Effect of wheel weight on measured power output - LotusTalk - The Lotus Cars Community
 
LinkBack Thread Tools Display Modes
post #1 of 5 (permalink) Old 03-26-2019, 08:57 PM Thread Starter
Registered User
 
gray's Avatar
 
Join Date: Mar 2010
Posts: 1,423
Effect of wheel weight on measured power output

I was doing some reading on reducing unsprung mass and came across an interesting post/graph.

A shop in Canada ran a few dyno runs using wheels of different weights to measure the actualized effect of wheel weight on measured power. It's a bit of an ad for HRE wheels (which are the lightest of the set they tested), and the lack of replicate runs irks me a little, but it's an interesting rough guide nonetheless.

https://mbworld.org/forums/c63-amg-w...orsepower.html

Quote:
Results:
1. (Blue curve) Factory wheels: 20”x9.0” with Pirelli 275/40-20 tires weighing 68 lbs combined per rear wheel. – Max hp: 371 hp, Max Torque: 375 ftlbs - (Baseline)
2. (Red curve) Aftermarket wheels: 20”x9.0” with Pirelli 275/40-20 tires weighing 72 lbs combined per rear wheel – Max hp: 369 hp, Max Torque: 373 ftlbs - (A [-] loss of 2 hp and 2 ftlbs)
3. (Green curve) HRE wheels: 20”x11.0” with Nitto 315/35-20 tires weighing 60 lbs combined per rear wheel – Max hp: 380hp, Max Torque: 384 ftlbs - (A [+] gain of 8 hp and 9 ftlbs and that is running a 2” wider wheel/tire combo)
Dyno Run - Effects of wheel/tire weights on wheel horsepower by HRE Wheels, on Flickr

Drives a Prius (clearly knows nothing about cars).

Lots Of Trouble Usually Serious
Leave Out The Unnecessary Stuff
gray is offline  
Sponsored Links
Advertisement
 
post #2 of 5 (permalink) Old 03-27-2019, 04:01 AM
Registered User
 
Join Date: Nov 2015
Location: massachusetts
Posts: 1,903
I would think it would show that on an inertial dyno but not a load dyno. Not meaningless but different
exigegus is online now  
post #3 of 5 (permalink) Old 03-27-2019, 05:17 AM
Registered User
 
eldonz's Avatar
 
Join Date: Jul 2014
Location: Concord, NC
Posts: 860
Quote:
Originally Posted by exigegus View Post
I would think it would show that on an inertial dyno but not a load dyno. Not meaningless but different
If it is a DynoJet dyno, like it says in the title, then it will make a difference. I converted a couple of DynoJet dynos, which are inertia dynos, so that they would calculate HP at the crank. It does require making a couple of measurements/approximations of the driveline inertial components. We were able to match the engine dynos very closely.

Later,
Eldon
eldonz is online now  
 
post #4 of 5 (permalink) Old 03-27-2019, 05:31 AM
Registered User
 
Join Date: Aug 2018
Location: Central Virginia
Posts: 343
Garage
Quote:
Originally Posted by exigegus View Post
I would think it would show that on an inertial dyno but not a load dyno. Not meaningless but different
Exactly this. If your dyno is measuring power output directly (water brake, eddy current, electro dynamic, or even a Prony brake), then there will be no difference in measured output between wheels of different masses, because the dyno is making a steady state measurement of torque, so the rotating masses have already been accelerated to the speed of measurement.

If your dyno is an inertia dyno, you're not actually measuring torque (or power output) directly. You're measuring the time the driveline takes to accelerate a large rotating mass to some maximum speed (storing kinetic energy in the roller). Anything that changes the total angular momentum of the system will change the rate of acceleration, and thus will change the measured power. I'd call it an error in reporting if the difference wasn't entered into the calculations.

In reality, the engine's power output is the same with both wheels and both dynos. The difference is how much kinetic energy you're storing in the system in the inertia dyno.

That said, yes it's an immediate performance gain for a car you want to accelerate and decelerate quickly to have lighter wheels because there's less angular momentum to store in the wheel when accelerating, and less to dissipate when decelerating. The car will handle and ride better too, because unsprung weight is the enemy of good handling and ride quality.

It's yet another reason that Chapman was fanatical about weight.
steelypip is offline  
post #5 of 5 (permalink) Old 03-27-2019, 05:54 AM
Registered User
 
Join Date: Aug 2018
Location: Central Virginia
Posts: 343
Garage
Quote:
Originally Posted by exigegus View Post
I would think it would show that on an inertial dyno but not a load dyno. Not meaningless but different
Exactly this. If your dyno is measuring power output directly (water brake, eddy current, electro dynamic, or even a Prony brake), then there will be no difference in measured output between wheels of different masses, because the dyno is making a steady state measurement of torque, so the rotating masses have already been accelerated to the speed of measurement.

If your dyno is an inertia dyno, you're not actually measuring torque (or power output) directly. You're measuring the time the driveline takes to accelerate a large rotating mass to some maximum speed (storing kinetic energy in the roller). Anything that changes the total angular momentum of the system will change the rate of acceleration, and thus will change the measured power. I'd call it an error in reporting if the difference wasn't entered into the calculations.

In reality, the engine's power output is the same with both wheels and both dynos. The difference is how much kinetic energy you're storing in the system in the inertia dyno.

That said, yes it's an immediate performance gain for a car you want to accelerate and decelerate quickly to have lighter wheels because there's less angular momentum to store in the wheel when accelerating, and less to dissipate when decelerating. The car will handle and ride better too, because unsprung weight is the enemy of good handling and ride quality.

It's yet another reason that Chapman was fanatical about weight.
steelypip is offline  
Sponsored Links
Advertisement
 
Reply

  LotusTalk - The Lotus Cars Community > Lotus Discussions > Suspension (including wheels, tires, brakes)

Quick Reply
Message:
Options

Register Now



In order to be able to post messages on the LotusTalk - The Lotus Cars Community forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
Please enter a valid email address for yourself.

Email Address:
OR

Log-in











Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page
Display Modes
Linear Mode Linear Mode



Similar Threads
Thread Thread Starter Forum Replies Last Post
Has anyone actually measured the decibel for Larini SC Sport exhaust? GTED Intake and Exhausts 3 05-27-2012 08:09 PM
Help! not allowed on track -anyone had arqray single tip exhaust level measured?? Drmike Intake and Exhausts 1 06-01-2010 02:40 PM
Has anyone ever measured the exact roll centers? kaz Suspension (including wheels, tires, brakes) 3 02-03-2009 08:12 PM
exhaust backpressure measured while on the dyno MikeW Intake and Exhausts 0 06-09-2006 10:49 AM
Weight vs. Gyroscopic effect BZRD69 General Discussion (Lotus related) 9 04-11-2006 07:48 AM

Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

 
For the best viewing experience please update your browser to Google Chrome