Bluem

Does anyone know the formula used to calculate the theoretical top speed so i can calculate what final drive ratio i need ???

Blake

The tire circumfrence (nominal) is:

Tire Circ=3.14*2*Width(mm)*Profile(series)/100+Rim Diam(in)*25.4)/25.4

And the speed in gear is:

Speed=RPM*Tire Circ*60/(Gear Ratio*Final Drive Ratio*12*5280)

I have a nice Excel spread sheet, if that will help.

Bluem

please post it

that would be realy nice

thanx

Gizmo

this witouth considering aerodynamic....

Blake

Okay, try this and let me know if it works. Obviously, it requires MS Excel to use...

http://rx-sevenandahalf.com/misc/RPMSpeed.XLS.zip

Blake

Obviously not. He was asking for theoretical top speed for gearing. Drag limited top speed had nothing to do with gearing, other than just having enough gear to get there. Calculating drag limited top speed requires you know the HP of the vehicle, Cd and Frontal Area. I have the formula somewhere, but it's really not very practical.

Howard Coleman

iTrader: (6)

the formula for top speed is:

rear wheel horsepower equals drag coefficient times frontal area in sq feet times miles per hour cubed . /divided by 146,600.

the fd has a drag coefficient between .29 and .31 (for the R1). frontal area is 19.26 sq feet. to get to flywheel hp from rear wheel you need to add 11% from driveline loss and 60 horsepower to drive the wheels at top speed.

this formula comes from Carol Smith's book "Tune to Win." i have used it for 22 years or racing and have found it to be right on the money.

so you will need to know your rwhp to solve for top speed...

then it is a simple matter of picking gearing.

howard coleman

PaulyDee

Howard,

How do you go about measuring the frontal area of a car. I have a FC and I am curious.

Pauly D

Blake

Frontal Area is a manufacturer spec, just like Cd. You can find it in the Yamaguchi books, among other places. It's still rather academic...I have no idea why anyone would need to estimate drag limited top speed when those who might need to know will have the practical means to do so (blasting down a super-long main straight somewhere). Gearing, however, is absolutely critical, as you need to maximize average power between shifts and make sure you don't have to shift at an awkward point or too many times. And almost any race car is gear limited in practice, so gearing will affect your actual top speed about 99 times out of 100.

Did anyone try the spreadsheet?

PaulyDee

I use the spreadsheet all the time. My car has a 5.12 diff/gear and different tire/wheel conbos result in different speed vs. RPM calculations. I also input the short 5th from the GTUs tranny which I don't have yet. Very useful, thanks!

DamonB

Frontal area is literally how big a "hole" the car punches in the air. If you view the car directly from the front everything you see is frontal area. If you were in a cartoon and drove your car through a wall the hole it left behind would be the frontal area. You can measure it yourself if you're patient...

Also keep in mind what frontal area and cd represent. Let's pretend that two cars have the same cd. One is a Miata and one is a Firebird. Even though their cd is the same (we're pretending) the Miata will be much more easily pushed through the air because of its much smaller frontal area. A large flat plate has the same cd no matter what its size when aimed perpendicular to the wind, but obviously a small plate is much easier to push through the wind than a larger one.

Cd is overemphasized; frontal area is more important in many cases. If designing from scratch the first thing you do is make the smallest package possible (least frontal area) and then monkey with the cd. In the far majority of cases a car with a higher cd but smaller frontal area would be easier to push through the air than one with a lower cd but more frontal area. Cd is like peak horsepower from a dyno: It's an important number but it doesn't tell the whole story. You wouldn't know it from reading advertisements though...

Blake

Are you using the most recent version? I've made quite a few upgrades in the last 5 years, since I first started giving it away. My favorite improvement has been the multiple cases, where you compare up to four gearing combos on the same screen. Very helpful when looking at two R&P gears and two trannys, displaying all four combos at the same time. Anyway, I'm glad to see someone is making use of it...

chairchild

If you go onto www.minifinity.com and go onto the tab labelled "downloads", there's a gear calculator - if you manually put in your own ratios, you can find out what your speed will be at what rpm (leave the drop-gear ratio at 1.0 though, this WAS designed for A-series engines afterall, so some bits won't be of any use to you lot)

chairchild

Found it http://www.minifinity.com/UpDownload...tit-lid-4.html

Gizmo

speed lost due to aerodynamic is an important aspect to gearing. On my ZX7RR, i had speed a speed difference of about 20km/h at about 140km/h. 20km/h is easy a go or not go into a corner.... My top speed was 320 km/h witout and 283 with it.