Tire Size and Rear End Ratio Question

[Beagle]

Don't forget the sandbox for traction.

[Thumpin455]

With all the cars I have, and buying tires for them over the years I have found something very interesting. Any given size of tire will vary in width as well as diameter from manufacturer to manufacturer, sometimes by almost two inches in diameter. An inch in diameter can have a somewhat large impact on top speed and cruise RPM. With Red being a small engine with limited power, more RPM in top gear will help him push more air. Red is drag limited more than RPM limited. The stuff I build is usually RPM limited so i think about things differently with all the bottom end and mid range torque my engines produce.

When I can't get online, or want to figure something out fast on a slow connection, its easy to do the math to see roughly what RPM you will be doing at what MPH with various gears and tire diameters.

(gear ratio in trans x final drive ratio x 336) / (tire diameter x RPM) = MPH
e.g.
2.41 x .67 x 336 = 542.54
27 x 2000 = 54,000

So you divide 54,000 by 542.54 and you get 99.53mph.

2.41 is the final drive, .67 is the OD ratio in a 2004R, 336 is the constant kinda like Pi. 27 is the diameter in inches and 2000rpm is target for MPH.

Increase or decrease the tire diameter, and you move the RPM range at a given MPH. Then the tire grows from centrifugal force which also changes things on you. How much the tire grows varies a lot based on its size, weight, RPM, and how its made.

The 100mph at 2000 rpm is why I don't run a 2.41 gear with a 2004R on the street, even though my 455s make more than enough grunt to pull that gear and drive around at 1800rpm, its putting a lot of strain on the trans doing that. The 455s make all the torque required to pull the 2.41 (even in a heavy car) and it might lose a tenth or two in ET compared to a 3.08-3.55 gear. They always slow down with more than 3.55 gears because of where the power band is. Like I said, different thinking.

All you need to do is change the 2.41 to 3.73 and whatever the OD ratios are in the trans, and you might go faster in a shorter trans gear. You are trading low speed grunt like my engines make for more torque at a higher RPM, not nearly as much as my engines make, but with the added parameter of revs per minute you can match the HP up top.

This is really fun stuff to me, I like to geek out on torque curves and how they affect HP, and gear ratios. To me torque over time is HP, because its measured in revolutions per minute. There is a huge difference in power between an engine that makes 400hp at 8500rpm vs an engine that makes 400hp at 3500rpm. If the former made as much torque at 8500 that the latter makes at 3500, it would be making well over 1000hp. That is the fun thing about math. I failed algebra twice in high school, something about teachers and I clashing.

I do all this math for fun just to see what I can make happen with a given vehicle, engine, trans, and final drive that I probably have laying around somewhere. Get into coefficient of drag, frontal area, lift, etc. and things get even more fun, how air reacts is very cool to learn, and its not very intuitive. Above 60mph drag gets increasingly more important with every MPH faster you go. LSR is a different bag of warm moist worms than drag racing, but with trap speeds over 100mph LSR stuff starts applying to drag racing. I picked up a solid .2 in ET and 4mph in trap speed by adding two block off plates to the grille of my 79 Formula. Something as simple as that can make a big difference, and the faster you go the more difference it makes.

Look at Cleetus McFarland's Leeroy and Ruby. Ruby makes less power than Leeroy, but Ruby is a lot faster. Two reasons, Leeroy is manual, Ruby is auto and torque multiplication is a thing, and Ruby has a lot less drag than Leeroy. That exposed cage, suspension, wheels, and all the turbo plumbing hanging off the engine increases the frontal area a lot over a smooth Corvette body. Every surface exposed to the wind is adding to frontal area, so Leeroy probably has more frontal area and drag than my 65 GTO does. Yes Leeroy is lighter, but he needs to increase the power by a lot to improve the 177mph traps speeds he was stuck at for a couple years.

Now you probably can't go changing the body because of class rules, but you can probably change things under the body. A belly pan (even a partial one) cleans up the underside, and yes you get drag from under there. Blocking off areas where air can 'pile up' inside the vehicle can improve CD and thus top speed. So its more than just tires, and you can find ways around the rules like Smokey Yunick did.

[pdub]

I do all this math for fun just to see what I can make happen

Andy (Red Kitty) played with the numbers in his post above and came up with a mighty great sounding technical guess at to what the smaller tires will do by how much, looking at Red's dyno chart etc. That's impressive to me, being able to figure all of that out. This all has me remembering, a perhaps shameful admission of sorts...

At the track I shift Red's 1st and 2nd by the way it sounds because it happens quick. I may feather the throttle a little bit near the redline in 2nd because the wheels are spinning anyway. I run through 3rd and 4th by the tach. I go as high as I think I can go without hitting the rev limiter. Red's rev limiter kills the spark, all of it. The nose comes down and I say damn, I'm wasting momentum. Or am I? Would I be better off hitting the rev limiter instead of short shifting? Which is quicker? And then, here comes what I sort of just now realized - in all of the many many runs I've made I have never looked at the tach going through the trap in 5th gear. Never have. On the big end I'm doing windshield/speedometer (GPS). So I don't know what rpms he's ever turned going through the trap and that's what should change for the better with the smaller tires.

[squirrel]

put a gopro in the car, pointing at the gauges....

[pdub]

The jury is in, the tires are on. That lowered Red by a measured 3/4 of an inch, paid for by a bigger gap around the tires in the fender wells. The old 40's filled up the wells with nothing to spare but they didn't scrape. I took him for a quick run between exits on the interstate to check the balancing, he does seem to pull surprisingly better in 4th gear. I like the feel, the ride and the handling. Good deal. But I ain't tried clearing the speed humps at the grocery store yet...

[Thumpin455]

Would I be better off hitting the rev limiter instead of short shifting?

on the pain of sounding like a dick... watch the damn tach or get a shift light.;) Find out where it runs fastest for various shift points. 1-2 no big thing as that is a quick one, 2-3-4 matter more, you know where the limiter is so shift before you hit it. Before I broke all my 4 speeds I raced them a lot, and if you aren't watching the tach/shift light you are doing it wrong. Maybe a hood tach so you can see it, or perhaps a HUD. As we get older this sort of thing gets harder to master, and I found myself short shifting the GTO last summer, but I was mostly screwing around, not racing. Even with 3.42 gears that old Goat rips through first in a real hurry, was worse with the 700R4, but the Th350 is done even before the pedal is matted. I am getting real old.. maybe I should sell a bunch of cars.

[pdub]

watch the damn tach or get a shift light

I was running Bluebelle the Fusion at Ohio with the speed limiter turned off. I was having a ball shifting her automatic transmission myself, trying to go slow faster. That was back when the ECTA rules were somewhat stricter. I took Bluebelle up there because Red had already gone 139 point something and and if you touched 140 in a car without a roll bar in it, you were uhhhh through running that car unless you put a roll bar in it. So Bluebelle for fun and fellowship.

With the speed limiter turned off and tuned to burn 93 gas she went 131 in what I figured to be my last run of the event toward the end of the day. But that time I hit the rev limiter in 1st gear, it felt like the whole show stopped until she went into 2nd. And she still went 131. So nothing else would do except to go around again and try to do better. I was one of the last cars to go before they said that's it for the day and I hit all the shifts perfectly that time. That got another tenth of a MPH at the trap. I know a tenth of anything is huge in drag racing but in a whole mile...maybe the wind changed too, I wasn't paying that any attention.