Corner Weights

[twinny]

Just been fortunate enough to have been taken to MIRA by a friend in our region to have my cars corner weights checked. The figures came out as follows.

N/S Front : O/S Front
261.0 244.6 505.6 Front : 39.4%
N/S Rear : O/S Rear
385.4 392.6 778.0 Rear : 60.6%
N/S total O/S total
646.4 637.2

Overall total weight = 1283.6kg

Car is a 1987 Targa with the addition of an aluminium strut brace. The OEM exhaust has been replaced by an H&S system which is lighter and eliminates the pre-silencer. Estimated fuel load around was around 10 gallons. Tools and compressor removed (these weighed approx 5kg). Spare wheel in place. No driver.

I had always felt my car was a little low on the front N/S, but looking at this, adjusting that height would make my front figures worse i.e. more than the exiting 15kg difference.

Any ideas guys? Also why is the total so much more than Porsches figures? Do they calculate the car without fuel/oil/ windscreen washer water?

 

[jam1976]

I always thought porsche figures were based on a full tank of fuel and an allowance for a driver weighing 11.5 stone app. not sure where i heard this, on the forum somwhere, i have the same problem my '89 3.2 is also lower on the n/s

 

[franklin]

Hi Twinny,

You are very honoured to have access to corner weight scales

Firstly kerb weight DIN 70020 is quoted at 1210kilos for '84-'89 Carrera's. This is with full tank, tools, spare wheel, ready to drive. This being said, numerous Coupes I've measured are between 1200 and 1320kilos after adding drivers weight which is the way I prefer to set ride heights, corner weights and geometry alignment (see Fred Puhn). This assumes anything from 70to90kilos of driver (chuckle).

1283kilos seams reasonable, although you have about 28kilos of fuel missing (0.77grams per cc) x 38litres (8.5gallons). Adding this you are in the money I believe.

The Carrera Workshop manual quotes a maximum difference between left and right of 20kilos for 911 and 10kilos for Turbo front and rear, so again you are in the money with a difference of 16.4kilos. Rear is even better at 7.2kilos.

Your cross weights NSF to OSR = 50.91% of total vehicle weight and OSF to NSR = 49.09%. To get more balance (simplistically) you need to increase the ride height on the OSF marginally (or split height increase between OSF and NSR). This increases the load slightly on the diagonal (OSF-NSR) and reduces it on the the (NSF-OSR).

You have most likely set your ride heights beforehand, setting the back first at 12mm with tolerance of plus/minus 5mm (workshop method) within 8mm of each other then coming forward to the front with 108mm ride height (workshop) plus/minus 5mm and 5mm of each other. You should have a 1degree nose down attitude measured at the sill. The corner weight balancing is referred to 'extended alignment' in the Porsche Workshop Manual and comes after ride height setting. Major changes in camber can affect ride heights and thus corner weights, so it becomes iterative.

It all boils down to how far you want to go. The overall weight ideally should be as lights as you can go, you are not a racer, so don't worry about the kerb weight. Best to concentrate on the ratios of that weight over the 4 wheels, concentrating on getting below across axle tolerances of 20kilos and cross weights for equal turn performance. Some people opt for a straight looking car (ride heights), others go for what scales say.

Either way you will have plenty of fun adjusting, running the car, adjusting, running the car (etcetera) to get it just right. So keep buddies with your friend with scales at Mira

chris

 

[nathan 1981 930 G50]

You should have weighed the corners with you in the car.

I think the NSF always feel light due to the camber on the road being the opposite of the camber of the wheel.

My '82 3.2 weighed in at 1190KG with just over half a tank of fuel in it.

My modded 930 weighed in at 1225KG with very little fuel in it. Should weigh less when I have fitted all my tuning parts to it

 

[twinny]

My NSF doesn't feel light. I've always believed my NSF to be a little low, ride height wise, maybe only a few mm when sat on level ground, but subjectively noticeable to me anyway.

Kerb weights quoted at the time (1987) I believe to be with all fluids and a full tank of petrol, but without passengers or luggage. Things changed in the mid 90's with EN regs.

My first point was/is my car is quoted at 1210kg in the handbook and gets nowhere near that in reality. I was curious where all the weight had come from or whether Porsche weighed a non-sport, completely standard coupe (no options) and quoted those figures for everything...Coupe Targa & Cab. These days it is widely admitted that when you cut the roof off the car gets heavier. I wonder what the true figures of each type should have been.

Secondly I wanted to see if my car was in balance i.e. within the 20kg between wheels on a common axle line as stipulated by Porsche - Clearly mine is and I now know what needs to be done to get it spot on (if I can be bothered).

Lastly the point about loading up the seats and luggage compartment only applies if I'm setting up the alignment. I think Id' leave this to JZ or one of the other guy's that know about this stuff. Looking at Bentley's you're applying a pretty even load, spread across and down the centreline of the car in order to get it sit down before checking/ changing the alignment. Overall the L/R, F/R and diagonal ratios shouldn't change too much I wouldn't have thought.

The fact remains my car has 1210kg stated in the handbook and yet it weighs in at nearly 1300kg. That's an awful lot of extra weight to be lugging around and I was just wondering what the effect might be on performance and handling and whether Porsche were being economical with the truth at the time, not unlike some of the performance car companies do with their BHP & torque figures these days.

 

[nathan 1981 930 G50]

Hi Twinny.

Re. looking level at the front.
The technician at the Harston OPC told me that as the cars were handbuilt not to worry about how the wheel relates to the wheelarch (if that is what you were referring too).

Loading up seats and luggage only applies if setting up alignment.
My car was setup by people that built race cars so they setup my corner weights with me in the car. Of course it depends whether you carry a passenger or a co driver.
I only intend to drive hard if I am off the public road and then I will be unlikely to be carrying a passenger.

Another downside of our cars is that the roll bars are not adjustable. Some USA companies make them but they do not look very German at all []