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Old 12th July 2011, 11:16   #16
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Default Re: Why rim size is as critical as tire pressures or tire sizes !!

Thanks everyone for your responses.

Why did I change my rims ?

- The older rims were 10 years old and were bent .As a result ,I had to fill air every 2-3 days !

How old are my tires?

- These tires have done 20 K Kms in 1 year and the tires are still in very good condition

Tire pressures

- I used to maintain 30 psi earlier on my 5J rims,but I had a couple of tires with damaged side walls and punctures.Also I used to see the dark bands on the outer edges of the tires indicating underinflation. All this disappeared at 34 psi .

On 5.5J rims
- I failed to find decent 5 j rims and hence had to forcibly opt for 5.5 J rims.
- My first set of alloys were 5.5 J rims with 155/65r15 tires ,which i exchanged within the first month of driving as I had similar issues where the car skidded when braking for a speed breaker .
I have never had any issues after that on my 5J rims or the 165/60 r13 tires.

What I believe is the problem ?

I think i need to to again experiment with tire pressures with these new rims .Guess 34 psi is too high and hence the aquaplanning.


More thoughts needed
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Old 12th July 2011, 11:50   #17
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Default Re: Why rim size is as critical as tire pressures or tire sizes !!

I read somewhere that rim size needs to be 90% of tread width for optimal performance. 139.7mm/165 translates to 84%. 155 tyres would have been perfect 90% according to this recommendation.


--Ragul
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Old 12th July 2011, 11:56   #18
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Default Re: Why rim size is as critical as tire pressures or tire sizes !!

The rims are not at fault here. It is the higher tyre pressure at fault. When you fill 34psi in cold, it will increase to 36 after driving for a fair distance. You will feel the ride harsh and bouncy and handling unpredictable.
I have been in one similar situation when I was driving from Bangalore to Mysore, my brother had filled the tyres at 34 psi and the ride was very bouncy and the steering was vibrating a lot. When I had to slow down for an road hump while driving at 90kmph, a slight tap from the brakes caused the tyres to lock. I slowed down after that and got the air pressure checked at the next fuel station, it was 37 and 38 in front two tyres while the rear two were at 36.
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Old 12th July 2011, 15:32   #19
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Default Re: Why rim size is as critical as tire pressures or tire sizes !!

+1 Speedmiester. The tyre pressure is too high for highway travel. Also the brakes lock the wheel not the other way around, thus check if they are in healthy state, sometimes during fitting they are fiddled with. Maybe they tighten the brakes to adjust the tires (pure speculation). Drive safe.
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Old 12th July 2011, 16:14   #20
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Default Re: Why rim size is as critical as tire pressures or tire sizes !!

Quote:
Originally Posted by speedsatya View Post
Team,

Tire pressure maintained : 34 psi
Quote:
Originally Posted by Poitive View Post
Am no expert, but high tyre pressures have a VERY big effect on road grip. Also, with prolonged use the tyre pressure increases.

What is the recommended pressure?

Now for the rant:
Do this sometime. Check the tyre pressure in the morning (with your own gauge) without driving the car at all. Then after a few kms, check it right away. Chances are that it would be 3-4 points (or more) higher.

If it was 34 in the morning, it could be a lot higher later, especially if it was a hot day.

However, the recommended tyre pressure is "cold tyre pressure". I usually opt to have about 2 points above recommended settings, since the car has run a bit by the time one has reached the fuel pump.
I totally agree with Positive, tyre pressure plays a very important role in the grip. I had a almost similar experience where in i had over inflated the tyres as running more than my usual 29 psi, i think it should be 31-32. I was too lazy to remove the pressure even after noticing the extra bumpy ride. It was a stationary truck on the flyover in my case and a small dab of brakes was enough to lock the wheels. It took me over by surpise as my tyres had never locked so easily at 80 kmph and there was very less i could do as i had only 3 options, to scrape the side of the car into the traffic, scrape the side to the walls of the flyover or go straight into the truck and go into the rear of the truck i did.
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Old 12th July 2011, 16:30   #21
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Default Re: Why rim size is as critical as tire pressures or tire sizes !!

Any co-incidence that OE tyres & rims are both on the smaller side ?

Quote:
Originally Posted by speedsatya View Post
How old are my tires?
- These tires have done 20 K Kms in 1 year and the tires are still in very good condition
I think one year, 20k of use is still sufficient aging. It's not about tread, but the entire structure is seeing aging.

Quote:
Originally Posted by speedsatya View Post
Tire pressures
- I used to maintain 30 psi earlier on my 5J rims, but I had a couple of tires with damaged side walls and punctures. Also ... underinflation. All this disappeared at 34 psi .

What I believe is the problem ?

I think i need to to again experiment with tire pressures with these new rims .Guess 34 psi is too high and hence the aquaplanning.
30 psi to 34 psi on same size tyres ? That's more than a 10% jump. Did you try 32 ?

But then, I find 34psi better on my 185's compared to the 30psi recommended for the stock 175's. The difference here is that I have upsized. In your case, you are using the same size tyre.



Quote:
Originally Posted by Ragul View Post
I read somewhere that rim size needs to be 90% of tread width for optimal performance. 139.7mm/165 translates to 84%. 155 tyres would have been perfect 90% according to this recommendation.
--Ragul
Ragul, tread width is different from cross-section. Tread-width is lesser than Cross-section.

Cross section (as seen in the tyre specs - eg 155, 165 etc - is the width at the widest part of the tyre.

Tread width is the width of the contact area.
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Old 12th July 2011, 16:32   #22
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Default Re: Why rim size is as critical as tire pressures or tire sizes !!

I am very particular on the tire pressure and thats the reason I have my own foot pump as well as a portable compressor.
Maruti recommends 30 psi on the 145/70r13 tires that came on the old zen .As i have upsized ,I was maintaining 2 psi higher and add another 2 psi for highway driving .

I never ever had any problems with the older 5J rims with the same tires.

Anyways ,I am using 32 psi and i am already seeing a drop in FE !

One lesson I learnt is that not only do tire pressures vary with tire sizes ,they also vary with rim sizes !!
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Old 12th July 2011, 16:57   #23
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Default Re: Why rim size is as critical as tire pressures or tire sizes !!

Is there any chance of dust or rust in the Brake drum/Disc causing what the OP experienced?

Asking because almost all alloy shops stock only 5.5J rims in 14" and 13" size, and these days many small cars sport after-market alloys with OEM size rubber.

Last edited by Daewood : 12th July 2011 at 17:18.
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Old 12th July 2011, 18:00   #24
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Default Re: Why rim size is as critical as tire pressures or tire sizes !!

Quote:
Originally Posted by speedsatya View Post
I was maintaining 2 psi higher and add another 2 psi for highway driving .
...........................
Anyways ,I am using 32 psi and i am already seeing a drop in FE !
Why the extra pressure for highway?

AFAIK, extra pressure is really needed when one loads the car a lot.

Higher pressure usually gives better FE. Why? The rolling resistance/grip is lesser. Which simply means that the tyre will skid easily. Over-inflation is often used by taxi drivers to save on fuel and pocket some amount. Is well known to be an unsafe practice to save some bucks. - Not recommended.
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