Tyre recommendations

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IanM
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Re: Tyre recommendations

Post by IanM »

gridgway wrote: Tue Dec 27, 2022 4:11 pm What do you mean by index load
185/70 has an index load of 89 (I don't know what is the unit, sorry) whereas the 195/65 has a load rating of 91.
Last edited by IanM on Mon Jan 02, 2023 2:45 am, edited 1 time in total.
911hillclimber
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Re: Tyre recommendations

Post by 911hillclimber »

TYRE LOAD INDEX CHART
Each load index number is tied to a real weight, measured in kilograms, on the chart. A tyre with a load index of 91, for example, can hold a maximum load of 615 kg at the correct PSI up to its rated speed.
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IanM
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Re: Tyre recommendations

Post by IanM »

Thanks hillclimber. Just found this:
https://www.blackcircles.com/helpcentre ... oad-rating
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gridgway
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Re: Tyre recommendations

Post by gridgway »

I'll dare go out on a limb here. I think there is some confusion. The load index is the max load for the tyre. It's not specific to the dimensions. Clearly it's related, but it's not correct to say that a specific width and profile of tyre has a specific load rating.

Also it's not specifically related to slow speed steering effort.

In general a higher load rating is going to be a tyre that is heavier in construction and thus stiffer so you want the right load rating for the car's weight.

Not enough and the tyre may fail, too much and the tyre may be too stiff as the dynamics of the tyre are integral in the suspension dynamics.

Back to my original question, I don't think that the load index is specifically related to turning effort at low speed.

I think that's primarily to do with the size of the contact patch.

The contact patch size is a function of the tyre dimensions, being radius and width.

Back to my previous suggestion, fatter tyres are harder to steer at lower speeds. Little to do with load index.

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KS
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Re: Tyre recommendations

Post by KS »

I swapped from 195/65x15 Dunlops to 185/70 Blockleys on El Chucho and...absolutely zero difference in steering effort at any speed. There was also no appreciable difference in the way the car handled out on the road or, for that matter, on the test track. What did make the biggest difference was tyre pressures, whichever tyre was fitted.
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Re: Tyre recommendations

Post by Lightweight_911 »

.

You also have to realise that tyres of a given 'size' from different manufacturers can vary significantly in some of their dimensions - for instance if you compare Pirelli CN36/Michelin XWX/Avon CR6ZZ in 185/70 x15:

CN36 - overall width 196mm & overall weight 8 kg

XWX - overall width 189mm & overall weight 10 kg

CR6ZZ - overall width 198mm & overall weight 10.2 kg


.
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- subtracting weight makes you faster everywhere”
911hillclimber
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Re: Tyre recommendations

Post by 911hillclimber »

^^^ My findings too, I've had around 6+ different tyre types and 3 different tyre sizes on my 73T over the last 32 years.

In all cases, the steering was only '911 light' as all the magazines go on about once, Goodyears (185 x 70 on 6" rims) after Bob Watson set the car up in 1990.
I called Bob when back home to say the steering was so light and wriggly there must be something loose. He laughed...

I put my heavy parking speed steering on too much caster, may be wrong.

That index is the max load the tyre can tolerate at the max speed given for long periods. Normal UK driving probably stresses the tyre about 30/40% of that capability.

Personally, I don't buy into the look of a tyre, the tread pattern, the profile is important to the look of the car itself, and if you need to go FAST then you should use a specialist tyre, KUMHO V70 soft compound is ridiculously grippy, no so in the rain and will do 3000 miles if lucky on the driven wheels. I went through 3 sets on my 911 when hillclimbing it, road legal, but looked a bit odd @ 50 x 205 on 7", the rims it is on now. (pic below)

The Dunlops are A rated in rain & noise.
Wet weather grip to me is very important, a lot depends on that!

You can do some nice road trips with the £1000 saving?

Image

Image
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Re: Tyre recommendations

Post by BILLY BEAN »

I think we need a thread on "Which is the best oil for my 911" as we have not had one for such a long time.
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911hillclimber
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Re: Tyre recommendations

Post by 911hillclimber »

Ha!
Or which wax is the best.
Interesting to read other's experiences, but it all depends what you do with the car and makes you happy.
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Re: Tyre recommendations

Post by IanM »

gridgway wrote: Wed Dec 28, 2022 5:03 am In general a higher load rating is going to be a tyre that is heavier in construction and thus stiffer so you want the right load rating for the car's weight.

Not enough and the tyre may fail, too much and the tyre may be too stiff as the dynamics of the tyre are integral in the suspension dynamics.
I entirely agree. You said it best.

For rear tyres, 215/60 is perfect (and homologated) if you have 7RS wheels but not 7R. The latter requires a tube and the 60 profile is too low for tubes. Dougal explained elsewhere that tubes should not be fitted with less than 70 profile tyres. But I think you can cheat with 65 profile but definitely not less than that. For 7R I think 205/65 is the best size but unfortunately classic brands like CN36 don't make these.



.
Lightweight_911 wrote: Wed Dec 28, 2022 9:28 am

CN36 - overall width 196mm & overall weight 8 kg

XWX - overall width 189mm & overall weight 10 kg
Incredible. I didn't know that.
Saving unsprung weight is a big advanatge.
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Re: Tyre recommendations

Post by IanM »

KS wrote: Wed Dec 28, 2022 8:50 am I swapped from 195/65x15 Dunlops to 185/70 Blockleys on El Chucho and...absolutely zero difference in steering effort at any speed.
I've just remembered that the light steering at parking speed that I experienced was with 185/65 HR 15 (88 load). Stock Koni's all round.

BTW, el Chucho was a SWB and 4-cylinder?
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Re: Tyre recommendations

Post by KS »

IanM wrote: Sat Dec 31, 2022 4:02 pm
KS wrote: Wed Dec 28, 2022 8:50 am I swapped from 195/65x15 Dunlops to 185/70 Blockleys on El Chucho and...absolutely zero difference in steering effort at any speed.
I've just remembered that the light steering at parking speed that I experienced was with 185/65 HR 15 (88 load). Stock Koni's all round.

BTW, el Chucho was a SWB and 4-cylinder?
SWB and 2.2-litre 'six'...
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Re: Tyre recommendations

Post by IanM »

KS wrote: Sat Dec 31, 2022 4:10 pm SWB and 2.2-litre 'six'...
OK thanks.

Approx weight distribution in %

SWB
40/60

LWB
42/58

:wink:
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Re: Tyre recommendations

Post by IanM »

Anyway, for my next purchase of tyres, I'm thinking 185/65 front and 205/65 rear. (with tubes all round, of course)

Most, if not all, 185/65s are HR speed rated (88LI), but as far as I know Firestone Roadhawk is the only brand that is available in VR.
221955988_365350328445490_265774475790958233_n.jpg
Below is the only reference picture I could find on the net of a 205/65 Firestone. The sidewall looks just right :drunken:
f94d910c-4e5a-4d56-b599-9adf57b43e0a.jpg
78e409a2-bea1-4456-a125-910f2330af90.jpg
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Re: Tyre recommendations

Post by Lightweight_911 »

.

When you have different width wheels front & rear it's normal practice to reduce the aspect ratio of the rear tyres so that the overall diameter remains roughly the same - a 205/65 x 15 tyre will be approx 1in larger in diameter than a 185/65 x 15 tyre ...

.
Andy

“Adding power makes you faster on the straights;
- subtracting weight makes you faster everywhere”
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