India and that was because many owners randomly just filled 30-32 PSI like they would have done in their normal cars. However, this made the 17" tyre's sidewall softer and weaker and led to lot of tyre bursts.

On the Mercs, 40-42 PSI gives the best combination of ride and handling.

This is probably why the Passat also recommended 42PSI.

Generally keeping 30 PSI works well... But I would be hesitant to do that on these luxury cars where they have specifically recommended something far higher.
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Old 28th June 2017, 14:15   #11
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Default Re: Rationale behind the 'recommended' Tyre Pressure?

Its certainly not just about the FE. For instance, I don't have to do a routine pressure check for my Vista. The moment the pressure in my front tires drops below 32 PSI, I physically "feel" it in normal driving also. The steering gets sluggish & heavy. Feedback from steering reduces. The recommended pressure for Vista diesel is 35 front 32 rear with load. So the recommended pressure must be a complex equation of suspension setup, vehicle weight & power, cornering characteristics, speed rating, wheel dimensions & tire profile.

I feel that the lower tire air pressures in the 90s were also dictated by the tire architecture. They were mostlyl nylon / radials but with TUBE inside. The pressure was also applicable on the tube itself which is certainly far more prone to failures of fatigue or explosive deflation in case of punctures. The sudden deflation of a tube-tire setup from a high PSI will certainly be more catastrophic. Today's tubeless radials more or less rule out the explosive deflation & so can safely run at higher pressures...
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Old 29th June 2017, 14:34   #12
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Default Re: Rationale behind the 'recommended' Tyre Pressure?

Quote:
Originally Posted by GTO View Post
Why do some cars have absurd tyre pressures? IIRC, the Passat had like a 42 PSI sticker (or in the whereabouts).
Quote:
Originally Posted by AMG Power View Post
The Passat came with 42 and even more weird was the Vento with 39 !!
Even more absurd is my humble Micra has a range 35psi to 45psi based on semi or fully loaded. Why would a 800 KG small cars with a puny engine and steel wheels would need that high pressure is still beyond my brain.

I read somewhere in TBHP, under inflation leads to tyre burst, compelling me to keep 36 psi.
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Old 29th June 2017, 14:45   #13
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Default Re: Rationale behind the 'recommended' Tyre Pressure?

The recommended pressure for my 2002 Zen is 24 psi! I don't remember when was the last time I filled 24 psi though. It has always been in the range of 28-30 psi. But anything over 32 psi makes it bouncy, especially the rear.

In fact, I cannot think of filling just 24 psi now and probably will not be able to fathom the sluggishness of the car and heaviness of the steering, esp in a non-assisted steering set up!

Regards,
Saket
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Old 29th June 2017, 15:28   #14
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Default Re: Rationale behind the 'recommended' Tyre Pressure?

Softer the side wall higher the PSI, i guess the newer tyre tech is also partly contributing as mentioned by AMG rulez. Real example the stock Thar tyres mention some 32 range, but if we use the same on offroad tyres with different sidewall ratings then the car just bounces around, i tried 28 and now running 26 without any tyre wear and reasonable comfort as before.

Other factors right from road conditions, suspension, body weight, dynamics, wear tear and efficiency... all add up to the so called recommended psi.
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Old 2nd July 2017, 15:24   #15
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Default Re: Rationale behind the 'recommended' Tyre Pressure?

Our Baleno_Lxi & Swift_Zxi come with 29psi recommended. I normally maintain them at 31psi.
I hate the 'flat' look of underinflated tyres & prefer tyres to look round (OCD )

Our '99 M800 had a 27psi recommendation & an old M800 I came across yesterday had 26psi mentioned on the door frame.
While the AStar is technically an Alto, it is an entirely different car in terms of weight & safety. Not sure if this is behind it
coming with a 32psi recommendation.

While maintaining the correct pressure helps increase engine-efficiency & tyre durability, for non-PS cars, it also helps ease
of steering. Have felt the difference with our M800.

Last edited by supremeBaleno : 2nd July 2017 at 15:25.
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