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The Diff In Coast

differential diff coast

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#1 John Woods

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Posted Jan 10 2019 - 08:19 AM

Have a suspicion many people have no idea how to use the diff in coast to apply an essential historically accurate realistic technique while driving.

It is easy to forgive a newbie when all they know is if you let off the gas the car turns in way too fast and spins out uncontrollably if they apply brakes or throttle.

Then when they complain someone here tells them to use 85/30/1, stiffen the front, increase brake bias, and just get used to it.

This leaves out any explanation of why it helps the driver if the car rotates when you let off the gas.

Rotating the car with differential coast is a driving technique described repeatedly by so far every credible source from Taruffi (50s), Jenkinson (50s), Clark (60s), Lauda (70s), taught by Bondurant School, Jim Russell School, Carrol Smith, and on up until recently Bob Boles, author of "Advanced Race Car Chassis Technology," HP Books, 2003 rev. 2010.

"Cars need rotation."

Generally, my humble fantasy preference is to not use brakes except softly to slow the car, along with downshifting, to threshold corner entry speed as brake markers are passed before a turn, counting down in rhythm with the downshifts 3-2-1 to turn-in at zero, which is where the throttle is closed, the diff goes into coast, the car rotates, throttle is applied and the suspension is locked on a vector that crosses over a point of choice at exit, while countersteer fixes the front on the default line of least resistance against the track camber thru the turn.

Others have their own ideas.

When the diff goes into coast the rear suspension relaxes and the mass at the back, no longer held in place by engine torque, heads out on its own vector at the sum of a tangent and the line of travel.

A driver can target a landmark alongside the track on the outside of a turn and by going off throttle set the rear's vector toward the target with the same certain confident precision that is required to get the car around the corner.

The rear's off-line attempt at escape is limited by its connection to the rest of the car and the tires, so all it can do is begin to "rotate" the car around its CoG and load the tire patch in an effort to relieve itself of a force with no place else to go.

This applies downward pressure at the tire patch on the outside rear which increases grip as loading approaches the maximum possible before any more excess force has no choice but to find another place to be.

From the driver's POV the best place for excess force to go is back into the springs to propel the car's sprung weight forward, which is what it will do when the car's suspension is accurately and correctly tuned.

As soon as rotation begins the front is steered into the track camber and hopefully, if toe bars and shocks are dialed-in, all four tires will settle at maximum lateral load, enjoy the same angle of attack, (neutral steering), and the car will easily negotiate the turn relying only on fully accelerating throttle from the pre-apex instant of coming back out of coast.

All in 4/5ths of a second or less.


Questions and guesses but okay, find an opening
:D

Edited by John Woods, Jan 13 2019 - 09:01 PM.


#2 Pete Gaimari

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Posted Jan 10 2019 - 09:13 AM

Here's the problem John.

Dave is quoted at saying a 45/85/1 diff setting is realistic. That may be in the real car but not in GPL. When talking about the original Papy physics we're talking about grip that's less than the real car. Proven by driving the Lotus 49 in iRacing and comparing it to the GPL version. Both developed by the same guy.

So, what happens when a noob to GPL use that 45/85/1 diff. He'll spin out in every turn. If he RF brakes it will be worse.

Early on we discovered that if we give some throttle with the braking we could use the 45/85/1 diff setting. By using throttle with braking we can lock and unlock the diff by the throttle setting. Of course, this has to be done bt LF braking. So, we had to use an unrealistic driving method to 67 F1 cars to use a realistic diff setting because the car doesn't have realistic grip.

It's much easier to use a 45/85/1 diff in the 65 mod because the cars have much more grip than the 67 F1 cars.

So, what do we have to do if we want to drive with a realistic method. Which means RF braking and no throttle with braking. We have to use an unrealistic diff setting to compensate for the unrealistic grip.

You make a choice on how you want to drive GPL and then use the diff setting that works for you. Nothing is black and white in simulations. Use what works for you.

#3 Alan Davies

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Posted Jan 10 2019 - 03:20 PM

Pete,
"Dave is quoted at saying a 45/85/1 diff setting is realistic."  As I have learned recently from another thread, F1 cars of the 1960's did not have limited-slip differentials as modeled in GPL; so there is nothing realistic about it at all. Or do the figures relate to other types of diff?

#4 leon_90

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Posted Jan 10 2019 - 04:03 PM

If I am not mistaken, Lee said that in GPL 'realistic' values should be 85 or 60 for Power and 30 or 45 for Coast. I do not remember what he said about clutches though :confused:

#5 Remco Hitman

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Posted Jan 10 2019 - 06:12 PM

60/45/2 is all you'll ever need :-)

#6 db312

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Posted Jan 10 2019 - 11:32 PM

If I can say something, I'm totally agree with Pete...

When I was driving, I wanted a car with a good balance and almost all, which was easy to control.
For me that was the only way to go fast and to be sure of the car's reaction.

I try to find more or less the same feeling with a simulation, and clearly, out of 85/60 and 30/45, the car become a hearse... Not funny at all !

But it's only my point of view...

#7 Pavel

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Posted Jan 11 2019 - 12:16 AM

View PostAlan Davies, on Jan 10 2019 - 03:20 PM, said:

Pete,
"Dave is quoted at saying a 45/85/1 diff setting is realistic."  As I have learned recently from another thread, F1 cars of the 1960's did not have limited-slip differentials as modeled in GPL; so there is nothing realistic about it at all.

Agree. For me 'realistic' diff setting is those that will make Salisbury LSD modeled in GPL behave more like Cam and Pawl LSD that were used on cars of the 60's. And I think it is 30/85/6 setting...

Edited by Pavel, Jan 11 2019 - 12:17 AM.


#8 mcmirande

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Posted Jan 11 2019 - 07:18 AM

Did somebody try different diffs setups in the 67x mod? In general I find that mod as very (and I think correctly) responsive to different setup settings.
Also, the grip was (imho) improved from the original 67's, with more grip at slow speeds and less at higher ones. And I guess it should allow more realistic setups too.

Cheers, Marcos

#9 Pete Gaimari

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Posted Jan 11 2019 - 10:19 AM

When I first started to drive GPL I worked up to using a 45/85/1 diff and all my PB's were set that way. I was a LF braker and used throttle with the braking. It was an easy way to drive and it's no wonder aliens like to drive that way.

After a while I wanted to drive in a more realistic 67 style of driving. I started to use RF braking and could no longer use the 45/85/1 diff. I tried but I was too inconsistent and had spins. I pride myself in never spinning out, so I needed to make changes. I went to what Greger used in his GH1 setups. 85/45/1. It was much easier to drive and I stayed with it while making changes in the setup to help it turn better but staying with the 85/45/1 diff. I finally got to the point of matching my PB times and even passing some of them.

My point is don't use a diff setting that you can't drive. Use a setting that's comfortable to drive and doesn't make to spin off. No matter how fast a setup is if you can't stay on the track you'll be slower at the end of a race.

#10 simmer

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Posted Jan 11 2019 - 11:40 AM

View PostRemco Hitman, on Jan 10 2019 - 06:12 PM, said:

60/45/2 is all you'll ever need :-)
    This has always been my go to Diff settings. Most every mod

#11 orlinos

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Posted Jan 11 2019 - 04:06 PM

I guess I am strange. I only play 1965 mod for now (still learning the craft). Brabham BT11 only. Mostly default setup, I only slowly lowered brake bias to 53% and changed the diff from 60/30/2 to 30/60/2 - and then again to 30/45/2 to avoid too much oversteer when braking.

Lotsa power when on the gas! :-D

#12 JonnyA

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Posted Jan 11 2019 - 04:11 PM

I hate understeer and end up running 30/85/x on pretty much every mod.

#13 Pete Gaimari

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Posted Jan 11 2019 - 04:58 PM

You can't just look at the diff setting. The rest of the setup matters too.

#14 mcmirande

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Posted Jan 11 2019 - 09:07 PM

I think a great variation in handling is possible with each set of diff values if you adjust the remaining setup.

For instance, you may use 30/85 but highly convergent rear wheels and highly divergent front ones and still having some understeer when in coast...

Marcos

#15 JonnyA

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Posted Jan 12 2019 - 05:41 AM

Pete: yes, but I find the diff values to be the biggest single factor.

#16 John Woods

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Posted Jan 12 2019 - 08:28 AM

Or the one between your ears.
Otherwise, yes.
:)

How the diff is set depends on how it is used?

Seems RFBrs tend toward tighter diffs?

(By which I mean a diff that as I understand tends to offer less rotation in coast from the driver's subjective POV).


:D

Edited by John Woods, Jan 12 2019 - 08:42 AM.


#17 Pete Gaimari

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Posted Jan 12 2019 - 08:41 AM

View PostJonnyA, on Jan 12 2019 - 05:41 AM, said:

Pete: yes, but I find the diff values to be the biggest single factor.


I agree but when someone gives their diff setting we have no idea how the car handles for them without seeing the whole setup.

View PostJohn Woods, on Jan 12 2019 - 08:28 AM, said:

Or the one between your ears.
:)

How the diff is set depends on how it is used?

Seems RFBrs tend toward tighter diffs?

(By which I mean a diff that as I understand tends to offer less rotation in coast from the driver's subjective POV).


:D

Now, we finally agree. :D

#18 JonnyA

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Posted Jan 12 2019 - 11:31 AM

Well Pete, I usually favour soft springs and a very hard front roll bar. But in a thread about diffs, I posted my diff preference.

#19 John Woods

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Posted Jan 12 2019 - 11:52 AM

The idea starting out was a thread about using the diff.
:)

Preference for specific ratios does seem to have implications about how the diff is used.
Or if its capabilities are used to their full extent or to what extent?

One important use of the diff is to cause the car to rotate as it transitions between throttle on-off-on while the car is turning.

That is a capability that can be taken advantage of to improve control and reduce lap times. We know it is an important capability because the whole host of experts has for years been telling us that, (or trying to).

Regardless of their diff setting, when newbies ask why does the car spin out when I let off the gas in a turn the first answer should be, "It is supposed to. Countersteer and floor it! Get used to that."
:P

As mcmirande's comment implies, tweaking toe settings in particular aligns car to diff rotation in compliment to or in spite of other settings.


:D

Edited by John Woods, Jan 12 2019 - 12:30 PM.


#20 twinpotter

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Posted Jan 12 2019 - 12:13 PM

Thanks for this thread.

Looks like I've been doing my setups all wrong 👋 My philosophy was to set my diff settings for different cars, mods and even set differently for different tracks. I thought the diff was an ongoing adjustable setting, with flexibility. I didn't realise that you found a setting to your driving style and pretty much kept it for all mods, cars and tracks.

Would you say that the key is to use your diff setting as your base. Then adjust the other car settings, on top of that 🤔

TP:





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