Monday, April 16, 2012

Installment #5 – Longer Wheelbase


No space here for a full lesson in physics, but automobile suspension systems, and their resulting geometry, are little more than a multi-variable balancing act.

In an earlier installment I discussed the importance of having a 25 lb. battery mounted in the front of a rear engined car.  In the case of my particular 912, there are two 25 lb. batteries ahead of the front wheels.  That applies even more leverage to the engine and most of the transmission which are mounted behind the rear wheels.  

The stock 1969 Porsche 912 weight distribution is described as “near perfect” and listed at 55% rear and 45% front.  Believe it or not, just that extra battery ahead of the front wheels makes it more like 54% rear and 46% front – even better.  

In keeping with Porsche’s thoughtful and effective design philosophy, all 1969 passenger car models were on a 2.25 inch (5.7 centimeters) longer wheelbase (length from rear wheel center to front wheel center) but the overall length of the body was left unchanged from the 1968 models.

This slight forward movement of the front wheels enhanced the positive effects of battery, tool, fuel and spare tire placement explained previously by putting the weight of the engine and transmission 2.25 inches further back from the centerline of the front wheels.  

And don’t forget that the two 25 lb. batteries on my 912 are ahead of the front wheels.  More distance to the rear axle means more leverage against engine and transmission weight behind the rear axle and a better balanced vehicle.  Simple physics































Above:  The left side rear wheel opening, torsion bar and adjustment cover 
plate (the circled white piece) on my 1969 Porsche 912.


If you’re really into this sort of thing, you can quickly spot the difference between 1969 911 and 912 Porsche models and earlier models by looking for the torsion bar access cover just ahead of the rear wheels.

As you can see in the photo above, looking toward the front of the car from near the left rear tire, the circular torsion bar cover (inside the black circle on the left side of the photo) is about 2.25 inches forward of the rear wheel body opening.































Above:  The same left rear wheel opening, showing the location of the torsion bar adjustment cover relative to the wheel opening.


That’s where the increased wheelbase happened on the 1969 model 911 and 912.

In the photo above, of the same spot but taken at a different angle and from a few feet away, the placement of the torsion bar access cover is very clear.  

If you look for the same feature in any pre-1969 Porsche 911 or 912, you will find the circular cover is practically on top of the rear wheel body opening.  

The photo below, of a red 1968 912, shows the torsion bar access cover is about as close as it can get to the rear wheel opening.  

That’s a visual give-away (among others previously discussed), so next time you see an older Porsche and wonder if it is or isn’t a pre-1969 911 or 912 model, look at the same spot.  You’ll see and understand the difference, now that you know what to look for.  




































Above:  Compare the placement of the right rear torsion bar access cover on this red  ’68 Porsche 912 with the photos above of my white ’69 Porsche 912.   


More importantly, just from reading this Installment you’ll also know more about why just that extra 2.25 inches of wheelbase is so important.  

Adding the extra wheelbase in exactly the right location became one more reason that all 1969 Porsche passenger cars, and the superb-handling 912 Porsche in particular, remain track and highway icons after all these years.