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Old 09-17-2015, 12:20 PM   #1
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Weight on front and rear

Does anyone know the approximate weight on the front and rear tires on a stock Boxster S?
Not the max load as listed in the owners manual, but actual weight.
I know my it won't be exact for my car, but it will be close enough for govt. work

I trying to figure out some track tire pressure recommendations using a formula that uses front and back weight

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Old 09-17-2015, 01:19 PM   #2
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Originally Posted by JayG View Post
Does anyone know the approximate weight on the front and rear tires on a stock Boxster S?
Not the max load as listed in the owners manual, but actual weight.
I know my it won't be exact for my car, but it will be close enough for govt. work

I trying to figure out some track tire pressure recommendations using a formula that uses front and back weight

My 2000 S manual is 2855 lbs from factory, your 2004 is probably 75 lbs more. Find front to rear weight bias & figure from there. Does the formula consider tire size?
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Old 09-17-2015, 01:24 PM   #3
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Originally Posted by BYprodriver View Post
My 2000 S manual is 2855 lbs from factory, your 2004 is probably 75 lbs more. Find front to rear weight bias & figure from there. Does the formula consider tire size?
Ok, what is the front/rear bias?

yes, it factors in the tire width
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Old 09-17-2015, 04:48 PM   #4
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Do a search for corner weights and some one will have list this when setting up their car.
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Old 09-17-2015, 08:39 PM   #5
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Originally Posted by jcslocum View Post
Do a search for corner weights and some one will have list this when setting up their car.
Corner weighting only works for coilovers. Since the ride height of the stock shocks can't be adjusted, there is no way to adjust the corner weights. Thus, there is usually not much of a point in putting a car with stock suspension on the scales.

With that understanding, finding a post with corner weight info might be better than nothing, but it won't be exactly the same as the stock shocks.
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Old 09-17-2015, 08:42 PM   #6
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I was just looking for a close idea of the front and back weights. I'm not trying to corner balance the car
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Old 09-17-2015, 08:44 PM   #7
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Originally Posted by jcslocum View Post
Do a search for corner weights and some one will have list this when setting up their car.
Corner weighting only works for coilovers. Since the ride height of the stock shocks can't be adjusted, there is no way to adjust the corner weights. Thus, there is usually no point in putting a car with stock suspension on the scales.

With that understood, finding a post with corner weight info might be helpful (better than nothing) but it won't be exactly the same as the stock shocks.

See this post for more info on corner weighting: Corner balance - suspension
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Old 09-17-2015, 08:47 PM   #8
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Here is some info on an S-model that might help: Corner Weights

Or at least close enough for government work!
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Old 09-18-2015, 06:51 AM   #9
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Originally Posted by thstone View Post
Here is some info on an S-model that might help: Corner Weights

Or at least close enough for government work!
Thanks Tom,
That's exactly what I was looking for.

The formula I got to estimate correct tire pressure in relation to weight and tread width is:

Pf = (weight/100) + 2
Pr = ((Wr*Tf)/(Wf*Tr)) * Pf ****corrected, was Pr = ((Wr*Tf)/(Wf*Tr) *****

Pf - front pressure
Pr - rear pressure
Wr - weight rear
Wf - weight front
Tf - tread width front
Tr - tread width rear

Using
3000 lbs as total weight
1650 lbs as rear weight
1350 lbs as front weight
255 as rear width
225 as front width

we get

(3000 / 100) + 2 =32 front pressure
((1650 * 225) / (1350 * 255)) * 32 = 34.5 rear pressure

That should be a good starting point for tire pressure for best performance

Yes, it will vary due to lots of variables such as tire compound, ambient temp, track condition, turns and lots more, but at least it give you some idea where to start

Factory spec is 29 front / 36 rear and that builds in more understeer. Raising the front in relation to the rear will reduce the understeer, so going to 34.5 rear & 32 front makes sense that it would be more neutral.

Of course, this would all have to be adjusted once the tires heat up. You can also check the temp of the tire tread on the inside, center and outside. They should be relatively even across the tread. You can adjust pressure based on those measurements. if the center is higher, lower the pressure a little and if the outside is higher, raise it a little.

Whats really interesting is for stock 17" tires 205 / 255 combo the calc give you 32 front and 31.4 rear! On stock 18" 225 / 265 its 32 front and 33 rear!

Yes, the weights are approximate for my car, but it is close enough for government work

Just my 2 cents FWIW
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Last edited by JayG; 10-03-2015 at 06:54 PM.
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Old 09-18-2015, 07:19 AM   #10
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I'm not so sure about the math on the rear...
Pr = ((Wr*Tf)/(Wf*Tr)
so, Pr=((1650 X 225)/(1350 X 255))
That gives us 371,250 / 344,250
I'm getting 1.078..
Where am I going wrong??
I'm very interested in this formula, too.

FWIW, to get a quick F/R weight distribution, measure between the wheel centers and mark the center of the car on the door sill with a piece of tape. Take the car to a truck scale and park the car on the scale wit the tape directly lined up with the split in the scale plates. Should do the trick for around $10.

Quote:
Originally Posted by JayG View Post
Thanks Tom,
That's exactly what I was looking for.

The formula I got to estimate correct tire pressure in relation to weight and tread width is:

Pf = (weight/100) + 2
Pr = ((Wr*Tf)/(Wf*Tr)

Pf - front pressure
Pr - rear pressure
Wr - weight rear
Wf - weight front
Tf - tread width front
Tr - tread width rear

Using
3000 lbs as total weight
1650 lbs as rear weight
1350 lbs as front weight
255 as rear width
225 as front width

we get

(3000 / 100) + 2 =32 front pressure
((1650 * 225) / (1350 * 255)) = 34.5 rear pressure

That should be a good starting point for tire pressure for best performance

Yes, it will vary due to lots of variables such as tire compound, ambient temp, track condition, turns and lots more, but at least it give you some idea where to start

Factory spec is 29 front / 36 rear and that builds in more understeer. Raising the front in relation to the rear will reduce the understeer, so going to 34.5 rear & 32 front makes sense that it would be more neutral.

Of course, this would all have to be adjusted once the tires heat up. You can also check the temp of the tire tread on the inside, center and outside. They should be relatively even across the tread. You can adjust pressure based on those measurements. if the center is higher, lower the pressure a little and if the outside is higher, raise it a little.

Whats really interesting is for stock 17" tires 205 / 255 combo the calc give you 32 front and 31.4 rear! On stock 18" 225 / 265 its 32 front and 33 rear!

Yes, the weights are approximate for my car, but it is close enough for government work

Just my 2 cents FWIW
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Old 09-18-2015, 07:51 AM   #11
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OOPS, missed a piece of the formula when I copied it. I corrected it in my post

Was Pr = ((Wr*Tf)/(Wf*Tr)

the correct formula is:
Pr = ((Wr*Tf)/(Wf*Tr)) * Pf

in your case - 1.078 * front pressure = rear pressure
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Old 09-18-2015, 08:05 AM   #12
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Quote:
Originally Posted by JayG View Post
OOPS, missed a piece of the formula when I copied it. I corrected it in my post

Was Pr = ((Wr*Tf)/(Wf*Tr)

the correct formula is:
Pr = ((Wr*Tf)/(Wf*Tr)) * Pf

in your case - 1.078 * front pressure = rear pressure
Excessive mental masturbation!
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Old 09-18-2015, 08:12 AM   #13
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Originally Posted by BYprodriver View Post
Excessive mental masturbation!
an exercised brain is a strong one

All seriousness aside, I have been trying to find the "right" tire pressures and this give me a good starting point for even balance. Had far too much understeer with the factory pressures.
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Old 09-18-2015, 08:20 AM   #14
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Originally Posted by JayG View Post
an exercised brain is a strong one

All seriousness aside, I have been trying to find the "right" tire pressures and this give me a good starting point for even balance. Had far too much understeer with the factory pressures.
986 has been around awhile, I could have told you where to start psi.
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Old 09-18-2015, 08:27 AM   #15
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Originally Posted by BYprodriver View Post
986 has been around awhile, I could have told you where to start psi.
Ok, where?
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Old 09-18-2015, 08:29 AM   #16
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thanks for the update. Math is now sound!
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Old 09-18-2015, 08:33 AM   #17
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Ok, where?
33 F 34 R always better to start on the high side if you have no idea, less shoulder wear, & faster & easier to reduce pressure than add.
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Old 09-18-2015, 08:58 AM   #18
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Yes, that is a good starting point, but you do need to factor in tread width
With narrower front tires 205 vs 225 with 255 or 265 rears, the rear needs to be lower than the front for neutral balance
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Old 09-18-2015, 09:40 AM   #19
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When I first got started with HPDE I thought that as long as you didn't roll over on the shoulder you were good. I didn't realize that by the end of the day my hot pressures were up in the 40's and I wondered why I was sliding around like crazy. The first time I had a "hot" session with my tires right in the sweet spot (around 32 front and 34 back hot, 225/45/17 and 255/40/17 Hankook RS3) I was like WOW. What a difference. A friend has a 996 with PS2s and ran these really high GT3 pressures and did not bleed throughout the day sp he was wickedly high in the 40's, once I got him to bleeding he was like WOW too and ran his best times.

The downside is airing them back up for the drive home... if it is a long drive like 3 hours they will cool back down and go below 30 if I don't give them a little air before leaving out.

Steve
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Old 09-18-2015, 09:49 AM   #20
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Originally Posted by steved0x View Post
When I first got started with HPDE I thought that as long as you didn't roll over on the shoulder you were good. I didn't realize that by the end of the day my hot pressures were up in the 40's and I wondered why I was sliding around like crazy. The first time I had a "hot" session with my tires right in the sweet spot (around 32 front and 34 back hot, 225/45/17 and 255/40/17 Hankook RS3) I was like WOW. What a difference. A friend has a 996 with PS2s and ran these really high GT3 pressures and did not bleed throughout the day sp he was wickedly high in the 40's, once I got him to bleeding he was like WOW too and ran his best times.

The downside is airing them back up for the drive home... if it is a long drive like 3 hours they will cool back down and go below 30 if I don't give them a little air before leaving out.

Steve
I found similar results at the track and the last few AX I did.
I have been running Hankook Ventus V12 evo2's. They are great street tires, love them for canyon carving, but when they hot at the track or AX, they get really greasy and slippery. I found I was dropping PSI a lot. It would help.
At higher temps and then pressure, the tire does not give you the maximum contact patch, hence poorer performance

I just got some 17" wheel for track and AX and am planning on putting RS-3's on them. They came with brand new Sumitomo HTR ZIII's and I will run with them for a but until they are spent.

Tire pressure is really important for traction and a little change can make a big difference. Porsche builds in lots of understeer as it is "safer" for many drivers

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