Tim.Wright wrote:Secondly, stiffer springs (and anti-roll bars!) increase your warp stiffness. All tracks have non negligible amounts of banking, curbs and random asperities during every corner which excite the warp mode of the suspension and here stiff setups cause 3 problems...
Good explanation of warp sensitivity and influence on roll stiffness distribution. In hindsight, softer warp stiffness is probably one thing F1 was trying to achieve with Front-Rear interconnected suspension.
EDIT:
But wait ! there's a different perspective! DaveW has suggested that some amount of warp sensitivity can be a good thing to positively influence handling for different dynamic states --
Ideally, I suppose, an F1 driver would like his car to understeer under braking, oversteer on turn in (to establish the turn quickly), moving to neutral at the apex and then to understeer again under acceleration (to get power down). Without going into details of how this might be achieved, I think that religiously maintaining zero warp loads though a corner would not be a brilliant idea.
Pierce89 wrote:Its crazy that more people on this board don't know about DaveW. He's literally one of the top.suspension tuners in the entire racing world.
Mmm... I think that Larry Holt, vice president of Multimatic deserves a mention. He has inspired many engineers, me included. 2016 was a good year for Multimatic, as it happens, see here. More could be said.
I suspect your performance issues are probably caused by tyre temperatures. JT's suggestion of a notebook is a good one....
Pierce89 wrote:Its crazy that more people on this board don't know about DaveW. He's literally one of the top.suspension tuners in the entire racing world.
Mmm... I think that Larry Holt, vice president of Multimatic deserves a mention. He has inspired many engineers, me included. 2016 was a good year for Multimatic, as it happens, see here. More could be said.
I suspect your performance issues are probably caused by tyre temperatures. JT's suggestion of a notebook is a good one....
You're too modest. I can't tell you how many time I've heard "guru" connected to your name by other engineers . I've always assumed that you were a big part of Multimatic's success. Just out of curiosity, do you personally do any of their OEM suspension work and is it anywhere near as interesting as race stuff?
“To be able to actually make something is awfully nice”
Bruce McLaren on building his first McLaren racecars, 1970
“I've got to be careful what I say, but possibly to probably Juan would have had a bigger go”
Sir Frank Williams after the 2003 Canadian GP, where Ralf hesitated to pass brother M. Schumacher
Wow, first off, thanks to everyones in-depth input. Especially Tims long and informative post. This is why I come to F1T .
To answer various questions:
Tim.Wright wrote:
Regarding the original post, unfortunately I couldn’t find a short concise way to respond. Mainly because I hate working with ‘rules of thumb’ and prefer a more fundamental approach. I have to say your reasoning/logic is pretty solid. Sounds like a Rallycross vehicle am I right? To explain better you need to go into detail about the pros/cons of hard and soft springing:
Yes the car resembles quite some features with Rallycross. It's actually called autocross over here but that's often confused with the tarmac-events that are more common in the US so I tend to avoid that term.
The class is a competition for fully custom built tube-frame chassis, racing on dirt-tracks throughout the country. Below is some short footage of what we usually drive (and don't mind the unusual spectators ).
Our car is the white one, with one of our colleagues demonstrating why we took the wider line in that corner.
Tim.Wright wrote:
[*]Objectively less roll will mean your outer, dominant tyre, will have an inclination angle more tilted towards the turn centre which for racing tyres typically (not always) means it will have more grip. I know you said you are happy with your roll kinematics but more camber on the outer wheel is almost always an advantage until you run into durability or braking stability problems. How much camber gain are you running out of interest? Also, if you roll centres are high (100mm) I can almost guarantee that your outer wheel camber isn’t what you think it is mid-corner .
Camber again, off course nonlineair but roughly 1.1 - 1.5 deg recovery for 2deg roll. Static setup around -0.5 -1.7 (asymm.). Castor -7.
RC are quite high;
roll centre (kinematic) front ~130-140mm, rear ~250 (I think? with my trailing arm level RC is at wheel centre height right?)
I try digging through the photo's taken mid-corner after we get home. That is by no means an accurate measurement, but it does at least give an indication if my wheels are neg/pos cambered mid turn. They seem to keep slightly negative, with our competitors outer wheels often quite positive.
Tim.Wright wrote:
So basically, based on your information, the fact that you are slower on the flatter tracks is likey because the 3 main disadvantages of stiff setups aren’t as present on the flatter tracks. Or in other words the advantages of stiff springs outweigh their disadvantages.
One remedy would be to try and exploit the advantages of a stiff suspension by running the car as low as possible and stiffening the suspension accordingly. I think also that your trailing arm rear suspension might be hold you back here. Why did you choose this type of axle? Rules?
Yes the rear axle choice was mostly dictated by the rulebook, as is the FWD layout. Front suspension style has no limits (more on that later).
Tim.Wright wrote:
I suspect you might be saturating the front axle in order to render the car stable due to a weak rear axle architecture. It would explain your mid corner US and traction problems.
What exactly do you mean by this?
Jersey Tom wrote:
There's a lot to balancing "mechanical grip" of the tires versus aero platform. Different tires can want different things at different tracks. And I'd say it's unrealistic to think that - starting from scratch - you're going to come up with the answer through some elaborate analysis in advance. Come up with a few different package options, try them at the track, see what works, and take notes on it.
I still maintain that keeping a good notebook is one of the most important things for any good racing or vehicle dynamics engineer. There are just far more unknowns or things you won't have elegant closed-form solutions to.
Fully agree, although for someone at the amateur level, sometimes the hardest part is trying to see the right pattern or draw the right conclusions from the data that's available . We do keep setup sheets and driver feedback forms from every heat, along with some GPS, gyro and ECU data that comes from our AIM datalogger.
PhillipM wrote:
Given that you're having issues at specific high grip tracks I'd be looking at whether you're under/overheating the tyres first anyway.
Now this quote caught my eye as overheating is something we should be able to deduce through some testing. Yesterday we raced the track that's by far the smoothest/slippery of the season, and where our problems are at their worst. This is a picture from yesterday:
Ironically, the tracks that are this smooth, tend to get more slippery and actually generate less grip than we do at a more "natural" dirt/gravel surface like the one in the youtube clip even when we switch from WRC tyres to rallycross/semi-slick tyres.
We first put on our soft compound tyre and looked at grip levels from lap to lap. See screenshot below. I'm undecided if the lateral acc shows any decisive pattern, but the long acc does show a downward trend in straight line acceleration:
(car slightly U/S)
After this heat we made no changes to the setup and put on a medium compound tyre. The acc. data to me looks more stable from beginning to end of stint, but overall grip levels are lower than the softer compound. The car understeered a lot (mid and exit) and again we had no traction.
For reference: at 90% of the tracks, once we get our setup right, we get consistent readings of 1.10-1.15lat, 0.47-0.50 long.
Additional info:
Now that I'm reading my original post again, I see I've left out a crucial bit of info, and furthermore yesterday I've noticed something else:
1. We're the only front-running team using a double-wishbone suspension at the front. All the others have kept to mcpherson struts. Rear axle is trailing arm for everyone. This is -probably- the reason why we see a big difference in wheel rates, others trying to counter body roll and positive camber gain while that's less concern for us.
Any comments on this left-out piece of info?
2. This might be an important clue that I've only thought of yesterday:
-We run zero ackermann (parallel steering).
-Most of our colleagues run OEM layouts, somewhere close to 100% ackermann
The reason we did this; when driving loose surfaces the drivers use part-throttle after initial turn in to allow the tyres to dig in and use it to "vector-steer" and help the car turn. This requires some finesse, as 0 throttle = understeer, part throttle = balanced and good steering respons, too much throttle = the usual FWD power-on understeer. I figured having the tyres vector-steering in parallel is more beneficial than having them work against eachother.
However, seeing that the above track looks more like tarmac than it looks like dirt, and the tyre threads don't have anything to dig into or spray away, perhaps the lack of toe-out is causing us concerns?
The driver tested with delaying his throttle application till corner exit to avoid power-on U/S but this didn't change much.
I'll check how much pro-ackermann I can dial back into the car to test with, but could this be part of the problem?
For a trailing arm the roll centre is more or less at ground level.
By saturating the front axle I mean you may have too much load transfer and it's causing the front tyres to saturate (slide) earlier.
Have you tried to calculate your lateral load transfer distribution? With a FWD car you want as much load transfer on the rear axle practical in order to keep the front wheels as equally loaded as possible.
Jersey Tom wrote:I'd mirror some other comments that to target "critical damping" and then just leave things be is going to be leaving performance on the table.
There's a lot to balancing "mechanical grip" of the tires versus aero platform. Different tires can want different things at different tracks. And I'd say it's unrealistic to think that - starting from scratch - you're going to come up with the answer through some elaborate analysis in advance. Come up with a few different package options, try them at the track, see what works, and take notes on it.
I still maintain that keeping a good notebook is one of the most important things for any good racing or vehicle dynamics engineer. There are just far more unknowns or things you won't have elegant closed-form solutions to.
Anyway, anything thing to think about as far as springs and such are concerned... I'd say you will hear people say that more stiffly sprung vehicles are overall more responsive, and very soft ones can be a bit lazy. And I would say this is true to an extent.
The limit is effectively a kart, or rigid bicycle model, or whatever. But even then, response isn't instant, it's limited by your tires and the inertia of your vehicle. Adding the dynamics of a sprung platform to the picture is like adding a spring in series. So if on a road car setup you went from 100 lbf/in springs, to 1000, to 10000, there's a limit to where getting more and more stiff does nothing for yaw response...
...unless of course you have a downforce car, in which case the more you pile on the better!
I just want to add in, do not forget that you are also tailoring to a human driver. There are some things that might be mechanically optimal that a human cannot adapt easily to. There is a big variable in the system with that.
JT, have you ever seen any data on the durability of tires on an unspung vs sprung vehicle? I have been curious about this based on some recent projects that are pushing some tire durability limits with aero. We have established some basic curve in terms of the aero loading they can endure but it falls off rapidly when you are into the bump stops for a while down the straight at Fuji.
gixxer_drew wrote:JT, have you ever seen any data on the durability of tires on an unspung vs sprung vehicle? I have been curious about this based on some recent projects that are pushing some tire durability limits with aero. We have established some basic curve in terms of the aero loading they can endure but it falls off rapidly when you are into the bump stops for a while down the straight at Fuji.
So are you asking durability vs. overall steady state loading vs. more dynamic content? I haven't seen any 'hard' data on that, no.
Typically tire durability machines aren't very dynamic creatures - slow actuation when doing track or duty cycle replays. There are a handful of flat track F&M testing rigs capable of much higher dynamic rates - but I don't know how often those are used for durability testing. I'd think rarely if ever.
Tire endurance is just a tricky beast with dependency on load, camber, starting pressures, build rates, wear rates, etc. And as you allude to - can be very nonlinear. Once you pass a certain threshold, tire life just falls off a cliff.
Grip is a four letter word. All opinions are my own and not those of current or previous employers.
Damping force can change the amplitude of the vertical load on the wheel and damping force can change vertical load at the moment of compression or rebound.
When the car is driving on a road bump, the response time of the car changes with an increase in transmissibility?
Damping force can change the amplitude of the vertical load on the wheel and damping force can change vertical load at the moment of compression or rebound.
I guess both statements might be correct, but neither is an illuminating way to think.
Greg Locock suggested in another place that you should read a text book, & he mentioned the Millikens' book. Good advice...
If we study the transmissibility ratios for different frequencies, do we need to know only the resonant frequencies? I think that for fast shock absorbers we need to know the transmisibility ratios in the high frequency range.
And if we have the vibration frequency of the sprung mass on one of the wheels close to the natural frequency, then we increase the spring stiffness or soften shock absorbers to increase the natural frequency and move the zone of high values of the transmissibility ratio. Or we increase the stiffness of the shock absorbers to reduce the transmissibility ratio at the natural frequency. To reduce the transmissibility ratio in the high frequency region, the value of the fast shock absorbers must be reduced.
A few years ago, one of my customers convinced himself that "best transmissibility" was obtained with zero damping.
It was not a good damping policy, I'm afraid....
What is the motion ratio of your front suspension, I imagine the wishbones reduce the wheel rate in relation to the spring rate compared to Mcphereson struts?
.
).
.
