Bumpstop stiffness, an explanation of it please!

[Celtic Pharaoh]

Hey everyone,

I was running the ASR Ferrari 412 T2 at Monza '88 last night and I took the opportunity to try and figure out how bumpstops and limiting suspension travel to use them. I first tuned everything else as best I could. I then found that the ride height in Motec was around 25mm at top speed and from what I've heard, the car should almost be bottoming out at the end of the longest straight to insure that the car is being as low as it can be as this was the pre-diffuser days.

So I lowered the car by 20mm. Once I did this, I got this reaction in Lesmo 2
https://streamable.com/topa5

So I thought "okay the car is bottoming out so I need to limit the suspension travel" although according to Motec, ride height was still above 15+mm. I did this, it didn't fix the problem. However, even though I thought it might make the problem worse, I increased bumpstop stiffness at the rear from 80 to 200 and then magically, the car stopped oversteering but I am failing to see how it solved issue since stiffer bumpstop = less grip. I then thought that maybe its because my bumpstops need to have a stiffness thats relatively equal to the spring rates whereas before the bumpstops were on the softer side while the spring rates were on the stiffer side and some sort of mismatch was happening.

But this is all speculation and like I said at the start of the post, I'm mostly ignorant on how they work so can anyone explain to me why increasing the stiffness of the bumpstop solved this problem and not made it worse? Thank you

 

[PhilS13]

I don't know the mod but I bet you lose rear grip because the diffuser gets too low and stops working. This can happen before bottoming out. Stiffening the bumpstops prevents the rear from going too low. Yes you "lose" some mechanical grip doing that but it is incredibly small compared to what you lose when the rear diffuser stops working.

Also the ride height in MoTec is not necessarily true ride height. You can have ground collisions even when MoTec shows otherwise.

 

[Bobby Pennington]

The thing about bumpstop's, bump rubbers is that they do 2 things. Like you said, they limit suspension travel by hitting "the stops" and they do provide a limited amount of dampening depending on the stiffness of the bump rubber you use.

What I saw in the video and then the solution you used to correct it, tells me that even though you went stiffer, you stopped the car from bottoming out and the tire sidewalls were able to take care of the rest of the dampening. The loads through there aren't too great like they would be at Spa with Eau Rouge, Pouhon, and Blanchimont, the 2 fast right handers at Mugello, turns 8 and 9 aka Arrabbiata1 and 2 or Suzuka's notorious 130R.

Think of the suspension as being one giant progressive spring. All the parts work together or against one another at differing times of suspension travel and your settings. When all you have left is the tire sidewall, it can get really sketchy. Try running at Spa and playing with the bumpstops with regards to Eau Rouge and Pouhon. I've found that if I set the bumpstops too stiff, it can be tricky almost just like bottoming out.

 

[Kek700]

Also the ride height in MoTec is not necessarily true ride height. You can have ground collisions even when MoTec shows otherwise.[/QUOTE]

Thats interesting, i invested an exorbitant amout of time on MoTec on this specific subject.
Finally having to give up and accept that i probably lacked sufficient knowledge to solve this.
If only i had known the MoTec was somewhat at fault. Even though i suspected it may have been the case. i could not think of a counter way to substantiate it. After this i tended not to get involved with MoTec data logging. This was only one of a number of things that did not make sense to me. I also tried the AC developers App but finding enough inforamation on that proved difficult to.
using MoTec. This is not a criticism of MoTec, just an uneducated observation.

Whilst being helped by a very knowledgeable gentleman on this subject he gave me this example of
using packers unwisely.

Important thing to realize when running car on packers is that softening the spring actually stiffens the suspension. For example, Z4 GT3 by default runs on rear packers almost out of the pits, softening rear springs will lower the car and it will sit on packers more effectively having more total stiffness. So the proper way to soften the rear on that car is to stiffen the rear springs!!

 

[Celtic Pharaoh]

I don't know the mod but I bet you lose rear grip because the diffuser gets too low and stops working. This can happen before bottoming out. Stiffening the bumpstops prevents the rear from going too low. Yes you "lose" some mechanical grip doing that but it is incredibly small compared to what you lose when the rear diffuser stops working.

Also the ride height in MoTec is not necessarily true ride height. You can have ground collisions even when MoTec shows otherwise.

Thanks for the explanation @PhilS13. I actually had assumed that the older F1 cars didn't have diffusers which is why I was confused. I'll need to double check wings app to see

I agree about Motec. I don't know how many times when driving GT3s, particularly the 911, I'd hear the front splitter scrape but Motec wouldn't show a 0 or a negative value for the ride height. Quite frustrating that the data being output isn't accurate!

The thing about bumpstop's, bump rubbers is that they do 2 things. Like you said, they limit suspension travel by hitting "the stops" and they do provide a limited amount of dampening depending on the stiffness of the bump rubber you use.

What I saw in the video and then the solution you used to correct it, tells me that even though you went stiffer, you stopped the car from bottoming out and the tire sidewalls were able to take care of the rest of the dampening. The loads through there aren't too great like they would be at Spa with Eau Rouge, Pouhon, and Blanchimont, the 2 fast right handers at Mugello, turns 8 and 9 aka Arrabbiata1 and 2 or Suzuka's notorious 130R.

Think of the suspension as being one giant progressive spring. All the parts work together or against one another at differing times of suspension travel and your settings. When all you have left is the tire sidewall, it can get really sketchy. Try running at Spa and playing with the bumpstops with regards to Eau Rouge and Pouhon. I've found that if I set the bumpstops too stiff, it can be tricky almost just like bottoming out.

Thank you for the detailed explanation Bobby
I sort of understand the idea behind how it all comes together. Like to insure optimal damping and handling, as the car pitches or rolls more, the suspension should get progressively stiffer as well.

The thing that confused me about this though is that I remember hearing from someone before that you never want the car to ride on the bumpstops mid corner or exit and that its only acceptable at top speed and going on the front bumpstops under braking. This is why I had thought that the car should actually drive worse through there when I stiffened them but Philippe's explanation about losing diffuser efficiency could be causing the problem.

I'll experiment with Eau Rouge, Pouhon and 130R

 

[Celtic Pharaoh]

Also the ride height in MoTec is not necessarily true ride height. You can have ground collisions even when MoTec shows otherwise.

Thats interesting, i invested an exorbitant amout of time on MoTec on this specific subject.
Finally having to give up and accept that i probably lacked sufficient knowledge to solve this.
If only i had known the MoTec was somewhat at fault. Even though i suspected it may have
been the case. i could not think of a counter way to substantiate it. After this i tended not to
get involved with MoTec data logging. This was only one of a number of things that did not
make sense to me. I also tried the AC developers App but finding enough inforamation on that
proved difficult to.
using MoTec. This is not a criticism of MoTec, just an uneducated observation.

Whilst being helped by a very knowledgeable gentleman on this subject he gave me this example of
using packers unwisely.

Important thing to realize when running car on packers is that softening the spring actually stiffens the suspension. For example, Z4 GT3 by default runs on rear packers almost out of the pits, softening rear springs will lower the car and it will sit on packers more effectively having more total stiffness. So the proper way to soften the rear on that car is to stiffen the rear springs!!

It is indeed frustrating! There is an alternative program that you can use to get telemetry known as AIM RaceStudio that does show more data channels than ACTI but I refrained from using it and sticking with Motec as it's a much better program. Performance is quite poor in comparison.

That's a very interesting fact about the Z4 I didn't know that!

 

[Kek700]

@Celtic Pharaoh
I'll experiment with Eau Rouge, Pouhon and 130R

This is another fruitless crusade i went on with MoTec. Trying to get my Nissan GT-R gt3 to turn in
Just past Eau Rouge ( can’t remember the name of that part ) under full throttle .
This was just for a fast lap, most of the time during a race it was of less importance. Probably
because the Nissan has such a slug of torque. That then became a real headache, of finding
the correct gearing to keep the car in its optimum torque curve thoughout the climb, with respect to
the rest of the long circuit.

I do sound like a moany git.

 

[Wayne Kerr]

It is indeed frustrating! There is an alternative program that you can use to get telemetry known as AIM RaceStudio that does show more data channels than ACTI but I refrained from using it and sticking with Motec as it's a much better program. Performance is quite poor in comparison.

That's a very interesting fact about the Z4 I didn't know that!

if you want to tune an aero car in AC, you have to use AIM. ACTI isn't as efficient because you don't get the aero dump into the motec file.

There isn't a "better" or "worse" program. They almost all do the same thing once you understand the syntaxes of how to write the math channels. for the purpose of AC, there's no way aim is an inferior software vs motec, just the info you get from the game alone makes it worthwhile to work with it.

 

[Celtic Pharaoh]

if you want to tune an aero car in AC, you have to use AIM. ACTI isn't as efficient because you don't get the aero dump into the motec file.

There isn't a "better" or "worse" program. They almost all do the same thing once you understand the syntaxes of how to write the math channels. for the purpose of AC, there's no way aim is an inferior software vs motec, just the info you get from the game alone makes it worthwhile to work with it.

I do miss the aero data from AIM. It can show real time downforce figures as well at different speeds and thats a huge help with determining front-rear aero balance. I just found it to lag quite a lot when I was using it where Motec is virtually perfect in that regard.

I'll try it again and see. Like you say, there is a lot more detail in AIM than can be found used ACTI

 

[Kek700]

You could always use the “aero” in the AC developers App. And record a lap in real time with
the appropriate software. Then look at it in slow motion. That would be a very simple solution.
I’m sure you already know this, probably worth a mention though.