Welcome to 911UK
The only place for Porsche, 911uk is the definitive enthusiast and resource site for the Porsche 911.
Registration is fast, simple and absolutely free, so join up today for full access to the site and benefit from latest member offers.

Porsche Classifieds
Sell Your Porsche on 911uk
Create a Free Classified Advert
Search Ads
Classified Adverts FAQ
Trade Classified Information
Buyer & Seller Fraud Protection
Consumer Rights Act
Pre Purchase Inspection (PPI)
Porsche Car Sourcing
Porsche Cars Wanted
Official Porsche Centre Reviews
Model
Stock
Porsche 911
992 : 2019- 0
991 : 2011- 21
997 : 2004- 64
996 : 1997-2005 32
993 : 1993-1998 5
964 : 1989-1993 2
Carrera 3.2 : 1983-1989 3
Carrera SC : 1977-1983 2
930 Turbo : 1975-1989 1
Early 911 : 1964-1977 1
Porsche Other Models
Classic : 1950-1965 0
Boxster : 1997- 30
Cayman : 2005- 21
Cayenne : 2003- 5
Macan : 2014- 5
Panamera : 2009- 0
912-914-924-928-944-968 1
959 - CarreraGT - RaceCar 0
Car Parts For Sale & Wanted
Other Items For Sale & Wanted
Wheels Tyres For Sale & Wanted
Number Plates For Sale Wanted

Porsche Services
Porsche Body Shop Repair
Paint Protection & Wrapping
Porsche Classic Insurance
Porsche Classic Parts
Porsche Classic Restoration
Porsche Design Collection
Porsche Engine Gearbox Rebuild
Porsche Heritage & History
Porsche News
Porsche Picture Gallery
Win a New Porsche 911

Porsche Parts
Body Parts, Body Styling
Brakes, Clearance
Electrical, Exhausts
Engine Cooling, Engine Electrical
Engine Rebuild, Heating Cooling
Interior Incar, Lighting
Rubber Seals, Service Parts
Steering, Suspension
Transmission, Workshop Tools
Early 911, 911 - 930, 928 - 968
964 - 993, 996 - 997, Boxster
Cayman, Cayenne, Panamera

Porsche Model Range
911 [992] 2018-Current
Porsche 911 [992]
911 [991] 2011-2019
Porsche 911 [991]
911 [997] 2004-2012
Porsche 911 [997]
911 [GT] GT1-GT2-GT3
Porsche 911 [GT]
911 [996] 1997-2005
Porsche 911 [996]
911 [993] 1993-1998
Porsche 911 [993]
911 [RS] RS-RSR
Porsche 911 [RS]
911 [964] 1989-1993
Porsche 911 [964]
911 3.2 1983-1989
Porsche 911 3.2 Carrera
911 SC 1977-1983
Porsche 911 SC
911 [Early] 1964-1977
Porsche 911 [Early]
Boxster & Cayman
Porsche Boxster & Cayman
Cayenne & Panamera
Porsche Cayenne & Panamera

911uk Site Partners

Post new topic   Reply to topic
Author Message
MisterCorn
Dijon


Joined: 08 Jan 2011
Posts: 7251
Location: Nottingham, England

2004 Porsche 996 Turbo

PostPosted: Wed Oct 16, 2019 11:25 pm    Post subject: Reply with quote

If any of you guys want to use our measurement kit for tests just give me a shout. If I can get other cars in next time we are at Bruntingthorpe I'll let you know.

MC
 
  
View user's profile Send private message Visit poster's website
   
maldren
Suzuka


Joined: 07 Oct 2016
Posts: 1182



PostPosted: Thu Oct 17, 2019 7:47 am    Post subject: Reply with quote

MC,
it would be interesting to try something like a figure of 8 that Kieran suggested, in the end we were limited on where we could test on the day but his help and input was much appreciated, please pass it on to him and of course, many thanks to you too,

Mike
_________________
Mike
2003 996.2 C2 Coupe Arctic Silver
 
  
View user's profile Send private message
   
Martin996RSR
Nürburgring


Joined: 08 Dec 2016
Posts: 397



PostPosted: Thu Oct 17, 2019 8:42 am    Post subject: Reply with quote

skinny_monkey wrote:

It would be interesting to see if increased roll stiffness reduces grip from increased weight transfer to loaded tyres in steady state. Ive heard arguments both ways.


Increasing the total (front and back) roll stiffness will generally improve overall grip as it reduces weight transfer and allows the inside wheels to make more of a contribution. The downside is that when the limit of grip is reached, the transition to slip is far more sudden than a car with low roll stiffness. It's basically a trade-off between driveability and ultimate performance. In the real world, on the road, high roll stiffness is a always going top be limited by the acceptability of the ride, as hard shocks/springs, and AR bars are always going to make you feel every last bump.
 
  
View user's profile Send private message
   
DynoMike
Albert Park


Joined: 25 May 2012
Posts: 1572
Location: The Cotswolds

2003 Porsche 996 Turbo

PostPosted: Thu Oct 17, 2019 11:45 am    Post subject: Reply with quote

Quote:
Increasing the total (front and back) roll stiffness will generally improve overall grip as it reduces weight transfer and allows the inside wheels to make more of a contribution. The downside is that when the limit of grip is reached, the transition to slip is far more sudden than a car with low roll stiffness. It's basically a trade-off between driveability and ultimate performance. In the real world, on the road, high roll stiffness is a always going top be limited by the acceptability of the ride, as hard shocks/springs, and AR bars are always going to make you feel every last bump.



I agree with this
_________________
2003 996 Turbo
Previous toy 1974 Mini 1000
 
  
View user's profile Send private message
   
skinny_monkey
Nürburgring


Joined: 16 Sep 2014
Posts: 457



PostPosted: Fri Oct 18, 2019 10:28 am    Post subject: Reply with quote

I understood increasing roll stiffness increased weight transfer. Say you had no suspension (infinitely stiff) and you pushed your car sideways to roll the body, the inside tyres would lift and put the weight on the outside tyres. If you had a very floppy suspension, and did the same, the body would roll but the inside wheels would stay on the ground.
Hence why if you stiffen only the rear ARB, you get oversteer due to increased weight transfer at the rear.
As you say tho, the increased roll stiffness decreases response time and makes the car more reactive.
At least, this is my understanding
 
  
View user's profile Send private message
   
Martin996RSR
Nürburgring


Joined: 08 Dec 2016
Posts: 397



PostPosted: Fri Oct 18, 2019 11:29 am    Post subject: Reply with quote

Basically, the stiffer an axle is, the less weight transfer takes place. I'm not quite sure how to prove this with a thought experiment, so you'll just have to trust me.
I realise it also seems counter-intuitive given the above, that increasing the roll stiffness at one end of the car is going to make that end less grippy, but it does.

These two nuggets are pretty much what all suspension tuning is derived from. Hopefully someone better at explaining things can post and give you a good explanation as to why these two things are true, but trust me, they are.
 
  
View user's profile Send private message
   
Jamie Summers
Österreich


Joined: 29 Oct 2002
Posts: 905
Location: Esher, Surrey

1992 Porsche 964 RS

PostPosted: Fri Oct 18, 2019 12:44 pm    Post subject: Reply with quote

You can see a clear correlation between roll angle and lateral g force in the data, which illustrates the points above very clearly.

My own car is quite an interesting case in point here. I was running the (understood to be) less mechanically grippy Goodyears, but the roll stiffness of the car meant that it was able to make the most of the tyres to translate into lateral g - still producing a good overall result.

Intuitively with a gripper tyre, say a Cup 2, the greater mechanical grip from the tyre would most likely allow a higher lap speed and higher lateral load, but would probably produce a greater roll angle.

If the limit of the tyre's adhesion was reached, by further increasing the roll stiffness would you be able to go faster still ??

Clearly at some point the laws of physics suggest the tyre reaches its maximum possible load - a function of contact patch / compound / temperature and road surface and then you're in the weeds regardless of how sophisticated your suspension is !!!!

......or am I being too simplistic ???
_________________
'06 Autofarm Cayman RS Development Car
‘99 996 C2
Ex '92 964 RS
Ex '93 968 CS
Ex '90 Clio Mk1 Cup Racer
Ex '90 964 C2
Ex '89 Carrera 3.2
 
  
View user's profile Send private message Send e-mail
   
Griffter
Spa-Francorchamps


Joined: 22 May 2016
Posts: 359



PostPosted: Fri Oct 18, 2019 2:11 pm    Post subject: Reply with quote

Martin996RSR wrote:
Basically, the stiffer an axle is, the less weight transfer takes place. I'm not quite sure how to prove this with a thought experiment, so you'll just have to trust me.


Is it that as the outside wheel of an axle is loaded up and compresses the suspension, a stiff axle (stiff arb for example) will try to lift the inside wheel as well, thus lowering the suspension across the axle and the centre of mass?
 
  
View user's profile Send private message
   
skinny_monkey
Nürburgring


Joined: 16 Sep 2014
Posts: 457



PostPosted: Fri Oct 18, 2019 10:13 pm    Post subject: Reply with quote

Martin996RSR wrote:
Basically, the stiffer an axle is, the less weight transfer takes place. I'm not quite sure how to prove this with a thought experiment, so you'll just have to trust me.
I realise it also seems counter-intuitive given the above, that increasing the roll stiffness at one end of the car is going to make that end less grippy, but it does.

These two nuggets are pretty much what all suspension tuning is derived from. Hopefully someone better at explaining things can post and give you a good explanation as to why these two things are true, but trust me, they are.


Are you sure that increasing the stiffness of an axle reduces weight transfer? I mean, university was 20 long years ago so fuzzy at best, but i can't see (everything else being equal) how that works?

My original question (probably poorly worded) came from the understanding that you cannot change total weight transfer from roll stiffness as it depends on centre of mass height (relative to roll centre), track width, weight, and lateral acceleration. But you can certainly tune weight transfer and hence grip from to rear with relative roll stiffness. So what happens to total grip if you stiffen (or soften) front and rear together equally... i guess nothing!


Griffter wrote:

Is it that as the outside wheel of an axle is loaded up and compresses the suspension, a stiff axle (stiff arb for example) will try to lift the inside wheel as well, thus lowering the suspension across the axle and the centre of mass?


Yeah this makes logical sense to me at least when using stiff ARB's - as the vehicle leans, then lifting the inside wheel will cause it to squat, thus changing the height of off the c of g.

But going back to my previous point, this lifting the inside wheel with the stiff ARB surely can only increase weight transfer to the outside wheel?

Last edited by skinny_monkey on Sat Oct 19, 2019 4:31 pm; edited 1 time in total
 
  
View user's profile Send private message
   
Martin996RSR
Nürburgring


Joined: 08 Dec 2016
Posts: 397



PostPosted: Sat Oct 19, 2019 6:42 am    Post subject: Reply with quote

My original question (probably poorly worded) came from the understanding that you cannot change total weight transfer from roll stiffness as it depends on centre of mass height (relative to roll centre), track width, weight, and lateral acceleration. But you can certainly tune weight transfer and hence grip from to rear with relative roll stiffness. So what happens to total grip if you stiffen (or soften) front and rear together equally... i guess nothing!


Nope, not nothing, total grip increases.

The grippiest theoretical suspension is completely solid, including the tyres (imagine magic solid tyres with no air, just a band of grippy rubber that also happens to not be able to compress at all), allowing no roll. This would give zero weight transfer and allow the tyres to reach their full potential. The downside is that you would have to have a perfectly smooth and level track to run this car on.

Forget about CoM height, roll centre height, track width, etc. Just know that making a car's suspension softer will make it tend to roll more. The mechanics needn't be any more complex than that.

Total grip is the amount of grip available when the car is in steady state, so should be regarded as 'total potential grip' rather than total grip being used at any one time, and is whatever the tyres are capable of with perfect suspension and zero weight transfer.

Lets now put a car into a corner and let it take a set. The outside suspension will compress as the car rolls and more of the car's weight will be pressing those tyres into the ground than the inside tyres. (remember the car's weight is constant, and if the suspension compresses more on the outside tyres then it must be less on the inside tyres)

The outside tyres will be working harder than normal and be able to give more grip, but the inside tyres with less weight on them lose more grip than the outside tyres gain. Ergo the softer the car, the more weight transfer and the less total grip.


Try this on for size and let us know how you get on.

https://youtu.be/qCE54-H2zkM

In particular, from about 3:20, he uses an illustration where at zero weight transfer due to steady state driving, the car has 100 'grip units'. During periods of weight transfer, the car has only 90 total grip units (explained in the video). This is why the more weight transfer you have, the less total grip you have, and why softer cars have more weight transfer and less total grip available and harder cars have more total grip available, basically due to being able to make the inside tyres work harder.
 
  
View user's profile Send private message
   
skinny_monkey
Nürburgring


Joined: 16 Sep 2014
Posts: 457



PostPosted: Sat Oct 19, 2019 11:17 am    Post subject: Reply with quote

I don't dispute anything the guy says - and we both agree (obviously) that a soft car rolls more than a stiff car, and also that the more weight transfer, the less total available grip.


Martin996RSR wrote:


The grippiest theoretical suspension is completely solid, including the tyres (imagine magic solid tyres with no air, just a band of grippy rubber that also happens to not be able to compress at all), allowing no roll. This would give zero weight transfer and allow the tyres to reach their full potential. The downside is that you would have to have a perfectly smooth and level track to run this car on.


What we don't agree on is that a stiff car reduces weight transfer. Going to your example of a completely solid suspension going round a corner. Your view is that it will never roll and so you get zero weight transfer. My view is that it will still roll, and that it now takes barely any roll at all to cause total weight transfer.

In my head, I imagine a brick with 4 grippy pads stuck underneath at the corners. Push the brick sideways (simulating force from lateral acceleration) and it will tilt, lifting the inside pads up. I.e weight transfer to the outside. If it doesn't tilt with this force applied, it just slides, which obviously you don't want.

When the car corners and tries to roll, it tries to compress the suspension on the outside. Stiff suspension will resist the car rolling by pushing those outside tyres harder into the ground (so they become more loaded hence more weight transfer) compared to soft suspension, which will just let the car body roll without loading the tyres.


Martin996RSR wrote:
... the softer the car, the more weight transfer and the less total grip.


This is also contrary to what i understand about arb's for example. I thought that if you stiffen the front arb (everything else being equal) you will increase the tendency for understeer. If you stiffen the rear arb, you will increase the tendency for oversteer. Therefore, increasing arb stiffness reduces grip (because of increased weight transfer)?

The way that stiffer ARB's can increase total available grip is by improving the balance front to rear - say if your car is prone to understeer, a stiffer rear arb means the rear takes more weight transfer and gives some more grip to the (relatively softer) front?


having said all that, I'm certainly not saying that running very soft suspension is a good idea. It would be horrible and wallowy, transient response will be trash and the more roll you have, the less control you have over your suspension geometry. So making a stiff car can improve grip (preventing undesirable camber or toe etc in compression). I actually run stiffer suspension and uprated ARB's on my track day mx5 Smile

Last edited by skinny_monkey on Sat Oct 19, 2019 4:27 pm; edited 2 times in total
 
  
View user's profile Send private message
   
skinny_monkey
Nürburgring


Joined: 16 Sep 2014
Posts: 457



PostPosted: Sat Oct 19, 2019 12:54 pm    Post subject: Reply with quote

Actually (having just checked wiki Smile ) it might be that I'm talking about load transfer whilst you are talking about weight transfer.

Load transfer is caused by the acceleration of the vehicle mass being reacted by the loaded tyres. This is caused by the lever effect of the centre of the sprung mass being above the tyre contact patch so in cornering, braking, or acceleration, you shift additional load to the compressed side.

Weight transfer is more of a geometric effect and refers to the change in the position of the c of g due to roll (so on a car that rolls a lot, the position of the centre of the car moves relatively, hence more weight transfer, and a car that doesn't roll the centre of the car doesn't move relatively, hence no weight transfer)

So in a way, we were both right!! Very Happy

Weight transfer vs load transfer https://en.m.wikipedia.org/wiki/Weight_transfer


However, load transfer is a more significant factor for grip which is why i thought we were talking about the same thing.

Edit: just been out for a little walk with the missus and i have come to agree with you Martin!! Smile This is how I've resolved it in my head, hopefully I'm on the right track!
Whilst a stiffer arb does increase load transfer at that end relative to the other end causing reduced grip on that axle, increased roll stiffness at both the front and rear does not affect load transfer. (This is where i was getting confused). It does, as you say, affect weight transfer and decreases it. This, together with the additional control you have over the suspension, therefore does increase your grip.

Sorry it took me a while!
 
  
View user's profile Send private message
   
peterg1955
Silverstone


Joined: 09 Apr 2017
Posts: 100
Location: Berkshire


PostPosted: Sat Nov 09, 2019 10:36 am    Post subject: in next month's magazine (out on 5th Dec) Reply with quote


_________________
2002 996 C2 black/black with aerokit and M030
 



thumbnail_20191109_100543.jpg
 Description:
 Filesize:  348.64 KB
 Viewed:  114 Time(s)

thumbnail_20191109_100543.jpg


  
View user's profile Send private message
   
Display posts from previous:   
Post new topic   Reply to topic   All times are GMT - 12 Hours
Goto page Previous  1, 2, 3, 4
Page 4 of 4

 
Jump to:  

You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You can attach files in this forum
You can download files in this forum
You cannot post calendar events in this forum