GPS or car sensed speed - Rennlist - Porsche Discussion Forums

Notices

GPS or car sensed speed

Reply

Old 03-18-2017, 01:53 AM
  #1
MJP911
Addict
Rennlist Member
Thread Starter
 
MJP911's Avatar
 
Join Date: Oct 2010
Location: New York, NY
Posts: 317
Default GPS or car sensed speed

I have an AIM Solo DL and SmartyCam connected to the car for data acquisition. I have been using the GPS sensed speed for my data overlays but I was wondering if the speed signal supplied by the car is more accurate. Thoughts?
MJP911 is offline  
Reply With Quote
Old 03-18-2017, 02:39 AM
  #2
Spyerx
Addict
Rennlist Member
 
Spyerx's Avatar
 
Join Date: Mar 2012
Location: SoCal
Posts: 9,911
Default

No, gps is more accurate. My car speedo reads quite optimistic once over 130mph.
Spyerx is offline  
Reply With Quote
Old 03-18-2017, 02:41 AM
  #3
38D
Super User
 
38D's Avatar
 
Join Date: Jun 2002
Location: About to pass you...
Posts: 5,069
Default

Gps speed is normally better for actual ground speed than wheel speed as you don't have to worry about tire slippage. Wheel speeds are used or over/under steer calc, seeing when the lsd engages, etc.
38D is offline  
Reply With Quote
Old 03-18-2017, 08:58 AM
  #4
MJP911
Addict
Rennlist Member
Thread Starter
 
MJP911's Avatar
 
Join Date: Oct 2010
Location: New York, NY
Posts: 317
Default

Thanks. And nice avatar, 38D!
MJP911 is offline  
Reply With Quote
Old 03-18-2017, 10:26 AM
  #5
Matt Romanowski
Rennlist Hoonigan
which cost no drachmas
Lifetime Rennlist
Member


Rennlist
Site Sponsor
 
Matt Romanowski's Avatar
 
Join Date: Dec 2001
Location: Manchester, NH
Posts: 8,369
Default

Tires also change circumference with load and speed, so the actual wheel speeds are not usually completely accurate. Like the guys above said, use GPS speed for car to car comparison, track metrics, and other performance measures. Then use actual wheel speed to look for tire lockup, diff lock/unlock, tire slip, etc.
Matt Romanowski is offline  
Reply With Quote
Old 03-18-2017, 06:37 PM
  #6
gobuffs
User
 
Join Date: May 2013
Posts: 126
Default

Originally Posted by Spyerx View Post
No, gps is more accurate. My car speedo reads quite optimistic once over 130mph.
agree on using GPS.

However, just because your speedo reads optimistically doesn't mean the speed the car is using is wrong. I've seen the speed the ABS reports be very close to GPS but be very different from the speedo reports.
gobuffs is offline  
Reply With Quote
Old 03-18-2017, 08:04 PM
  #7
2BWise
User
 
2BWise's Avatar
 
Join Date: Nov 2004
Location: Northville, MI
Posts: 1,260
Default

ABS speed is calculated differently from the speedo. Often times the speedo is referencing trans or diff speeds for output. ABS uses wheels speeds and then processes them to get actual vehicle speed, but there's a whole lot of fuzzy logic that goes into it.
2BWise is offline  
Reply With Quote
Old 03-18-2017, 09:03 PM
  #8
amso3

Rennlist
Small Business Partner

 
amso3's Avatar
 
Join Date: Apr 2004
Location: North Port, FL
Posts: 1,599
Default GPS vs wheel speed

Accuracy is going to be different on a straight vs a hard braking zone. GPS refreshes at 10 Hz and in a hard braking zone speed can change a lot in 1/10 sec. So GPS is an approximation where wheel speed (approxomately 40 pulses per rev of the wheel) is more accurate relative to changing speed. As Matt said, if you are comparing car to car, GPS would be the way to go. If you are looking for changes in data with your car, wheel speed might be better. If you are looking for bragging numbers, use the higest! Here are three views of data of a Cayman at Road America
Attached Images    
__________________
Jerry Austin P.E.
AIM Authorized Dealer since 2008
Racing Data Engineer
National PCA Instructor
84 911 3.6 track car
www.austinmotorsportsllc.com
amso3 is offline  
Reply With Quote
Old 03-18-2017, 09:42 PM
  #9
ProCoach
Rennlist Member
Rennlist
Small Business Partner

 
ProCoach's Avatar
 
Join Date: Jul 2008
Location: Durham, NC and Virginia International Raceway
Posts: 9,757
Default

Originally Posted by amso3 View Post
Accuracy is going to be different on a straight vs a hard braking zone. GPS refreshes at 10 Hz and in a hard braking zone speed can change a lot in 1/10 sec. So GPS is an approximation where wheel speed (approxomately 40 pulses per rev of the wheel) is more accurate relative to changing speed. As Matt said, if you are comparing car to car, GPS would be the way to go. If you are looking for changes in data with your car, wheel speed might be better. If you are looking for bragging numbers, use the higest! Here are three views of data of a Cayman at Road America
^^This^^
ProCoach is offline  
Reply With Quote
Old 03-19-2017, 12:48 PM
  #10
Spyerx
Addict
Rennlist Member
 
Spyerx's Avatar
 
Join Date: Mar 2012
Location: SoCal
Posts: 9,911
Default

You guys are completely right. But the question asked is which is more accurate for actual measured speed. That will gps for all the reasons listed above.

But yes wheels speeds tell more to the story then just raw speed. I use them for other car dynamics. But for general comparisons of "speed" it's gps.

So the real answer is "it depends"!
Spyerx is offline  
Reply With Quote
 

Related Topics
Thread
Thread Starter
Forum
Replies
Last Post
ShakeNBake
Trucks and Trailers
12
11-06-2017 03:54 PM
hsmith
Racing & Drivers Education Forum
10
08-04-2017 12:18 PM
Lemming
Data Acquisition and Analysis for Racing and DE
13
12-13-2016 02:45 AM
johnsopa
Data Acquisition and Analysis for Racing and DE
1
07-14-2016 09:47 AM
TXE36
Data Acquisition and Analysis for Racing and DE
7
05-25-2016 11:56 AM

Thread Tools
Search this Thread
Posting Rules
You may not post new threads
You may not 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 Off
Pingbacks are Off
Refbacks are Off

Forum Jump


All times are GMT -3. The time now is 05:13 AM.


Copyright ę 1998 - 2017 Rennlist.com 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: