How to determine Track Width

I need a bit of tech help.

I am changing a rear solid axle on a car for a more robust one. This 'new' more robust axle is narrower than the original one.   I will/can relocate the spring perches, (old school leafs)  to fit

I am also changing the road wheels (front & rear) to match the PCD of the hubs on the 'new' rear axle, and accommodate the wheel offset.  I recognize the need for spacer/adapters. 

So how does one measure the track width? Is this centre to centre of road wheel? 

Thanks in advance

B

I think the centerline of the road wheel is the modern measurement, but it doesn't seem to have been that consistent in the past. Sometimes it's measured as hub face to hub face, especially on live axles where that's a really easy thing to measure and not have to calculate.

Thanks Keith

I suppose I could also measure brake drum face to the other drum face?  to verify?

The original track width of the car has a width delta of  approximately 3/4" with the rear being  narrower.  I should maintain this difference?

With all being equal I suppose the  wheels/tires combo should fit into the 'ample' wheel wells? 

This modification isn't anything extreme, however I just want to understand as I move forward.

Thanks again

B 

I think the brake drum face is the common measurement. If you change the relative track front/rear, you will change the weight transfer ratio and the handling balance will shift to more grip at the widened end. Your call on whether that's good or bad. 
 

It's hard to generalize about the wheels/tires fitting. 

Front engined car?

You can alter front/rear weight distribution by changing track at the front/rear.
Front engined cars are commonly front heavy, increasing front track or decreasing rear track will shift front
percentage to rear.

Not by a huge amount, but a few percentage points.

In reply to clshore :

No. You can change the amount of weight transfer at one end by changing the track - and it will affect the total weight transfer, not shift it - but the weight distribution (a static measurement) will only change if the new axle has a different mass than the old one. Again, it will be the total weight that changes.

Your post reads like how sway bars can shift the percentage of weight transfer from one end of the car to the other by changing the roll couple. It might be that's what you're thinking of.

In reply to Keith Tanner :

Not talking about weight transfer, talking about static weight distribution. 
The weight distribution at each tire patch is determined by the relative distances from the vehicle CoG.
The sum of the moments, distance x corner weight, = total weight.
When front track is increased, the distances of those 2 increase compared to the rear 2,
and front corner weights decrease, while the corner weights of the rears increase.
Example:
Vehicle weight = 2,000 lb
Wheelbase = 70"
Front Track = 50"
Rear Track = 50"
CoG = (25,35)

ordered (LF, RF, LR, RR)
Distances from CoG to each tire patch (43.012", 43.012", 43.012", 43.012")
Corner weights = (500, 500, 500, 500)

Now increase front track to 52"

Vehicle weight = 2,000 lb
Wheelbase = 70"
Front Track = 52"
Rear Track = 50"
CoG = (26,35)

Distances from CoG to each tire patch (43.6", 43.6", 43.012", 43.012")
Corner weights = (496.6, 496.6, 503.4, 503.4).

Not by much, but some.

Of course, changing wheelbase can accomplish same, but track changes can be done using wheels with different
width and/or offset.

Okay, if we're going for the technically correct answer - you're assuming a CG at ground level, which is pretty unlikely. Plugging in a height of 17" (what my notes say should be accurate for a Miata, so we can take that for generic small sports car), we get corner weights of 497.1 and 502.9, or a shift from 50.0:50.0 to 49.71:50.29.

Note that the accuracy of a set of Longacre scales in this range is 0.1% or 1 unit, whichever is greater. Our scales could be reading 499/501 on the square car and 498/502 on the wide front track. So we're flirting with the ability to actually measure this change. If our Longacre scales had the heavier load cells with the 0.25% allowable error, we wouldn't be able to measure it.

Meanwhile, that change in front track will decrease the weight transfer across the front axle by about 26 lbs (3.85%) and a drop in total weight transfer to match. That means more front grip and I suspect we'd see a much greater dynamic change.

From the standpoint of static weight distribution on a level surface, the height of the CoG does not matter.

But as you've noted, with 3D kinematics involving lateral acceleration, CoG height becomes more important.

A small % of advantage, consistently applied over the length of a race, can make the difference.
 

Wow, you chaps have taken this re fit of mine further than I expected.  

I guess I need to really research a bit more to make sure I don't muck this up totally. 

FWIW on this same car I changed the ride height and the handling, (which is poor anyway) went downhill rapidly, particularly in 'bouncy' cornering.  

Thanks again

Bruce

You're not going to muck it up monumentally by changing the track width by a reasonable amount. clshore's statements about static weight distribution are more in the theoretical realm than the actual - the bigger effect will come from the change in weight distribution in cornering. Just figure the wider axle will have more grip than it did before.

Your ride height change effects likely came from running out of travel.

Keith Tanner said:

Your ride height change effects likely came from running out of travel.

Keith, I appreciate this I believe you are spot on. 

I was altering the ride height and was bumping out. I have since corrected this. 

It would appear that not all 'lowering blocks' are created equal. 

Thanks

Bruce