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The Salisbury Differential


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#1 hagapito40

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Posted Jan 26 2015 - 08:21 AM

I found this video animation very useful to understand the Salisbury diff working...maybe the most misunderstood part of the setup (IMO)

https://www.youtube....h?v=WeLm7wHvdxQ

This read helps too...
http://www.intothere...fferential.html


And this is fo me the best diff tutorial available...
Attached File  LSDiffs.doc   73.5K   39 downloads

#2 Fat Rich

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Posted Jan 26 2015 - 09:20 AM

That video doesn't explain ramp angles, this one is probably closer:

https://www.youtube....tion_3763234919

It's interesting that Assetto Corsa models a different limited slip diff in it's Lotus 49 with a setting for the preload spring as in your first video, whereas GPL seems to model the pressure ring version of LSD (and models it badly according to David Kaemer from Papyrus).

Edit: I may have remembered David Kaemer's comments wrong, had a look for the source and can't find it.

Edited by Fat Rich, Jan 27 2015 - 10:24 AM.


#3 hagapito40

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Posted Jan 26 2015 - 09:51 AM

Yeap! that is more complete even with the same principles...i will keep it with the doc tutorial... :)

i dont know how the historic Lotus 49 works...

View PostFat Rich, on Jan 26 2015 - 09:20 AM, said:

... (and models it badly according to David Kaemer from Papyrus).

and I don´t know if the right word is "badly" but wrong kind of diff type in terms of historical accuracy...but if you can achieve the same results, i don´t think it was a wrong choise for the math model.
Anyway...i don´t not much about it, just trying to put some tools that help me :) so thanks for the video pal!

Edited by hagapito40, Jan 26 2015 - 09:52 AM.


#4 John Woods

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Posted Jan 26 2015 - 10:52 AM

Here is a link about tuning a Vari-Loc "salisbury" Diff: http://icpcitation.c...iloc_theory.htm

Edit: (quoted comment deleted here)

Thanks for starting this thread.

Edited by John Woods, Jan 26 2015 - 06:30 PM.


#5 David Wright Lo67

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Posted Jan 26 2015 - 02:23 PM

View PostJohn Woods, on Jan 26 2015 - 10:52 AM, said:


His opinion about GPL no longer matters anyway, at least no more than anyone else, (no disrespect intended at all).

I'm sorry but you are showing a great deal of disrespect and I certainly don't agree with you.  Dave is by far the best placed to give an opinion on GPLs physics.

#6 John Woods

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Posted Jan 26 2015 - 03:13 PM

Truly my comments were meant with no disrespect, as I said.
Dave Kaemmer is no longer a participant, and does have a vested interest.
That's all.

The comments attributed to him by others, (I assume), refer to those made in an interview years ago, as I recall in the context of promoting his own future prosperity. That doesn't mean he was in error about Grand Prix Legends, and anyway no one else knows what he meant by his very general comments.

David, I am happy to delete my comment, and intended to, if you will work with me on that. (You delete your quote and I'll delete my comment and this post).
Had no intention either of offending anyone else. Sorry.

Edited by John Woods, Jan 27 2015 - 05:24 AM.


#7 Fat Rich

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Posted Jan 26 2015 - 05:09 PM

Apologies, I may have inadvertently started all this.... I have a vague recollection that David Kaemer wasn't happy with a few aspects of GPL's physics and that the diff modelling was one of them, but I can't remember his exact words :blush:

As John pointed out, those comments were made years ago and were indeed fairly general if I remember correctly. DK seems to have moved on from GPL fairly quickly and focussed on Nascar Racing 2003 and ultimately iRacing, probably because GPL wasn't the commercial success Papyrus hoped for and possibly needed it to be.

It's become clear over the years that DK is a perfectionist and is always looking to improve the physics in his simulations, I believe iRacing is on it's 5th or 6th "New Tire Model" and he's probably still looking for improvements. His desire to simulate every aspect of a racing car's physics is probably why GPL was so advanced when it launched and still stands up as a sim today.

#8 John Woods

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Posted Jan 26 2015 - 06:28 PM

No, I started it. Or at least I picked it up and took it too far.

The GPL Diffs are what they are and have worked well for years, as proved by our use of them.

#9 Pete Gaimari

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Posted Jan 26 2015 - 07:04 PM

All I can remember Dave saying is a realistic diff in GPL would be 45/85/1. A difficult setting to drive with the present physics.

Now that I think about it. I'm not sure if Dave said that, or someone else said he said that.

#10 John Woods

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Posted Jan 27 2015 - 06:03 AM

Huttu's later setups used looser coast side diffs.

All the default setups use tighter coast side diffs, favoring  85/30 and 60/45. Recall also Alison noting that none of the beta team members could drive tight diffs even tho they tried, apparently because they thought they were supposed to.

What is or is not realistic depends on where you place the gage.
If the scale is the setup menu, then maybe DK's 45/85 and Huttu's fun setups are more closely in line with what may have been used when real engines roared.

However another scale to be considered is what a GPL driver can make a car do, and whether that closely and appropriately reflects the performance characteristics of a race car...any race car.
By the second standard whatever the setup menu says is secondary to what the car is doing for a driver, who is in any realm the ultimate judge of what is going on.

Just my opinion, (woh boy), what is appropriate realistically depends more on the user's rig, especially the size of the display, the controller, FFB settings, and what the user can do with it all, than an alleged realistic setup specification.

(Before anyone misunderstands this last comment, understand that my quickest/fastest times were made back when the display was 19in at 42deg FOV hooked to a Logi black MOMO, suggesting to me all my subsequent hardware enhancements have been in vain with regard to getting any better at lowering my lap times).

Edited by John Woods, Jan 27 2015 - 06:43 AM.


#11 Pete Gaimari

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Posted Jan 27 2015 - 08:10 AM

Greger used throttle with the braking. An unrealistic way to drive a realistic diff, because the physics are wrong.

#12 John Woods

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Posted Jan 27 2015 - 08:40 AM

View PostPete Gaimari, on Jan 27 2015 - 08:10 AM, said:

Greger used throttle with the braking. An unrealistic way to drive a realistic diff, because the physics are wrong.

How do we know the diff physics are wrong?

Throttle while braking = trail braking? Nuvolari didn't let off, was it because of a bad diff?

Edited by John Woods, Jan 27 2015 - 08:56 AM.


#13 David Wright Lo67

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Posted Jan 27 2015 - 09:46 AM

View PostJohn Woods, on Jan 26 2015 - 03:13 PM, said:

Truly my comments were meant with no disrespect, as I said.
Dave Kaemmer is no longer a participant, and does have a vested interest.
That's all.

The comments attributed to him by others, (I assume), refer to those made in an interview years ago, as I recall in the context of promoting his own future prosperity. That doesn't mean he was in error about Grand Prix Legends, and anyway no one else knows what he meant by his very general comments.

David, I am happy to delete my comment, and intended to, if you will work with me on that. (You delete your quote and I'll delete my comment and this post).
Had no intention either of offending anyone else. Sorry.

I'm not offended John.  These are your views and you are perfectly entitled to them.  I have heard similar views from others when Dave criticized GPLs tyre model.

I just don't agree.  I don't agree Dave's primary motivation is selling his latest sim.  I believe his primary motivation is improving sim physics.  He did this when he created GPL and I have no doubt he continued to improve the physics of each sim that came out after GPL.

But even if someone is selling you something, it would be perfectly natural to tell your potential customer about improvements the latest product offered.  It's your implication that Dave would lie about flaws in GPLs physics which I find disrespectful.  

Anyway I'm happy to agree to disagree.  No harm done.

#14 Pete Gaimari

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Posted Jan 27 2015 - 10:09 AM

View PostJohn Woods, on Jan 27 2015 - 08:40 AM, said:

View PostPete Gaimari, on Jan 27 2015 - 08:10 AM, said:

Greger used throttle with the braking. An unrealistic way to drive a realistic diff, because the physics are wrong.

How do we know the diff physics are wrong?

Throttle while braking = trail braking? Nuvolari didn't let off, was it because of a bad diff?

I didn't say the diff settings are wrong. I said the physics are wrong, and the diff doesn't work as it should, because the rest of the physics are wrong. (grip)

Using throttle with braking is not trail braking John. Trail braking is applying the brakes hard while the car is still straight approaching a turn. As you start to turn in you release some of the brake pressure, and continue to do that to the apex. That's why it's called trail braking. You're trailing off the brakes. Using throttle with braking is something else, and is exaggerated in driving GPL by aliens. It's used to tighten up the diff during braking, because to not do it the rear of the car would be too loose. ( bad physics)

#15 John Woods

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Posted Jan 27 2015 - 10:10 AM

Totally did not mean to imply DK would lie about anything and agree about his integrity and his intention.
Really don't remember what he said and was only referring to comments by others, who tend to use his reputation to back their claims, (which has not been the case by anyone so far here on this thread), so that's why my intention originally was to make the now deleted comment, and then delete it because it was way off topic except in kind of an oblique way.
Its understandable he would think his latest efforts are superior to things published earlier, and entirely understandable he would promote his latest effort during an interview in part for that purpose. (Or maybe the timing of the interview was coincidental). Either way, anyone's comments should be considered in the context in which they were made.
But that is not the same as dissing his own work, which others do by referring to his comments in support of their claim the physics are all wrong and we need a new model for the 67s. In retrospect, he may now think it might have been better to have phrased his comment in a more positive and diplomatic way.

"Some cars do not react well to trail braking, especially those prone to lift off oversteer - although there will be more grip available at the front wheels while trail braking, the rear will be more prone to break lose. Beware!"
-from drivingfast.net


So, what cars are prone to lift off oversteer? Rear/mid-engine with a short wheelbase and wide track, as I understand from other real racer forums, and the remedy in the form of a compensation, no matter what car, real or otherwise, is to keep a little throttle on during trail braking.
Also, modern technique given vastly improved brakes is not the same as old school technique with brakes that failed to work when abused repeatedly. Recall Clark's pads looked almost new after a race because, apparently, he didn't use them.

Drawing below is from F1 Dictionary.

Attached Files


Edited by John Woods, Jan 27 2015 - 11:12 AM.


#16 Fat Rich

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Posted Jan 27 2015 - 10:18 AM

View PostJohn Woods, on Jan 27 2015 - 08:40 AM, said:

View PostPete Gaimari, on Jan 27 2015 - 08:10 AM, said:

Greger used throttle with the braking. An unrealistic way to drive a realistic diff, because the physics are wrong.

How do we know the diff physics are wrong?

Throttle while braking = trail braking? Nuvolari didn't let off, was it because of a bad diff?

Throttle and gas at the same time is a legitimate technique on loose surfaces, the rally guys use it to turn the car in some circumstances. But on tarmac it only gets used to warm the tyres and brakes before the start of the race, and even then it's used very carefully. If real racers drove like some sim racers they'd have no brakes, tyres and probably no transmission left by the end of the race ;) Mashing the throttle and gas will sometimes save the car from spinning in some sims, I doubt it works very well in real life.

Trail braking is where you use the brakes deep into the corner so continue to slow the car while turning in to the apex, as appos...............and as I'm writing this Pete has posted and explained it better already :D

It seems from a recent interview posted on SRMZ that Jim Clark was trail braking and that may explain some of his speed ( he also said the Lotus 49 had fixed gear ratios, something all sims are still getting wrong!), Mark Donohue was also well known for using it and explaining it with the Circle of Traction diagram. But it was a fairly new technique in the 60s as far as I know.

Edited by Fat Rich, Jan 27 2015 - 10:28 AM.


#17 John Woods

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Posted Jan 27 2015 - 11:21 AM

From autoevolution.com:

"Mario Andretti, a bloke who probably knows a thing or two about manhandling a car, since his motorsport victories include races ranging from F1, NASCAR and IndyCar, once said an interesting thing about brakes: 'It is amazing how many drivers, even on the Formula One level, think that the brakes are for slowing the car down.'

The more complex application: You can use LFB for trail braking, which means that you can keep the brake pedal pressed while cornering as you switch from a certain amount of throttle (before the corner) to a smaller one (during the corner) and then back to “power on” mode again. This will help you keep the car’s balance (you will no longer transfer the weight in a violent way) and therefore be able to tackle the bend more efficiently."

Edited by John Woods, Jan 27 2015 - 11:21 AM.


#18 Pete Gaimari

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Posted Jan 27 2015 - 11:42 AM

Not done in 1967. Stay in the correct era John.

#19 Pete Gaimari

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Posted Jan 27 2015 - 11:42 AM

View PostPete Gaimari, on Jan 27 2015 - 11:42 AM, said:

Not done in 1967 F1 cars. Stay in the correct era John.


#20 Fat Rich

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Posted Jan 27 2015 - 12:09 PM

View PostJohn Woods, on Jan 27 2015 - 11:21 AM, said:

Also, modern technique given vastly improved brakes is not the same as old school technique with brakes that failed to work when abused repeatedly. Recall Clark's pads looked almost new after a race because, apparently, he didn't use them.

He definitely used them :), but because he was trail braking into the corners he didn't need to use them as hard as the guys who braked only in a straight line. I use a bit of trail braking in GPL (a lot in the GT 67 mod) because it shortens braking distances and keeps a bit more weight on the front tyres and helps the car on turn in. That's why some people say my setups are understeery, they're actually oversteery when you trail brake ;). Of course too much trail braking will put too much load on the front wheels and you'll get understeer again, that's why setups are a very personal thing.

View PostJohn Woods, on Jan 27 2015 - 11:21 AM, said:

From autoevolution.com:

"Mario Andretti, a bloke who probably knows a thing or two about manhandling a car, since his motorsport victories include races ranging from F1, NASCAR and IndyCar, once said an interesting thing about brakes: 'It is amazing how many drivers, even on the Formula One level, think that the brakes are for slowing the car down.'

The more complex application: You can use LFB for trail braking, which means that you can keep the brake pedal pressed while cornering as you switch from a certain amount of throttle (before the corner) to a smaller one (during the corner) and then back to “power on” mode again. This will help you keep the car’s balance (you will no longer transfer the weight in a violent way) and therefore be able to tackle the bend more efficiently."


You can use either foot for trail braking, it's where you're using the brakes that defines whether you're a trail braker or not. I'm strictly a right foot braker in GPL because I believe they didn't use their left foot in those days, many of the cars had the steering column in the way so you couldn't. You might be able to in the Chapparal GT car because it had a centrifugal clutch / torque converter and your left foot isn't doing much, although I think they were operating the movable rear wing and front flap with a pedal.

I left foot brake in other sims in modern cars with flappy paddle gear shifts, there's no question it's easier and quicker than right foot braking because there's always a slight delay getting your foot from the gas to the brake. But I still refuse to use both gas and brake at the same time because to me it feels gamey.

Not trying to tell anyone how to play their game, if ultimate laptime is what you're after then use whatever is fastest even if it's unrealistic. :)

Edited by Fat Rich, Jan 27 2015 - 12:13 PM.





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