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Old 11th July 2011, 14:46   #1
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Default Why rim size is as critical as tire pressures or tire sizes !!

Team,

Just wanted to share with you my experience yesterday which could have had my car featured on the accidents in india thread .

Car : 2001 Zen
Kms :1.9 L kms
Condition : better than most new cars
Tires : 165/60r13
Rims : Evo 5.5J since the last 1500 kms .Had Aura 5 J rims for 1.88L kms
Tire pressure maintained : 34 psi


I was driving at around 80 kph with 5 on board and the boot full with bags being used as head rest by the rear seat occupants and just enough view onthe CRVM

Usually i would be around 140 kph on this runway stretch ,but as i am not so happy with the handling change with the 5.5J rims ,i was doing almost half the usual speeds.Suddenly,2 buffaloes decide to eat the grass on the other side of the road and they just run across the road.As I spot them from quite some distance ,my first reaction was to downshift to 3rd gear and then brake as the road appeared wet on that stretch .

To my horror ,a small tap on the brakes in enough for the wheels to lock and the car just skids .As soon as i realise this i let go the brakes and dont turn the steering at all aiming straight at the buffaloes.But ,luckily the buffaloes decide to follow some traffic sense ( which the locals usually dont have ) and they stop .My LHS RVM grazes the buffalo ,but no one inside the car( other than me ) have any idea of how close were we to a big accident !!

I have been in panic stop situations earlier as well and at much higher speeds,but never before has the car stepped out of its line or not listened to me .

Its been the new 5.5 J rims that I changed recently to replace the old 5 J rims that have been the reason .I know its the rims for sure as I had one more similar situation the ither day when I almost rear ended another car in heavy rain when the car skidded again !!

Now my question is what should i do ?
1 - Change the rims ? - But we dont find 5 J rims these days
2- Maintain lower tire pressures ?

But this has also shown how important tire rim sizes are .EVen though the recommended rim sizes for 165/60r13 is 4.5 J to 5.5 J ,it was 5 J that gave me the best handling .

The reason I have posted this is because i see a lot of small car drivers blindly go for 5.5 J rim alloys just because dealers recommend this to them .And ,in the process endanger themselves as well as other road users .

I could have easily skidded into someone else on the other side of the road
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Old 11th July 2011, 14:53   #2
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Default Re: Why rim size is as critical as tire pressures or tire sizes !!

Thanks for sharing Satya. Do you think going for wider tyres would have helped when you switch from 5J to 5.5J?
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Old 11th July 2011, 15:07   #3
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Default Re: Why rim size is as critical as tire pressures or tire sizes !!

Dunno about that ,but I didnt opt for wider rims out of choice :( .

But yeah ,every tire size has got that "right" rim width if you want to make the best use of the tire .

After this incident ,i was very careful when braking in rain. I see so many small cars with OE tires and wide aftermarket rims
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Old 11th July 2011, 15:48   #4
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Default Re: Why rim size is as critical as tire pressures or tire sizes !!

@Satya, why do you think it is only the half inch increase in rim size that has caused this ? My guess is you have changed the tyres when you changed the rims too.

If what you are saying is true, then most of the SUV owners who have upgraded to alloys here wuold have reported similar problems.

Have you noticed any difference in the handling under normal conditions, after you changed the wheels ?

Last edited by condor : 11th July 2011 at 15:50.
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Old 11th July 2011, 15:50   #5
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Default Re: Why rim size is as critical as tire pressures or tire sizes !!

My Zen had 165/60/R13 Bridgestone Potenze GIII's shod on 5.5j rims but I never ever experienced such behaviour under panic braking or otherwise. My only grouse was fouling of the rear right tyre with the fender.

Why did you upsize to 5.5j though? I remember you had 5.5j earlier and then downsized to 5j.

Last edited by moralfibre : 11th July 2011 at 15:52.
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Old 11th July 2011, 15:55   #6
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Default Re: Why rim size is as critical as tire pressures or tire sizes !!

If its the same old set of tires then its possible that the extra width caused them to stretch and harden than when they were on 5J rims, whereas a wider tire would have fit more comfortably?
But i am not sure how many tire sizes you get in R13.
Probably the other alternative would be, like you said, to reduce the tire pressure.

EDIT: Looking at moralfiber's post, the above theory looks unlikely though. Moral, do you remember what tire pressure you used to maintain?

Last edited by amitoj : 11th July 2011 at 15:57.
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Old 12th July 2011, 07:31   #7
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Default Re: Why rim size is as critical as tire pressures or tire sizes !!

Quite unlikely that bigger rim size was cause of this problem.

Wrong sized tyre for the rim could be part of problem. Higher than recommended pressure (34psi) could be bigger cause. Oil spill or mud on wet road surface is another variable. Mud may not be visible but when it has been dry for a long time and the first drizzle hits the surface becomes very slippery.

Again does not look like issue solely due to upsized rims.
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Old 12th July 2011, 08:00   #8
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Default Re: Why rim size is as critical as tire pressures or tire sizes !!

satya I quite don't think the problem here is with the wider rims. In-fact wider rims = more contact area between the tires and tarmac, this in turn increases rolling resistance. I quite don't think it has to do with the wider rims, however with wider rims the center of gravity should have changed too.

Might be something else which you could be overlooking.
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Old 12th July 2011, 10:35   #9
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Default Re: Why rim size is as critical as tire pressures or tire sizes !!

Quote:
Originally Posted by rjstyles69 View Post
In-fact wider rims = more contact area between the tires and tarmac,
Riju, I dont think that is the case.
The contact area between the tires and tarmac is going to the same for a given tyre (size).

Note the diagram here.

Why rim size is as critical as tire pressures or tire sizes !!-rimwidtheffect.jpg

Image shows schematic of same tyre, different rims. Top rim is (say) optimum rim, and bottom one is narrower rim.

Difference in rim width is marked in blue.

When the tyre is seated on a narrower rim, the area along the inner dia (both walls) get squeezed inwards, to be able to fit the narrower rim.

This will introduce an amount of additional stress/strain on the side-walls of the tyre.

Similarly if you are wider rims, the side-walls have to be pushed apart to be able to fit the bigger/wider rims.

So narrower rims & wider rims are both sub-optimal. But between the two, I would think that the narrow rims are less dangerous. (As also seen in OE rims being on the narrow side).

Guess the ideal rim size would be a little wider than the width of the un-mounted tyre.

But either way, the rim size should not cause the problems mentioned by Satya.

Comments welcome !
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Old 12th July 2011, 10:42   #10
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Default Re: Why rim size is as critical as tire pressures or tire sizes !!

With wider rims the chances of the tyre coming off the rim is more. Also the tyres on wider rims may flex a bit more.

I think in this case it's cause the older tyres were mounted on narrower rims. Old tyres on narrow rims would have deformed into a more round shape. When mounting them on wider rims they would not be optimum.

Another factor could be if the new alloys have different offset?

Last edited by Vid6639 : 12th July 2011 at 10:56.
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Old 12th July 2011, 10:52   #11
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Default Re: Why rim size is as critical as tire pressures or tire sizes !!

@Viddy - Satya moved from narrower to wider rims.

But I would agree with your point about not using old tyres with new rims when the size is different (new rim size is different than old rim size).
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Old 12th July 2011, 10:55   #12
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Default Re: Why rim size is as critical as tire pressures or tire sizes !!

condor my post was based on the fact that you have chosen the right up-size for the tires.As for me I wouldn't only up-size the rims/alloys, tires are a part of the equation too.Like viddy said , if you chose to retain the stock tire size and increased the width of the rim the chances of the tire coming of the rims are quite high.
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Old 12th July 2011, 10:56   #13
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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
Tire pressure maintained : 34 psi
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.
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Old 12th July 2011, 10:58   #14
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Default Re: Why rim size is as critical as tire pressures or tire sizes !!

Quote:
Originally Posted by condor View Post
@Viddy - Satya moved from narrower to wider rims.

But I would agree with your point about not using old tyres with new rims when the size is different (new rim size is different than old rim size).
That was a typo. I corrected it now. Tyres will bed in with one set of rims over time, so when you change the rim width it may not work out.
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Old 12th July 2011, 11:03   #15
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Default Re: Why rim size is as critical as tire pressures or tire sizes !!

Riju, now it makes sense

Rim-to-tyre-matching already highlighted by Satya:
Quote:
Originally Posted by speedsatya View Post
But yeah ,every tire size has got that "right" rim width if you want to make the best use of the tire .
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