Race Car Tech Discuss anything related to road racing and auto X.

formula for theoretical top speed

Old 07-18-04, 04:14 PM
  #1  
Concept Motorsports
10 Year Member
Thread Starter
 
Bluem's Avatar
 
Join Date: Jun 2002
Location: San Juan, Puerto Rico
Posts: 686
Likes: 0
Received 0 Likes on 0 Posts
Question formula for theoretical top speed

Does anyone know the formula used to calculate the theoretical top speed so i can calculate what final drive ratio i need ???
Bluem is offline  
Old 07-18-04, 06:12 PM
  #2  
Rotary Freak
10 Year Member
 
Blake's Avatar
 
Join Date: Aug 2001
Posts: 2,267
Likes: 0
Received 0 Likes on 0 Posts
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.
Blake is offline  
Old 07-18-04, 09:25 PM
  #3  
Concept Motorsports
10 Year Member
Thread Starter
 
Bluem's Avatar
 
Join Date: Jun 2002
Location: San Juan, Puerto Rico
Posts: 686
Likes: 0
Received 0 Likes on 0 Posts
please post it

that would be realy nice

thanx
Bluem is offline  
Old 07-18-04, 10:30 PM
  #4  
Universal Genius
10 Year Member
 
Gizmo's Avatar
 
Join Date: Feb 2001
Location: tt
Posts: 611
Likes: 0
Received 0 Likes on 0 Posts
this witouth considering aerodynamic....
Gizmo is offline  
Old 07-18-04, 11:19 PM
  #5  
Rotary Freak
10 Year Member
 
Blake's Avatar
 
Join Date: Aug 2001
Posts: 2,267
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by King_fish111
please post it

that would be realy nice

thanx
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

Last edited by Blake; 07-18-04 at 11:28 PM.
Blake is offline  
Old 07-18-04, 11:28 PM
  #6  
Rotary Freak
10 Year Member
 
Blake's Avatar
 
Join Date: Aug 2001
Posts: 2,267
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by Gizmo
this witouth considering aerodynamic....
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.
Blake is offline  
Old 07-19-04, 08:31 AM
  #7  
Racing Rotary Since 1983
iTrader: (6)
 
Join Date: Oct 2001
Location: Wisconsin
Posts: 5,348
Received 43 Likes on 28 Posts
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
Howard Coleman CPR is offline  
Old 07-19-04, 10:41 AM
  #8  
Full Member
10 Year Member
 
PaulyDee's Avatar
 
Join Date: Mar 2001
Location: Indianapolis
Posts: 245
Likes: 0
Received 0 Likes on 0 Posts
Howard,

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

Pauly D
PaulyDee is offline  
Old 07-19-04, 10:57 AM
  #9  
Rotary Freak
10 Year Member
 
Blake's Avatar
 
Join Date: Aug 2001
Posts: 2,267
Likes: 0
Received 0 Likes on 0 Posts
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?
Blake is offline  
Old 07-20-04, 06:48 AM
  #10  
Full Member
10 Year Member
 
PaulyDee's Avatar
 
Join Date: Mar 2001
Location: Indianapolis
Posts: 245
Likes: 0
Received 0 Likes on 0 Posts
Thumbs up

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!
PaulyDee is offline  
Old 07-20-04, 07:27 AM
  #11  
Lives on the Forum
15 Year Member
 
DamonB's Avatar
 
Join Date: Feb 2001
Location: Dallas
Posts: 9,617
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by PaulyDee
Howard,

How do you go about measuring the frontal area of a car.
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...

Last edited by DamonB; 07-20-04 at 07:45 AM.
DamonB is offline  
Old 07-20-04, 12:15 PM
  #12  
Rotary Freak
10 Year Member
 
Blake's Avatar
 
Join Date: Aug 2001
Posts: 2,267
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by 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!
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...
Attached Thumbnails formula for theoretical top speed-picture-1.gif  

Last edited by Blake; 07-20-04 at 12:24 PM.
Blake is offline  
Old 07-20-04, 08:30 PM
  #13  
50mpg - oooooh yeah!
5 Year Member
 
chairchild's Avatar
 
Join Date: Mar 2003
Location: UK
Posts: 527
Likes: 0
Received 0 Likes on 0 Posts
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 is offline  
Old 07-20-04, 08:37 PM
  #14  
50mpg - oooooh yeah!
5 Year Member
 
chairchild's Avatar
 
Join Date: Mar 2003
Location: UK
Posts: 527
Likes: 0
Received 0 Likes on 0 Posts
chairchild is offline  
Old 07-22-04, 08:10 PM
  #15  
Universal Genius
10 Year Member
 
Gizmo's Avatar
 
Join Date: Feb 2001
Location: tt
Posts: 611
Likes: 0
Received 0 Likes on 0 Posts
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.
Gizmo is offline  
Related Topics
Thread
Thread Starter
Forum
Replies
Last Post
MazdaspeedR1
The Bad & Fugly Businesses
27
05-22-18 05:58 PM
ZaqAtaq
New Member RX-7 Technical
2
09-05-15 08:57 PM
Donald Hampton
2nd Generation Specific (1986-1992)
9
09-03-15 02:00 PM


Thread Tools
Search this Thread
Quick Reply: formula for theoretical top speed


Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service

© 2019 MH Sub I, LLC dba Internet Brands

We are a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for us to earn fees by linking to Amazon.com and affiliated sites.
 
  • Ask a Question
    Get answers from community experts
Question Title:
Description:
Your question will be posted in: