Track and Wheelbase definition?

Jim Holden · #1 (**permalink**) 02-25-2008, 10:11 AM

Can anyone out there refer me to the technical definition for wheelbase and track? In other words, what do you actually measure to get a reading for each?

I assume that wheelbase is the center of the king pin to the center of the rear axle.

But where do you measure track? Flange to flange? Or is it center of the contact patch to the center of the other contact patch? etc. etc.

Small block Cobras were quoted ("back in the day") as being 51.5" track front and 52.5" rear. What were they measuring?

Jim Holden

mr0077 · #2 (**permalink**) 02-25-2008, 10:14 AM

I thought I read that wheelbase was front wheel center to rear wheel center and track was the distance from center of tread to center of tread for a given axle.

strictlypersonl · #3 (**permalink**) 02-25-2008, 11:23 AM

Track is the distance from the center of the left tire tread to the center of the right tire tread.

speed220mph · #4 (**permalink**) 02-25-2008, 11:35 AM

Further, front and rear track, or tread width, are not necessarily the same. They are not on a Cobra.

Jim Holden · #5 (**permalink**) 02-25-2008, 11:37 AM

So, if for example, you were using six inch rims, you would ad 3+3 to the flange to flange number to arrive at the track number? Or do you have to take into account camber angle as well? But then there's back spacing to be factored in as well, isn't there.

Jim

speed220mph · #6 (**permalink**) 02-25-2008, 12:03 PM

Jim: Track is measured at the center of the wheel level with the ground. It's independent of wheel offset or backpsacing unless you're trying to use the wheels to gain a particular track. Increasing backspacing--moving the wheels in--reduces track and vice versa.

Jim Holden · #7 (**permalink**) 02-25-2008, 01:42 PM

Speed220:

I should have been clearer where I was going with this. As I indicated, the orignal small block cars had a track; front 51.5" and rear 52.5" as reported in several magazine articles of the time. Assuming a 6" wheel, I am trying to get to the flange to flange measurement of the original cars and to do that, I have to know the backspacing of the wheels, I would think. As for the various replicas out there, I would think that this measurement could be all over the place given different offsets and different body widths, etc.

Jim

speed220mph · #8 (**permalink**) 02-25-2008, 02:57 PM

Simple enough, Jim.

All you need to know is the tread you're wanting to obtain and the wheel-hub surface measurement from side to side. Subtract the two and divide the result by two. If the desired tread is a bigger number, say 52.5" versus 50.0", the answer would be a positive 2.5". Divided by two, the offset of each wheel from center would be 1.25". With a zero offset wheel, backspacing would be 3.00" with a 6" wheel plus allowance for the bead. But in this case, back spacing would be 3.00" minus 1.25", or 1.75" to set the center of the tires on the ground at zero camber to 52.5" apart, which is track, or tread width. Does this make sense?

mr0077 · #9 (**permalink**) 02-25-2008, 02:59 PM

Jim, I think to get at the flange-to-flange dimension, the wheel offset is what you want; you add twice the wheel offset to the track to get this dimension.

Backspacing is the distance from the inside lip of the wheel to the mounting flange, and offset is the numerical distance (plus or minus) from the wheel centerline to the mounting flange, with a positive offset placing the flange closer to the outside of the wheel. If you had a -2" offset on the rear wheels and the rear track was 52.5", the flange dimension would be 52.5" + [2 x (-2")] = 52.5" - 4" = 48.5".

Jim Holden · #10 (**permalink**) 02-26-2008, 10:41 AM

So, if the 6" 70 spoke wire wheel (according to Dayton) has a back spacing of 3.25" you would add 1/4" x 2= 1/2" to the track number (52.5" for the rear for example) to get the flange to flange measurement of 53"... Right?

Jim

I feel like I need (as in the old Arlo Gutherie song) 8x12 glossy photos with circles and arrows and a description on the back.

speed220mph · #11 (**permalink**) 02-26-2008, 11:18 AM

Quote:

Originally Posted by Jim Holden

So, if the 6" 70 spoke wire wheel (according to Dayton) has a back spacing of 3.25" you would add 1/4" x 2= 1/5" to the track number (52.5" for the rear for example) to get the flange to flange measurement of 53"... Right?

Jim

I feel like I need (as in the old Arlo Gutherie song) 8x12 glossy photos with circles and arrows and a description on the back.

Start with your math: 1/4" X 2 = 1/2". Because the backspacing is more that 1/2 rim width--hope you accounted for the bead--subtract 1/2" from 52.5" to arrive at a flange-to-flange distance of 52.00"! Your spoke wheel in this case is what was called "laced in" in the old days, so the center of the tires moves in on both sides from the hub flange by 1/4" per side. Simple . . . right?

strictlypersonl · #12 (**permalink**) 02-26-2008, 11:21 AM

Unfortunately backspacing is not measured from the bead, it's measured from the outer edge of the casting. Typically a wheel is 3/4"-1" wider overall than its "nominal" width. Your 6" wheel might be 7" total width so that 3.25" backspacing puts the wheel mounting surface 1/4" in from the centerline of the wheel. Which is why I much prefer to specify mounting-surface-to-centerline rather than backspacing.

speed220mph · #13 (**permalink**) 02-26-2008, 11:30 AM

Bob: That includes you and me. Problem is when you're starting with a wheel and don't have the specs like in your drawing, you've got to measure the bead from the edge of the rim.

Jim Holden · #14 (**permalink**) 02-26-2008, 11:37 AM

I hit the wrong key, obviously 1/4 x2= 1/2".

...and Bob comes through with the 8x12.

For the sake of the discussion I was omitting consideration of wheel flanges, etc. and essentially viewing this as Bob has suggested i.e.; that the centerline of the 6" wheel is 3" in and (inasmuch as I don't have any other dimensions than the backspacing) the backspace is 3.25", I would have to add the negative offset number, less the centerline number to the track to get the flange to flange number.

Maybe I can coerce Bob into going into his data base and coming up with the magic number for the 6" Dayton 70 spoke wire wheel... and even a diagram.

Jim

mr0077 · #15 (**permalink**) 02-26-2008, 11:45 AM

Jim, your arithmetic is right, but I'm not sure about the assumptions. If I understand right, backspacing is the distance from the inside edge of the wheel lip to the mounting flange of a wheel, but offset is measured from the center of the wheel to the mounting flange, ignoring the distance from the outside edge of the wheel to the inside bead seat, which is about 1/4". If you need the exact flange-to-flange measurement, you really need the offset of the Daytons, or maybe get one and measure it.

I suspect that the 3.25" back spacing includes 0.25" from the inside edge of the rim to the inside of the wheel lip (where the tire bead mounts), then 3" from the inside of the lip to the center of the wheel, meaning that the offset on the Daytons could be zero. If so, the rear flange-to-flange dimension is 52.5", same as the rear track. See this link for wheel backspacing and dimensioning info:
How to measure Backspacing - GETAHELMET.COM
and this link for offset info:
Wheel Tech - Offset

Aww, never mind...Bob and Tom beat me to it and with better info!

speed220mph · #16 (**permalink**) 02-26-2008, 11:56 AM

Jim: If the back spacing of the 6.00" wheel is 3.00" there is no offset. Makes things simple for you. Flange-to-flange measurement is the same as track.

mr0077 · #17 (**permalink**) 02-26-2008, 12:23 PM

Tom, if the backspacing of a 6" wheel is 3", there will be a negative offset due to the width of the lip (distance from the outer edge of the rim to the bead seat). If the width of the lip on a 6" Dayton is 0.25, and the backspacing is 3.25", there would be no offset.

Don · #18 (**permalink**) 02-26-2008, 12:39 PM

The US Government definition:

Footprint is defined as the product of track width (measured in
inches and rounded to the nearest tenth of an inch) times wheelbase
(measured in inches and rounded to the nearest tenth of an inch) divided
by 144 and then rounded to the nearest tenth of a square foot.

For purposes of this definition, track width is the lateral distance between
the centerlines of the base tires at ground, including the camber angle.
For purposes of this definition, wheelbase is the longitudinal distance
between front and rear wheel centerlines.

Section

speed220mph · #19 (**permalink**) 02-26-2008, 01:31 PM

Ken: You're right, except I was taking the 1/4 in. into account. Likewise, if bs is 3.00 in. and the flange isn't considered, offset is -0.25in.

Simple things can cause so much confusion it funny.

mr0077 · #20 (**permalink**) 02-26-2008, 01:44 PM

Tom, from the definitions I've seen, if you lay a wheel on a flat surface, inside edge down, backspacing is effectively the distance from the mounting flange to the flat surface, but the bead seat is maybe 1/4"-1/2" above the flat surface. The wheel width is from bead seat to bead seat, sooo, the distance from center of the wheel to the flat surface (backspacing) would be half the width plus the 1/4"-1/2" lip,...A 6" wheel with a 1/4" lip has the centerline 3.25" above the flat surface; with a 3.25" backspacing the flange is lined up with the centerline for zero offset, right?

Oh well, what the hey...