Changes to LMP2 in January 14 Build

All P2 drivers: there are significant changes coming to the Dallara P217 in the January 14 build. Please review these notes from Christian Challiner of iRacing:

In the 14/01/2021 patch the LMP2 will receive a few changes, we understand that these are quite close to the Daytona 24h and that a lot of you have already been working on setups. Because of this, we'd like to give you a heads up on what these changes are, what they mean and what you will need to do to 'fix' your existing setups as well as take advantage of the changes we have made.


So what are the changes?


- Power has been reduced by 40 hp (-6.66% reduction relative to 2020 specification). This is to match the car to the 2021 regulation change for LMP2's globally. https://sportscar365.com/lemans/wec/lmp2-power-red...-be-applied-across-all-series/


- Tyre warmer temperatures have been increased. This change reduces the time taken to bring the tyres up to operating temperature. You will find that out laps are now significantly less frightening.


- Camber limits have been revised to better match real world limits. Front and rear camber limits are now -2.9 (front) and -1.9 (rear). This will be the change that has the most impact upon your setups if you are running higher cambers.If you're only skim reading this post and want to know how to account for this, see the last few paragraphs.


- Two new adjustments have been added to the garage screen, 'Pushrod Length' and 'Third Pin Length'. These require more detailed explanation so I will talk about them individually, though functionally they are similar.


Pushrod Length


This adjustment will allow you to change the ride height of the car independently of the spring perch options at either end of the car. However, when adjusted in conjunction with the spring perch offset this also shifts your ratio of bump/rebound damper travel. What this means is that you now have greater control over when you will either top out the shock (this should ALWAYS be avoided) or when you contact the bump stop that is at the end of each corner dampers travel.


Ok so how do I use this to my advantage? Why would we want this?


Allowing users to alter the bump stop engagement means that you can 'time' when you land on the bump stop and means that you can use the bump stop to achieve greater ride height control.


In practice, this will allow for the use of softer third spring rates without track contact which results in greater straightaway speeds at tracks such as Daytona while improving corner exit traction through the use of softer spring rates. I have adjusted the iRacing Daytona baseline setup to give you an idea of how this can be utilised.


This setup now runs softer front and rear third springs (which is good for the low speed infield) without excessively scraping the racetrack on the banked sections since the right side bump stops are engaged and holding the car off the track surface.


If this is perhaps unclear to this point, think of the bump stop as an additional extra stiff spring that is located at the end of the damper travel. By altering your pushrod length and spring perch offset together, you move the engagement point of this spring earlier or later in travel. If you engage the bump stop earlier, your 'at speed' ride height will increase. If you engage the bump stop later, your 'at speed' ride height will decrease.


There is a lot of fine tuning that can be necessary in this area to achieve the best performance and you should experiment with different third spring rates, pushrod lengths and starting damper deflections to find the best solution. This is why my recommendation for Daytona is to start with the values within the iRacing setup, as these will get you in the ballpark of where you need to be!


Note, pushrod length adjustments are paired left to right, this means that if you alter the 'Front Pushrod Length' the length of both the LF and RF pushrod lengths are adjusted together.


Third Pin Length


This works in much the same way as the pushrod length only instead of changing the position of engagement for the corner spring bump stops it will change the point of engagement of the third spring unit bump stop. Currently, the bump stops for these are quite far out, which means you likely wont engage them even with the shortest pin lengths used except in extreme cases. This may change in the future and I will notify you if that's the case. For now, just know it works in the same way as the pushrod length adjustment but that currently, it's much less effective relative to the pushrod length adjustment. Certainly, I would not spend a lot of time in this area.


My recommendations for starting pushrod length and third pin lengths (for normal tracks) are as follows:


Front Third pin length: 201.0 mm

Front Pushrod length: 190.0 mm


Rear Pushrod length: 166.5 mm

Rear Third pin length: 181.5 mm


For a 45 mm front and rear static ride height this will result in approximately equal bump and rebound travel of all corner dampers and each third spring unit. This is a good baseline to work from. As mentioned previously, the Daytona setup uses different values to get you started at that track specifically.


So how do I make my old setups work now?


My advice is note down your current static ride heights and spring deflections prior to the patch. Then, using the pushrod and pin lengths from the iRacing Daytona setup or those mentioned above reset your cambers to the new rule limitations and readjust your ride heights using spring perch offsets/torsion bar turns (both corner spring and third spring). Don't forget to reset your toe-in. You should find that your old setup now passes tech and drives very similarly to previously.


However, the other option that is available is to work from the iRacing Daytona setup and transfer your existing aero, ARB and differential configurations to this new setup which will take minimal time to implement.


Finally, I would like to mention that the power reduction will not result in difficulty passing GTE cars at Daytona. You should still enjoy a 20 km/h straightaway speed advantage and lap times will increase by approximately 1 to 1.5 seconds. You may also find that you can reduce then number of diff clutch plates you are using since you are putting less power to the ground and require less drive side locking to handle this.


For the sake of simplicity I'm considering the following as the wheel rate (combined rate of all springs measured at the hub) at one end of the car in a purely straight compression as you would see going down the straight at Le Mans. Let's just say this is for the front of the car, though it works exactly the same at the rear as well.


The diagram attached shows four options/configurations:


Note - the large 'knees' in the graph signify the point of bump stop engagement.


Green - A stiff linear spring with zero bump stop engagement. To achieve this in sim, you would fit a 'stiff' third spring and use a 'short' pushrod length. A shorter pushrod length will increase the available bump travel of the damper at a given ride height, thus increasing the bump stop gap and resulting in the bump stop remaining unloaded throughout the entirety of the practically usable suspension travel.


Blue - This option is a 'medium' rate third spring with a pushrod of 'medium' length. This option has reduced available bump travel before the bump stop is engaged but as you can see, the 'final' force produced is the same. What this means in practice is that this car at 'low' loads will sit down further initially (so lower) relative to the 'green' car but once it lands on the bump stop it will then ride on a much stiffer rate that will result in the same end of straightaway ride height. This means that relative to the 'green' option the suspension is softer in low load cases such as the Daytona infield and will have relatively more available mechanical grip and corner exit traction, especially in bumpy sections of track. Yet despite this, ride height control at high loads/speeds remains the same. Essentially, you can get a good compromise in vastly different corner types which can equate to better performance. Though, you will note that the bump stop engagement is quite harsh and this abrupt change in rate can be unsettling if it's happening during a corner. You should aim to either be on or off the bump stops in high speed corners and not transitioning on and off them during the corner, this will lead to poor performance and likely oscillation.


Red - This option is the same 'medium' rate third spring as the 'blue' option only with a 'long' pushrod length. The longer pushrod length results in less available damper travel in compression for a given starting ride height and thus the bump stop is loaded earlier. As you can see, this results in essentially greater resistive force from the suspension and would manifest itself as an increase in ride height at high loads compared to the 'blue' option. When you are changing your pushrod lengths if you are resetting the car back to the same static ride heights it is this which you are changing and why you will notice a difference in your dynamic ride heights and straightaway speeds!


Purple - This option is a 'soft' third spring used in conjunction with a relatively longer pushrod length than that in the 'blue' case. As you can see, this results in the bump stop being engaged earlier, yet the final force is the same and thus, so is the end of straightaway ride height! You will also note that in this case the gradient of the purple line is not identical to that of the blue line after the bump stop is contacted, this is because the final gradient is the COMBINED rate of all springs plus the bump stop and thus, since the third spring rate was softer, so is the final rate produced despite using the same bump stop.