TSW Bathurst 20" first install

[50PONY]

Question??????

Will be running TSW Bathurst Gunmetal on a Magnetic GT.
I have chosen 20 x 9 up front and 20 x 11 rear. I want to maximize the concavity.

My question relates to the rubber. I am told that you should run 255/35/20 up front and 305/30/20 up back. Why would they suggest 35 in front and 30 in the rear. The short answer I was given was to maintain the same rolling diameter. Does this make sense?

Cheers Brett

Sponsored

 

[plc268]

Question??????

Will be running TSW Bathurst Gunmetal on a Magnetic GT.
I have chosen 20 x 9 up front and 20 x 11 rear. I want to maximize the concavity.

My question relates to the rubber. I am told that you should run 255/35/20 up front and 305/30/20 up back. Why would they suggest 35 in front and 30 in the rear. The short answer I was given was to maintain the same rolling diameter. Does this make sense?

Cheers Brett

If you keep the same diameter, it won't mess with your speedo too much. Personally, I run 275/35/20 in front, and 305/35/20 in the rear. The diameter in the front goes up by about .5 inches in the front and 1.5 inches in the rear in my setup.

The benefit of my setup, is that the visual appearance of the staggered setup will be more striking with a taller diameter tire in the rear.


But yea, doing a 255/35 and 305/30 is a good setup as well.

 

[50PONY]

Thanks Patrick.
Can you explain why 35 profile in the front would be the same rolling diameter as 30 in the rear. Still can't understand why two 20" rims with different profile tyres could have the same rolling diameter .

 

[plc268]

Sure. There's a little bit of math (and converting from millimeters to inches) to figure out the tire diameter. Or there's various calculators on the net.

255/35/20 = 27" diameter (20 inch rim, 3.5 inch sidewalls)
275/35/20 = 27.58" diameter

305/30/20 = 27.2" diameter
305/35/20 = 28.4" diameter

The aspect ratio (profile) is a roundabout way of measuring the sidewall, but it's tied to the width of the tire. For example, the width of a sidewall on a 255/35/20 is calculated by taking the width (255 mm) and multiplying it by the aspect ratio (35%). Multiplying this, gets you the width of the sidewalls in mm, in this case: 89.25mm. Converting it to inches gives you ~3.5 inches.

Now, a tire diameter is figured out by taking the rim size (20 inches) and adding two sidewalls together (20+3.5+3.5=27 inches).

A wider tire will have a larger sidewall with the same aspect ratio. 305/35 will have a 106.75 mm sidewall, for example.

Hopefully that helps you in understanding how the tire sizes work.

 

[50PONY]

Patrick

You're a legend, first person who has made it make sense to me.
I have taken a photo of your explanation for later reference.

Again thank you

 

[Need4SpeedMotors]

Sure. There's a little bit of math (and converting from millimeters to inches) to figure out the tire diameter. Or there's various calculators on the net.

255/35/20 = 27" diameter (20 inch rim, 3.5 inch sidewalls)
275/35/20 = 27.58" diameter

305/30/20 = 27.2" diameter
305/35/20 = 28.4" diameter

The aspect ratio (profile) is a roundabout way of measuring the sidewall, but it's tied to the width of the tire. For example, the width of a sidewall on a 255/35/20 is calculated by taking the width (255 mm) and multiplying it by the aspect ratio (35%). Multiplying this, gets you the width of the sidewalls in mm, in this case: 89.25mm. Converting it to inches gives you ~3.5 inches.

Now, a tire diameter is figured out by taking the rim size (20 inches) and adding two sidewalls together (20+3.5+3.5=27 inches).

A wider tire will have a larger sidewall with the same aspect ratio. 305/35 will have a 106.75 mm sidewall, for example.

Hopefully that helps you in understanding how the tire sizes work.

Very good explanation sir.

 

[RSPEC-015]

Hi Brett,

You're in NSW, like me, so you will have to meet the NSW RMS requirements for alternative wheels and tyres.

There are 2 Vehicle Standards Information guidelines which provide information on what modifications can be done without needing an Engineers Certificate.

VSI9 Rev 4 is the more conservative and that states the overall diameter of the tyre can not change more than 15mm over the original tyres.

For us, with 255/40/19 for the front, means you can go up to (686.6 + 15) = 701.6mm.

With 275/40/19 for the rear, means you can go up to (702.6 + 15) = 717.6mm.

With your front tyre selection 255/35/20 the new overall diameter will be 508 + (2x255x0.35) = 686.5mm which means you are OK and pretty much spot on with the original overall diameter.

If you go 275/35/20 the overall diameter will be 508 + (2x275.0.35) = 700.6mm so will still be OK with the guidelines.

I would just check what is best tyre width for a 9" rim but I'm pretty sure 275 is OK.

For the rear with 305/30/20 the overall diameter will be 508 + (2x305x0.3) = 691mm which is OK but could give a sagging in the rear end type of appearance.

I think a better choice would be 295/35/20 for which the overall diameter would be 508 + (2x295x0.35) = 714.6mm which is OK.

This will have the rear a little bit bigger than the front so you get the rake pretty much like standard, especially if 275/35 tyres are OK for the front.

A 305/35/20 rear will be outside the guidelines at 721.5mm.

275/35 and 295/35 are readily available in NSW plus both front and rear have the same aspect ratio. I have the 19" MRR350's and just had the tyres fitted today. I have 275/35/19 F and 295/35/19 R. They're off to the detailer for Ceramic Pro next week and then they'll go on the car.

Hope this helps.

Jim

 

[50PONY]

Hi Brett,

You're in NSW, like me, so you will have to meet the NSW RMS requirements for alternative wheels and tyres.

There are 2 Vehicle Standards Information guidelines which provide information on what modifications can be done without needing an Engineers Certificate.

VSI9 Rev 4 is the more conservative and that states the overall diameter of the tyre can not change more than 15mm over the original tyres.

For us, with 255/40/19 for the front, means you can go up to (686.6 + 15) = 701.6mm.

With 275/40/19 for the rear, means you can go up to (702.6 + 15) = 717.6mm.

With your front tyre selection 255/35/20 the new overall diameter will be 508 + (2x255x0.35) = 686.5mm which means you are OK and pretty much spot on with the original overall diameter.

If you go 275/35/20 the overall diameter will be 508 + (2x275.0.35) = 700.6mm so will still be OK with the guidelines.

I would just check what is best tyre width for a 9" rim but I'm pretty sure 275 is OK.

For the rear with 305/30/20 the overall diameter will be 508 + (2x305x0.3) = 691mm which is OK but could give a sagging in the rear end type of appearance.

I think a better choice would be 295/35/20 for which the overall diameter would be 508 + (2x295x0.35) = 714.6mm which is OK.

This will have the rear a little bit bigger than the front so you get the rake pretty much like standard, especially if 275/35 tyres are OK for the front.

A 305/35/20 rear will be outside the guidelines at 721.5mm.

275/35 and 295/35 are readily available in NSW plus both front and rear have the same aspect ratio. I have the 19" MRR350's and just had the tyres fitted today. I have 275/35/19 F and 295/35/19 R. They're off to the detailer for Ceramic Pro next week and then they'll go on the car.

Hope this helps.

Jim

Jim thank you for this information. Sadly I have had to read and re read it for it to sink in. Maths is not my strong point. I will have a chat before any final decision on rubber. You may have just saved me from ending up with a saggy stang.

Cheers Brett

 

[RSPEC-015]

No worries. If you stick with 275/35/20 & 295/35/20 you'll be right. You can check HybridGT's Build thread as he has Velgens and went with that tyre choice. His wheels are 20x9.0 and 20x10.5.

Sponsored