Spring rates, weight distribution and other things
Spring rates, weight distribution and other things
I would love suggestions on suspension tuning books. Especially anything that focusses on Macpherson strut style suspension since it is used on most of the cars that I am involved with.
I do have Tune to Win and have been putting it off for far too long. I'll at least crack open the spring rates section tonight and see how far I get.
I would really like to get some input from the engineering side though on a thread that has had a major influence on the MR2 community.
I think everyone should be able to view this thread. If not I will try to copy and past key points.
This thread has a lot of good information and I believe much of it to be true. However it has resulted in many people running spring rates that heavily oppose weight distribution.
As xhead discusses in post #5 he uses the inverse of the weight distribution to choose his wheel rates.
Now the logic for doing so does make some sense in that running a stiffer rate at the lighter end should theoretically help balance out the car however it doesn't seem like it's as simple as that.
Next in post #7 he says that FWD cars run very stiff rates and bars in back and softer up front. Therefore a car with more weight bias in rear should do the opposite. I don't follow FF cars enough to know but I didn't realize this was the case. If they do run stiffer rates I would assume it would be because the overall suspension would need to account for cargo and people. Or perhaps he is talking about more racing oriented FF cars? Either way does a car with 60f/40r weight distribution running more spring in the rear automatically mean that a car with 40f/60r should do the opposite? I know there are a lot of other things to consider beside which end wants to break loose first.
In post #18 he starts to address some of these other concerns such as ride frequency, Center of suspension and center of gravity. However he seems to dismiss them as nearly irrelevant for performance and only really related to comfort. One thing that I know is very far off about his calculations is that he says the CG is near the rear axle. Just from jacking my car up many times I can tell you that the CG is much further forward than that.
Then in post #31 talks about how his builds do not use swaybars and that he does everything with spring rates. Now this leads to my real questions although I would love input on everything.
First question is if running such higher front rates to balance the car will really benefit the handling more than losses you might see by ignoring ride frequency and center of suspension?
Second question and this one really bothers me. He says that he does not run swaybars because they decrease mechanical grip and instead uses spring rates because that doesn't reduce mechanical grip. Yet earlier he discusses using spring rates to balance the understeer/oversteer tendencies of the car. So in one post he is saying he uses spring rates to change the balance of mechanical grip then in another post says he doesn't like sway bars because they reduce mechanical grip.
A given spring rate will have a given effect on mechanical grip correct?
He does at one point suggest that the downside is that the decrease in mechanical grip is not linear and that may be what causes the understeering issues but I am having a hard time understanding why.
"A swaybar transfers load from the inside tire to the outside tire and thus reduce mechanical grip as they add spring rate."
I don't believe this is technically true. At least no more true than a spring that is transferring that load through the chassis. Perhaps this is where I'm confused.
Now I can see his point about changing traction conditions changing the rate with a sway bar since the amount of roll will change before you reach the limit of traction. However I can't help but wonder how much. He is trying to keep roll under 1.5 degrees. At the shocks that is about 36mm or 1.4" difference in travel from left to right. Now if you compared racing slicks on perfect pavement and 1.5 degrees of roll to say black ice and .2 degrees of roll it is true that the sway bar will have increased the spring rate notably more on the high traction surface but if you compare reasonable traction like he is talking about like even in rain the G force is still going to be pretty substantial and the body roll shouldn't be that much less than before. So maybe it's 60% as much body roll. That would be .9 deg and at the shocks 22mm or .866 in from left to right. 36-22=14mm or .55" from left to right means a 200 lb/in sway bar would be increasing the spring rate by 110lb/in. I guess this ties into his statement above but you aren't transferring load between the inside and the outside. If you had a front swaybar and no rear you would be transfering load from the back tires to the front tires making it more likely to understeer.
On the other hand if you had a 200 lb/in rear bar as well it would maintain the balance plus or minus the variation in body roll and whatnot right?
So from an engineering standpoint what are peoples thoughts on a swaybarless setup with rates inverted front to rear?
What are peoples thoughts on the explanation and logic behind the setup?
Obviously this setup has worked very well for some very competitve racers but now I see a lot of people using this setup for all kinds of cars including daily drivers. While I could see it having it's place and if you find success with it then all the power to you but I feel like there are also some gaps in the theory and would like to understand if I am right and if so better understand what those gaps are.
Sorry for the incoherent rambling. It's hard to work through this stuff in your head and try to ask questions all at the same time.