I have been getting a lot of request via private Email to compare tire sizes
for folks. I figured I would just let everyone know how I do it. I will still
be happy to answer questions about tire sizes, but you can have a lot of fun
doing this yourself with a spreadsheet.
Here are the instructions to set up a spreadsheet for tire sizes and speeds.
The formulae are industry-standard from BFG. They were taken from a Team T/A
tech sheet. After I made the spreadsheet, I ran the BFG examples to check it.
I did mine in Excel, but rather than try to send the Excel file, I thought it
would be more useful to send instructions that should work in your spreadsheet
program.
Assume for these formulae that your first cell is the upper left and it is
called "A1". [Cell "eye-one" looks like "I1"]. Actually, you will probably
start lower on the page so you can get your titles on the top. These formulae
are all on one line, using columns A through L, so you can fill down the
formulae and entered data to fill as many lines as you like, then change tire
sizes, gearbox ratios, final drive ratios, or engine speed on each line to
compare. You can figure the speeds in gears for your car, or check the effect
of a different tire size.
This example uses a 225-50-16 tire. For each column, the example will give:
1) The cell designation.
2) The title for the column.
3) The formula used for that column. "enter" means that the user enters this
data.
4) The number which appears in the cell as the result of the formula or entry.
[Comments about the example]
A1
Section Width (mm)
enter
225
B1
Aspect Ratio
enter
50
C1
Wheel Diameter (inches)
enter
16
D1
Wheel Width (inches)
=E1*0.7
6.20 [This is the industry-standard measuring rim width of 70% of the section
width. If you use a wider rim, your tire will measure wider. I don't
like to use a wheel narrower than this for autocross and like to be above
80%. I think you might have problems with wheels wider than 100%.]
E1
Section Width (inches)
=A1/25.4
8.86 [This is just a straight mm/inch conversion.]
F1
Section Height (inches)
=1*B1/100
4.43
G1
Overall Diameter (inches)
=(F1*2)+C1
24.86
H1
Revolutions per Mile
=20800/G1
836.74 [This gives a number about 3% higher than straight math calculations
would suggest. I can only assume that BFG's testing confirms the "flat
spot" theory that has spawned so many arguments in the past. I choose
to just accept BFG's formula.]
I1
Engine RPM
enter
5000
J1
Gearbox Ratio
enter
0.96 [This is a slight overdrive, maybe fifth gear]
K1
Final Drive
enter
3.55 [This is a typical "rear-end" ratio]
L1
Speed (MPH)
=(G1*I1)/(K1*J1*336)
108.54 [My spreadsheet displays all calculated numbers rounded to the nearest
hundredth, but all numbers are held in the spreadsheet to many decimal
places for calculation. This gives hell of a lot more accuracy than we
need.]
Go ahead and try this out. If you get numbers different from my example, let
me know and I will look for errors.
It is left as an exercise for the student to print out "speeds in gears" graphs
for the student's car from a spreadsheet modified from these formulae. :-)
Phil Ethier, THE RIGHT LINE, 672 Orleans Street, Saint Paul, MN 55107-2676
h (612) 224-3105 lotus@pnet51.orb.mn.org
w (612) 298-5324 phile@pwcs.stpaul.gov (list goes here)
"The workingman's GT-40" - Anthony Colin Bruce Chapman
--
Login name: phile In real life: Philip J Ethier
Phone: 298-5324
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