1 Topic by st_rage (edited by st_rage 2010-06-07 05:52 PM)

  • From: Atlanta, GA
  • Registered: 2009-09-01
  • Posts: 561

Topic: G Forces?

I've created a crude excel model to predict the relative value of handling vs Hp changes on lap times.  I used the lengths of straights and the turn radii at CMP along with the car inputs: values for acceleration, braking and handling g's.  The output is the projected lap time.

The model presumes that you drive like a poorly programmed but very precise robot.  Full acceleration, full breaking, then full lateral g's in the turns.

At the moment I'm using car inputs that are derived from old Car and Driver articles (skidpad, 0-60,  0-100, braking distance).  The times that the model predicts are much lower than our fast lap times.  I've reduced the braking g's and acceleration to reflect what I think we get on the track (~.6 and .2g's), but I'm surprised at how low I need to get the lateral g's (~.5) to reproduce our lap times.

So the question is, for all you folks that have a g-meter of some sort, what is the relationship between the g's you see in a turn on the track and the skidpad numbers for your car?

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  • Registered: 2009-10-25
  • Posts: 1,993

Re: G Forces?

Someone wrote an formula that took a cars data and correlated it to Ring times.....it worked pretty well except for heavy-quick cars like GTR

Cornering speed is EVERYTHING.....the more speed you carry through a corner the more speed you'll have at the end of the next straight.....cornering speed gets misconstrued as having a great motor from the end result.....

I would say most Lemons racers with decent tires-setup should pull close to 1 g in corners on track & maybe more braking....for example our 928 was pulling over 1.25g in the carousel at Sears Pointless and consistently 1.2g elsewhere.....lighter more nimble cars can pull 1.3g.....  Once I added real R comp tires...1.5g+

Richard Doty
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  • From: Atlanta, GA
  • Registered: 2009-09-01
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Re: G Forces?

~1g was what I was expecting.  I'll need to check my math.  Even using stock integra numbers (only a .76 skidpad!) the model is predicting laps times well below the overall best lap for the last CMP race.

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4 Reply by edward (edited by edward 2010-06-07 08:09 PM)

  • Registered: 2009-12-13
  • Posts: 44

Re: G Forces?

I have no idea what the Angry Hamster Z600 would do on a skid-pad since the car has been significantly modified. (We have RX-7 suspension and a motorcycle engine where the passenger seat used to be)

I can however tell you that we were able to pull 0.6g to 0.7g under braking and 0.15g to 0.2g acceleration. In the corners we were able to sustain 0.7g to 0.8g. In some corners we were over 1g with peaks approaching 1.3g.

Edward

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Re: G Forces?

i have no idea what our Gs are, but i do know that in a 1998 CVPI we have pulled mid-low 1:06s @ CMP for the spring 09 race. can you reverse the formula to extrapolate possible Gs for the track?

  • Registered: 2009-05-01
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Re: G Forces?

Here is some real-time Gs (2 axis) and corner speed data in comparison to lap position for Sears Point:

http://vimeo.com/10044143

I have datalogs of the same laps too. We were running on 2 race old Hankook RS2s. They still had a LOT of tread left (like not even to the wear bars yet) after 2 full races, at least 24 hours on track. So I'm thinking they're probably not the grippiest tires out there. And the suspension on our Talon isn't so great either, the struts/shocks are all blown.

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  • Registered: 2009-05-01
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Re: G Forces?

BTW, the flaw in your model is the full breaking/full accel/full cornering assumption. Simply not possible. Even a perfect traction circle transitioned model is well beyond what any of us mortal drivers could accomplish. If you watch my video, you'll see the G's usually just ramp up and down over a fair amount of track. Say you start at full accel. The transition from full accel to full braking will take time/distance. When you transition from braking to cornering you'll fall off the traction circle a good bit unless you're perfect. Even in cornering you'll be off full values unless the corner is nice and steady and you can dial it in.

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  • m610
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Re: G Forces?

I see 0.9 - 1.0 G's regularly in my track day/DD Prelude, on street tires, but I've seen it get as high as 1.1+. The books say my skid pad numbers should be 0.6 G's.

Here's a video showing some numbers while driving: http://vimeo.com/10455143

Skip ahead to 1:45 and then to 16:00.

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  • From: Atlanta, GA
  • Registered: 2009-09-01
  • Posts: 561

Re: G Forces?

gielamonster wrote:

BTW, the flaw in your model is the full breaking/full accel/full cornering assumption. Simply not possible. Even a perfect traction circle transitioned model is well beyond what any of us mortal drivers could accomplish. If you watch my video, you'll see the G's usually just ramp up and down over a fair amount of track. Say you start at full accel. The transition from full accel to full braking will take time/distance. When you transition from braking to cornering you'll fall off the traction circle a good bit unless you're perfect. Even in cornering you'll be off full values unless the corner is nice and steady and you can dial it in.

The model does set an impossible bar.  I thought that the best lap times might asymtotically approach it, but my times are 13 seconds slower on a ~1 minute lap.  My simple model may be too simple, but it does show the importance of lateral acceleration.  .1 g of lateral grip is worth more than doubling you hp at CMP.

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  • m610
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Re: G Forces?

Try simple scenarios first, like a skid pad. See if you can get realistic results there.

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  • Registered: 2009-10-25
  • Posts: 1,993

Re: G Forces?

the problem with skidpad is its a constant turn......vs typical corners you are in for 1 to maybe 3 seconds is typical...so the tires are load for far less time & that allows them to achieve far higher peak cornering loads than they ever could on a skidpad.....  For example my old street 928 could hit 1.15g but is rated at maybe .82g on the skidpad......

So the shorter the time in the corner and the higher the speed, the more G forces you will pull.....  A good example is this...turn 2 at Thunderhill is like a skidpad...peak continuous G forces since you are in that corner for 6+ seconds.....but something like turn 1 or 8 you take at high speeds for around 1-2 seconds....so the forces and speeds are FAR higher!!

Richard Doty
1984 Porsche 928 "Estate"
Porsche- "there is A substitute" Racing
Dirt Poorsche Racing #2