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Tyre pressures #1661235
14/10/2022 13:03
14/10/2022 13:03
Joined: Feb 2006
Posts: 5,385
Essex
Trappy Offline OP
Club Member 1834
Trappy  Offline OP
Club Member 1834
Forum is my life

Joined: Feb 2006
Posts: 5,385
Essex
I've just came across an article talking about changing tyre pressures to suit a change in tyre (wheel) size. The thinking being that a reduction in tyre sidewall size increases its stiffness, and therefore increases its load capacity, thus less air is required to maintain the same load capacity.

The proposal is that one should look up the load capacity of the original tyre by its required pressure and reference this against the load capacity of the new tyre size where the required pressue would be listed.

For my car, I've moved from the original 205/50/16 to 235/40/17 and so I'd need to look up the load capacity of the 205/50/16. According to the website, this is 1277lbs as the stock pressure is listed as 39psi.

Cross referencing this 1277lbs load capacity against the pressure required by a 235/40/17 gives me 35psi

Given that this calculation doesn't factor in the differing tyre walls across different tyre brands and models, this could easily acount for the bone shattering ride some owners have complained of over the years when running larger diameter wheels... Throw in heavier wheel and tyre combinations, stiffer sidewalls and stiffer suspension and its only going to get worse.


F****** b****** thing...
Re: Tyre pressures [Re: Trappy] #1661243
14/10/2022 15:16
14/10/2022 15:16
Joined: Mar 2007
Posts: 15,349
Auld Reekie
Edinburgh Offline
Club President, member225
Edinburgh  Offline
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Posts: 15,349
Auld Reekie
This is interesting Ryan, thanks for sharing that.

I was advised several years ago with my 225/45/17s to let a few lbs out for exactly the same reason of crashiness and it's proved to be effective with no visible detriment to the wear.


BumbleBee back:)
Re: Tyre pressures [Re: Trappy] #1661246
14/10/2022 15:51
14/10/2022 15:51
Joined: Mar 2008
Posts: 5,040
highlands
jimboy Offline
Club Member 857
jimboy  Offline
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Forum is my life

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Posts: 5,040
highlands
I’ve been running with 35 psi for some time now makes for a more comfortable ride. I certainly will not be doing anything like 155 mph so 35 psi will be quite safe.


I'm an old git & happy with it,most of the time
Re: Tyre pressures [Re: jimboy] #1661248
14/10/2022 15:55
14/10/2022 15:55
Joined: Mar 2007
Posts: 15,349
Auld Reekie
Edinburgh Offline
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Edinburgh  Offline
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Auld Reekie
Originally Posted by jimboy
I certainly will not be doing anything like 155 mph


You don't want your insurance going up, Jim wink


BumbleBee back:)
Re: Tyre pressures [Re: Trappy] #1661250
14/10/2022 17:05
14/10/2022 17:05
Joined: May 2018
Posts: 758
born in Italy I live in German...
carmine Offline
club member 1915
carmine  Offline
club member 1915
Enjoying the ride

Joined: May 2018
Posts: 758
born in Italy I live in German...
yes, I run with 235-40-17 with 2.55 Bar at the front, and 2.45 Bar at the rear.

watch this

https://www.fiatcoupeclub.org/forum/ubbthreads.php/topics/1640951/re-tyres-pressures#Post1640951

Re: Tyre pressures [Re: Trappy] #1661257
14/10/2022 18:32
14/10/2022 18:32
Joined: Dec 2005
Posts: 4,063
Pothole City
GrahamL Offline
Forum is my job
GrahamL  Offline
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Posts: 4,063
Pothole City
IMO 39 psi is too high even on the standard wheels, 33-36 is more like it depending on the quality of roads you drive on.

Re: Tyre pressures [Re: carmine] #1661259
14/10/2022 20:49
14/10/2022 20:49
Joined: Mar 2007
Posts: 15,349
Auld Reekie
Edinburgh Offline
Club President, member225
Edinburgh  Offline
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Forum veteran

Joined: Mar 2007
Posts: 15,349
Auld Reekie
Originally Posted by carmine
2.45 Bar at the rear.


That's quite high, do you carry a lot in the boot?


BumbleBee back:)
Re: Tyre pressures [Re: Trappy] #1661262
14/10/2022 22:25
14/10/2022 22:25
Joined: May 2018
Posts: 758
born in Italy I live in German...
carmine Offline
club member 1915
carmine  Offline
club member 1915
Enjoying the ride

Joined: May 2018
Posts: 758
born in Italy I live in German...
no, i don't carry much, i'll lower the pressure tomorrow, thanks for the tip.

Re: Tyre pressures [Re: Trappy] #1661310
17/10/2022 08:38
17/10/2022 08:38
Joined: Jun 2007
Posts: 4,583
Malaysia..Glasgow at heart
Kelv27 Offline
Club member 1998
Kelv27  Offline
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Malaysia..Glasgow at heart
I think back in the 90s, tyres had much softer sidewalls.

Re: Tyre pressures [Re: GrahamL] #1662814
08/01/2023 14:15
08/01/2023 14:15
Joined: Sep 2007
Posts: 945
Behind Enemy Lines
Master_Mariner Offline
Club member 583
Master_Mariner  Offline
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Enjoying the ride

Joined: Sep 2007
Posts: 945
Behind Enemy Lines
Originally Posted by GrahamL
IMO 39 psi is too high even on the standard wheels, 33-36 is more like it depending on the quality of roads you drive on.


Can absolutely relate to Graham,

See my subtle attempts to reduce "crashiness" in previous posts.

The number one best factor was Joe (Country Cruising) reducing the pressure all around after a shakedown test drive. Have driven them at lower pressure ever since- no issues of lost grip. No issues of advance tyre wear. But noticeably less bouncy.
The number two factor was replacing my Toyo Proxes for the "newer" Toyo design (I'm on the plus alloy wheel design size wheel so limited choice) and this has had a felt difference.

Additional factor - quite some time gently tightening up old trim screws in the cabin, and pads to the doors and handles where they meet plastic on plastic. You'd be surprised of the improvement!

Regards all,,

MM


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