Suspension kinematics...

[Jersey Tom]

not what YOU want ,but what the tyre wants !

No.. what YOU want. The tire doesn't care. Peak lateral force is only one small piece of critical tire data. For that matter, there are definitely times when designing just for peak lateral grip will cost you dearly. If you're doing serious design you have to ask yourself questions like.. what kind of on- and off-center understeer buildup you want.. how much do you want the balance to change on- and off-throttle.. what kind of tire rate gains.. what yaw natural frequency range is acceptable? How much phase lag between steering input and any number of outputs?

You're the engineer. YOU decide what handling and performance characteristics will work best for you. Selecting the right tire, kinematic package, springs and dampers to meet YOUR requirements all play in to this.

so to stick on a different type of tyre should never ever result in better performance as the kinematics should bee not optised for this tyre.
If sticking on a different tyre does indeed result in better performance ,your old
tyre was either complete crap or you ran with wrong kinematics ,setup,weight distribution,etc.

Doesn't necessarily mean the old tire was junk. Tires are improved over time. Sometimes there's no combination of kinematics at your disposal that will fit your needs. NASCAR is a good example, running left- and right-side tires, while the front and rear suspension types are radically different.

Jersey Tom: I've never seen tire data personally. Is there any way you can qualitatively describe what generally changes between different tires? Obviously, different tire contructions would produce different results, but do the different compounds, of the same construction, drastically shift the peaks on the various plots or is it usually just scaled, or both?

Changing something as simple as tread compound will definitely change the peak grip level, response, peak location, etc.

Good example on a FSAE car is to drive a Goodyear D2691, D2692, and D2696 back to back. Peak grip levels will vary, but the car will handle with night and day differences in stability and response. All 3 tires are just different compounds.

[PlatinumZealot]

Ok..thanks for the replies..

So suspension kinematics are to best position the wheels and the suspended car.. I get the gist of that answer. I am still left clueless in the not so much finer details though.

Street cars for example. From my limited experience each car can use many tyres within a certain size range. And you wouldn't normally change the suspension layout unless you change to some outlandish 12 inch wide tyres or something (camber plates, spacers etc). Still that same suspension was designed to meet certain movement goals with a predetermined range of tyres, If you get what I am saying.

I have a BMW E30 right... it has rear swing arm and front Mcpherson struts and when I look under the E90 3series.. I see a multilink in the rear and a mcpherson strut with double linkage control arms in the front. The E30 and the E90 can use the same tyres. Naturally, I know the newer E90 has finer control of the suspension movement just by driving it. So I can say the engineers had an Idea of what was the optimal/preferred movement and they worked towards it in the BMW E90.

Still, I don't know what that desirable movement is but I know there is must be an intent and drive to come close to that desired movement. That is why I asked here, to find out what that movement should cause.

I hope I am not annoying.. but lemme rephrase..

Generally, what are the best movement characteristics of a great suspension design in a road race car setting like Lemans or Formula 1?

(Front wheel movent, Rear wheel movement, body roll motion, dive motion, etc just in light detail since I wasn't to specific with my question) Thanks.

[marcush.]

hen or egg ...?

I guess we both want to express the same and my words were aimed at having the unknown and misconceptions (maybe non realistic simulations and testresults) ruled out.

I´d go back on my post and say ,the best kinematics is the one giving you the best lap time with your driver in a)qualytrim over a single lap and b)best average lap time in given circumstances-say track,temps etc).

[Jersey Tom]

Generally, what are the best movement characteristics of a great suspension design in a road race car setting like Lemans or Formula 1?

(Front wheel movent, Rear wheel movement, body roll motion, dive motion, etc just in light detail since I wasn't to specific with my question) Thanks.

I'll add at this point that kinematic suspension design also entails controlling shock movement, load paths, and aerodynamics.

There is no "general" answer to your question though. Look at the F1 grid today, there's a variety of suspension arrangements. Look at F1 in the past 5, 10, 15 years.. there have been an assortment of different trends.

Generally I'd say you want to avoid roll- and bump-steer as much as possible, but even then there may be times that's not the case. Maybe it's possible to dial enough enough roll-steer on the rear suspension to minimize body side-slip angle for downforce retention on an open wheeler. Maybe on a tin top there's advantage the opposite way, promoting high sideslip angle for high aero sideforce.

Generally though, kinematics of a SLA suspension (like that on a F1 car) works against you. It's far from "optimal." As the chassis goes into roll, the tires roll away from the turn. Generally speaking (but not always), some amount of camber into the turn is desired as it tends to produce higher peak grip levels.

On the front you can mitigate this with caster, though you run into a limit of how much you can offset the spindle axis to keep steering efforts reasonable. You also have no such option in the rear.

You can put heaps of negative camber in the car, but while that helps the outside tire in a turn, it works against the inside tire. It also generally isn't good for tire life. You can run a very short swing arm length and have no camber change in roll, but then you have monstrous camber change in bump and pitch. There's very little in terms of free performance. Almost everything comes down to a trade-off and finding a balance.

Although books like 'Tune to Win' and 'Race Car Vehicle Dynamics' have rules of thumb to go by.. I avoid them. Each chassis, each tire set, and each track you race is specific enough that I do it as a case-by-case thing. But then again I also avoid designing at the component level initially.. I'm a much bigger proponent of taking a "parametrized" approach and looking at a variety of potential camber curves first before doing too much of the real component level layout.

Suspension design is really a blank canvas. To put it in perspective you might as well be asking, "What's the best way to paint a picture?" Or, "What's the best car?" , "What's the best chicken recipe?" , "Who's the best sports player?" It's all open to interpretation.

[marcush.]

absolutely

but speaking of bumpsteer etc ...
You have to compromise everywhere ,thats the point.Those guys who compromise on the non important things and do almost nol compromise on the deciding factors will be in front ,simple as that.Just which are those deciding factors is the tricky bit to fathom. surely in a touring car you would weigh up things a lot different.
With no suspension movement to speak of in F1 + ultra low CG,things like camber curves ,scrub,bump steer ,caster,swingarmlength,roll have no real MAJOR influence on vehiocle performance.
But imagine a damper which could build up maximum force in 0.1mm travel...and then having 0.2mm slack in the system as you add up 5 bearings and bellcrankc transmitting the force from damper to the rim....adding flex and give thru installation stiffness and a weak torsional rigidity creeping in from a tub wich contain nomex weave taking up water after that torrential rain in one of those testdays...


btw ...I very often heard about Lap time calculation for fuel load added...with the fuel tank sitting so low in the car ...isn´t it rather helpingh CG height fillingup the car ?It will take quite a few gallons of fuel till you reach CG height ,don´t you?.So up to there ,all performance loss is only worse power to weight ratio (inertia)minus lowering CG....
minus lowering CG height....