Author Topic: Best final drive ration vs wind resistance for top speed  (Read 2871 times)

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Offline KYWes

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Best final drive ration vs wind resistance for top speed
« on: August 16, 2020, 08:47:49 AM »
Newbie alert!

Considering a stock production vehicle e.g. not something built specifically with LSR pursuits. Specific vehicle is a 1995 Z28 Camaro with a factory stated drag coefficient of .34 and a frontal area of 22 sq ft, 400 HP, and 3.73:1 rear end gearing. Redline is approximately 6,000 rpm.

Anyway, I've looked at gearing and top speed calculators as well as formulas regarding how much horsepower is needed to achieve X speed. For any given HP, theoretically, a lower numerical gear ratio would result in a higher top speed whereas a higher numerical final drive ratio would increase acceleration but lower top speed.

However, as speed increases so does wind resistance, exponentially, resulting in the engine running out of sufficient horsepower/torque to overcome said resistance when running a numerically low final drive ratio.

Conversely, the same engine with a higher numerical final drive ratio would multiply the effective torque thus enabling the engine to overcome the wind resistance but then the engine reaches it's redline quicker and would wind out sooner thus limiting the top speed.

So my query is what does conventional wisdom say regarding what final drive ratio is the best balance for top speed e.g that ratio that keeps the engine in it's power range while allowing the RPMs to stay sufficiently high so that top speed is accomplished?

Disclaimer: I read an article many, many years ago in a car magazine that looked at this question. If I recall correctly they determined that for a stock production car of say 300 HP, e.g. not getting into specific LSR design, a final rear end ratio of around 3.73:1 was the best balance to achieve that car's top speed. To that end I've read that my car's top speed is achieved in 4th gear (1:1) not 5th (.75) nor 6th (.5) as math would indicate.

So I thought I'd try a forum such as this one to see what the prevailing wisdom is.
« Last Edit: August 16, 2020, 08:51:10 AM by KYWes »

Offline Eddieschopshop

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Re: Best final drive ration vs wind resistance for top speed
« Reply #1 on: August 16, 2020, 10:48:51 AM »
You answered your own question. The best ratio will be based on the engines happy place and the overall combination. There isn?t much of an answer beyond that.

Offline Stainless1

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Re: Best final drive ration vs wind resistance for top speed
« Reply #2 on: August 16, 2020, 11:08:47 AM »
That's a question with a million opinion answer... no matter what your rear ratio, it is power vs aero... what rpm does you engine make the best power.... how much power does it take to go a certain speed....
Years ago we ran a motor that made the car go 202... 12 runs in a row... rear ratio changed up and down 6%, then 3%, up a jet down a jet from calculated... didn't matter.... 202 and change... until the next to the last run it went 206 on a qualifier against our own 202.... 8 mph tail wind... wanna guess what it did the next morning?  204 record...
Added a little N20 and it went 228.... power vs aero changed....
 
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Offline tortoise

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Re: Best final drive ration vs wind resistance for top speed
« Reply #3 on: August 16, 2020, 12:06:54 PM »
Having the power to go a given speed, and having a gear that puts you at peak power at that speed doesn't necessarily enable you to get to that speed. Even with the right final drive, if the spread between the two top gears used is too wide, you won't be able to pull the top gear. It also may be necessary to be able to rev past peak power, something many cars can't do.

Offline DallasV

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Re: Best final drive ration vs wind resistance for top speed
« Reply #4 on: August 16, 2020, 01:05:43 PM »
weight seems to be left out of this equation. If you are spinning the tires before you run out of horsepower because of lack weight where frontal force is higher than down force, it doesn't really matter what gear ratio you have. on the other hand if you have more weight than the HP/torque can pull then shorter gears are nice but you never attain top speed. it's quite a balancing act and I just want to throw another monkey wrench in this question
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Offline RichFox

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Re: Best final drive ration vs wind resistance for top speed
« Reply #5 on: August 16, 2020, 01:58:20 PM »
I have been told. "Keep throwing gear at it until it slows down. Then throw horsepower."

Offline jl222

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Re: Best final drive ration vs wind resistance for top speed
« Reply #6 on: August 16, 2020, 02:16:09 PM »
 
  Try Bonneville Pro by Racing Secrets. That's what I use for the 222 Camaro.

  I tested it against Summer Bros streamliner info from a book that had info on speed, hp frontal area, CD, ect.. And it was right on.

  Program allows inputs for different class cars, bville tracks and conditions , hp, frontal area, cd, weight gear ratio, tire roll out, ect.

 After inputting data it gives results. Great for figuring best gear ratio.

  $100 a few years ago.

                      JL222

 

Offline jacksoni

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Re: Best final drive ration vs wind resistance for top speed
« Reply #7 on: August 16, 2020, 04:18:15 PM »

  Try Bonneville Pro by Racing Secrets. That's what I use for the 222 Camaro.

  I tested it against Summer Bros streamliner info from a book that had info on speed, hp frontal area, CD, ect.. And it was right on.

  Program allows inputs for different class cars, bville tracks and conditions , hp, frontal area, cd, weight gear ratio, tire roll out, ect.

 After inputting data it gives results. Great for figuring best gear ratio.

  $100 a few years ago.

                      JL222

 

Agree on Bonneville pro. As with most, good data gives good results but you can get an idea with guesses. I think this is now only available through MaxraceSoftware, Larry Meaux.  Follow these links and then send email to Larry about purchase of the individual program. Often recently have been package only (but excellent deal if you are interested)
https://www.speed-talk.com/forum/viewtopic.php?f=69&t=57671  Review and screen shots of program
https://www.speed-talk.com/forum/viewtopic.php?f=69&t=57670

To send him an email, go to a post by him near bottom of the second thread above. High light his name and click on it. That takes you to his profile page and a link to send an email. You may have to register with Speedtalk to get to that but is easy and free unless you want to go up to higher memberships, which are not necessary for most things.
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Offline SPARKY

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Re: Best final drive ration vs wind resistance for top speed
« Reply #8 on: August 16, 2020, 05:35:29 PM »
Torque x Trans ratio x Rear ratio= Tractive Effort   
  pick any tire size you want to start with and assign it a value of 1   5% larger tire is .95 5% smaller tire is 1.05

Then it it becomes TE x Tire Value  then put a combination together that lets you run your target speed

As stainless says it can have a million replies  but with the above you can compare what you are putting to the salt so you can determine if you want more or less GR that will let you try to put your engine drive train combo at the desired speed.
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Offline jl222

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Re: Best final drive ration vs wind resistance for top speed
« Reply #9 on: August 16, 2020, 06:34:25 PM »
Torque x Trans ratio x Rear ratio= Tractive Effort   
  pick any tire size you want to start with and assign it a value of 1   5% larger tire is .95 5% smaller tire is 1.05

Then it it becomes TE x Tire Value  then put a combination together that lets you run your target speed

As stainless says it can have a million replies  but with the above you can compare what you are putting to the salt so you can determine if you want more or less GR that will let you try to put your engine drive train combo at the desired speed.

  OK 2000 ft lbs 1.85 final gear x 1=3700?

Offline Interested Observer

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Re: Best final drive ration vs wind resistance for top speed
« Reply #10 on: August 16, 2020, 08:22:21 PM »
Sparky's equation as stated above is nonsense.  He knows better and must have been out in the sun too long last week.
Suggest you look at the following: http://www.landracing.com/forum/index.php?topic=14044.msg254330#msg254330
Unless you have a credible power curve, you are just wasting your time diddling with gear ratios.  The drag power can be reasonably estimated.