Remind how to calculate gear ratio again?

[Tom Grossmann]

With F500.us down I was unable to access the gearing calculator.

I'm going to be doing a test day next month; I've got 15/60 sprockets for the car and the car uses a Polaris P85 and Polaris secondary. The 15/60 sprockets are what was on the car when it was run at Waterford Hills. I run 18" tires

I'll be running LVMS outside road course, I estimate the car's top speed may be as low as 115mph but possible as high as 120. As I'm trying to get a feel for how the car will function on a road course I'd like to get the AMW motor though as many sessions as possible. Part of the plan is to keep the car down to around 7000 rpms.

Any help calculating the gearing would be greatly appreciated.

Note I've got a Rotax waiting go in so the AMW going kablamo isn't a great tragedy but I'd still like to get 3-4 sessions in.

[BorkRacing]

google is your friend

[t walgamuth]

As for calculating gear ratio, count your teeth and divide the small number into the larger. For calculating the ratio needed, take your maximum rpm desired times the gear ratio, then put in the tire diameter and figure backwards to get max speed in mph. I am not sure if I answered your question.

[lance3556]

Main Gear tooth count	Secondary Gear tooth count	Ratio	Jack Shaft RPM@ .84:1 ratio	Jack Shaft RPM@ 1:1 ratio	Axle RPM @ .84:1	Axle RPM @ 1:1	MAX Speed (19.5") Tire @ .84:1	MAX Speed (19.5") Tire @ 1:1	MAX Speed (18") Tire @ .84:1	MAX Speed (18") Tire @ 1:1	AIM Data Setup for speed2 18 inch
110	32	3.44	6804	8100	1979.35	2356.36	115.65	137.68	107.78	128.31	16.64
110	34	3.24	7047	8100	2178.16	2503.64	127.27	146.28	118.60	136.32	17.68
110	35	3.14	7047	8100	2242.23	2577.27	131.01	150.58	122.09	140.33	18.20
110	36	3.06	7047	8100	2306.29	2650.91	134.75	154.89	125.58	144.34	18.72
110	37	2.97	7047	8100	2370.35	2724.55	138.50	159.19	129.07	148.35	19.24
110	38	2.89	7047	8100	2434.42	2798.18	142.24	163.49	132.56	152.36	19.76

Here is a chart, not sure how to embed a XL file but I use 57.5 inches for tire diameter for 18 inch tires and 61.7 in. for 19 inch tires and can look at speed based on calculated clutch shift. You can fill in the math. I would send the file if I had your email.

[t walgamuth]

As for calculating gear ratio, count your teeth and divide the small number into the larger. For calculating the ratio needed, take your maximum rpm desired times the gear ratio, then put in the tire diameter and figure backwards to get max speed in mph. I am not sure if I answered your question.

Or you could use an excellent chart.

[Tom Grossmann]

Thanks Lance.

For the Formula Ford guys; the CVT throws a monkey wrench in the normal calculation as we are in effect running an automatic transmission. For my Datsun I've got gearing charts in 100 rpm increments but for the CVT I couldn't remember what overdrive range it went into and the typical amount of belt slippage.

[Tuiguy]

I have always used the formula:
RPM x Tire dia in inches/gear ratio/336 (a constant)

i.e. 8100 rpm X 19.5" divide by 3.44, divide by 336 = 136.65 mph

[Daryl DeArman]

i.e. 8100 rpm X 19.5" divide by 3.44, divide by 336 = 136.65 mph

It's the gear ratio calculation that appeared to be throwing off the OP. The typical primary drive increases in effective diameter as engine speed increases, while the driven component decreases in effective diameter as vehicle speed increases. The tuning is basically how fast/slow each one changes and how much.

Maybe just looking for a range or what the maximum overdrive utilized is. (.84?)

[mmi16]

One has to remember - the Driven Clutch in the CVT set up is Torque Sensing.

As conditions are encountered where more torque is required - the CVT will backshift to supply the additional torque required.

At VIR, the straight from Oak Tree to the Roller Coaster has multiple situations where the Driven Clutch will be upshifting and backshifting.