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Friction In The Gpl Diff


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#1 john roberts

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Posted Oct 16 2010 - 08:51 AM

Hi guys and gals ,

here is a simple chart that shows the level of friction with each angle and clutch , making it easy to change clutches and still keep the friction level (or close to it) from one side of the diff while changing the other .

i'm expecting the topic of the diff to crop up when the GT mod is released and this might help people understand whats going on with different diff settings .

happy racing ,

john

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#2 Bob Simpson

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Posted Oct 16 2010 - 11:42 AM

How does "friction coeffient" compare with the "locking percentage" shown in GPL Setup  Manager?

A 30 ramp with 6 clutches shows as 6 on your chart, and as 30% in GPL Setup  Manager.

#3 john roberts

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Posted Oct 16 2010 - 01:27 PM

View PostBob Simpson, on Oct 16 2010 - 11:42 AM, said:

How does "friction coeffient" compare with the "locking percentage" shown in GPL Setup  Manager?

don't know .

john

#4 Lee200

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Posted Oct 16 2010 - 04:40 PM

John's chart is just a different way of showing what Setup Manager does.

Apparently, the formula for John's chart is:  (1+Number of Clutches)*cos(Ramp Angle) which he calls "Friction Coefficient".

Setup Manger uses Gene Fryman's "Locking Percent" which is the same thing multiplied by 5.

Edited by Lee200, Oct 16 2010 - 04:41 PM.


#5 john roberts

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Posted Oct 16 2010 - 04:53 PM

View PostLee200, on Oct 16 2010 - 04:40 PM, said:

John's chart is just a different way of showing what Setup Manager does.

thanks Lee .

john

#6 Lee200

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Posted Oct 16 2010 - 08:36 PM

No problem John.

I've never quite understood the locking %, but supposedly a completely locked axle will have a 100% lock while a free wheeling axle will be 0%.  However, current GPL setup limits of ramp angle and clutches restrict locking percentages between about 1 to 34% which seems low to me.  But Gene was very definite that this is how GPL works.   :idunno:

#7 David Wright Lo67

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Posted Oct 17 2010 - 04:28 AM

View PostLee200, on Oct 16 2010 - 08:36 PM, said:

No problem John.

I've never quite understood the locking %, but supposedly a completely locked axle will have a 100% lock while a free wheeling axle will be 0%.  However, current GPL setup limits of ramp angle and clutches restrict locking percentages between about 1 to 34% which seems low to me.  But Gene was very definite that this is how GPL works.   :idunno:

Seems very low to me too.  Typical locking % for a cam and pawl LSD is 50%.  Salisbury (clutchpack) LSDs can go much higher.  Porsche for example typically used 75% or 80% locking in their racing cars in the late 60s.

Edited by David Wright Lo67, Oct 17 2010 - 04:31 AM.


#8 john roberts

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Posted Oct 17 2010 - 07:39 AM

here is a chart that shows "locking percent" as used in gpl setup manager , is this what you wanted Bob ?

john

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#9 Lee200

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Posted Oct 17 2010 - 07:42 AM

View PostDavid Wright Lo67, on Oct 17 2010 - 04:28 AM, said:

Seems very low to me too.  Typical locking % for a cam and pawl LSD is 50%.  Salisbury (clutchpack) LSDs can go much higher.  Porsche for example typically used 75% or 80% locking in their racing cars in the late 60s.

Thanks David,

Gene and I had some discussions several years ago about LSDs and "locking percentage" and I'll try to find my notes on that.  Carroll Smith has a good discussion of real world LSDs in his book, "Tune To Win".

I don't know how GPL's locking percentage corresponds to real world LSD locking.  Like many things, Papy may have used an approximation to simulate it so there may not be a direct correlation.

#10 Lee200

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Posted Oct 17 2010 - 07:43 AM

View Postjohn roberts, on Oct 17 2010 - 07:39 AM, said:

here is a chart that shows "locking percent" as used in gpl setup manager , is this what you wanted Bob ?

Yes, this is what Setup Manager uses.   :thumbup:

#11 john roberts

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Posted Oct 14 2012 - 08:44 PM

as the can ams have 9 clutches ..

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#12 JohnnyAck

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Posted Oct 15 2012 - 02:59 AM

Those percentages make it even less understandable imo. I drive and look at tyre temps after changing the diff to know what helps or doesnt. To easily explain the diff settings, all you have to say is that 30/30/6 is the closest to a live axle direct drive configuration, and 85/85/1 is the least, imo. For can ams I am using 60/85/1 and 85/85/1. I have got one wr so far with that. I wish i knew why those odd numbers are the best.

#13 Cheapracer

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Posted Oct 15 2012 - 09:00 AM

View Postjohn roberts, on Oct 14 2012 - 08:44 PM, said:

as the can ams have 9 clutches ..

They didn't have clutches!

#14 Pete Gaimari

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Posted Oct 15 2012 - 09:52 AM

Dave has been quoted as saying that 45/85/1 is the most realistic diff for GPL. If he really thought that? He should have given the tires more grip, so the car acted more realistically. You should be able to drive the car with that diff setting, and not have it feel so loose off throttle.

#15 Cheapracer

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Posted Oct 15 2012 - 11:51 PM

30 with 3 or 4 "clutches" on the power side seems generally realistic but the coast side is a bit all over the place depending on variables in the tracks. It's an issue in GPL that track builders have no standard baseline for camber. If 2 track builders built the same track different diff settings would be required (and different lap times would be had).

The grip is ok in GPL, there's a solid argument that it's too much, it's the traction circle/what happens at the point of and after you lose grip that's not correct.




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