chishifu

iTrader: (5)

Hi, all this made lots of sense until the part with lowering front tire pressure or raising rear tire pressure will decrease oversteer. You said in the prior sentence that softening rates in the front or stiffening rates in the rear will induce greater relative adhesion in the front, which contradicts, a lower front tire pressure decreasing oversteer. A lower front tire pressure basically equates to a lower front wheel rate which should give the car more grip in the front end... thus increasing oversteer. Am I wrong in thinking this way?

So looking at tire pressure affecting wheel rate, lower tire pressure = softer wheel rate, which means lower relative tire pressure in the front should grip better and cure understeer correct? or no...

Sorry to keep asking the same question over and over again... just as a mechanical engineering student and a FSAE member, it's not making sense. About the lower car handling better, weight transfer is a function of lateral G's and track width. Thus lowering the car decreases the size of the moment arm and the amount of weight transferred which helps overall traction.

*edit: by lift forces do you mean normal forces?

gunslinger

Hi,

Sorry for the confusion, just a brain f*rt on my part; intermixing over- and under- steer in my sentences. It's what I get for typing between meetings. You are correct and I edited my remarks to show this.

Yes, lowering the car reduces the center of gravity and weight transfer is proportional to the height of the car. Therefore the weight transfer won't be as drastic as those with a higher car and the moment arm, or torque, generated while accelerating (in any direction) will be less.

And lift forces are the normal forces exerted by the ground.

Beau

iTrader: (4)

Can anybody comment on how lowering a car affects the suspension geometry and the roll centers?

Back in my Mustang days I recall that lowering the car pretty much made poor suspension geometry horrible because the roll center in the front would be pushed about 6ft under ground when lowered X inches.

I've been curious about this topic with Z's for awhile.

chishifu

iTrader: (5)

Thanks so much... this all actually makes sense to me. But those links in one of the previous posts state just the opposite... any ideas to that one?

NGZ

iTrader: (1)

Was there an answer to this question?

NGZ

iTrader: (1)

I guess here's a related question - is the purpose of "increasing front tire pressure" only to increase the load capacity of the tire? I.e., when considering the tire's adhesive capacity while cornering, all this setting takes advantage of is increasing load capacity, rather than using the spring property of tires?

If this is the case, is adjusting tire pressure an optimization with 2 conflicting factors:
1) the spring rate of the tire: increased pressure = increased spring rate = less adhesion = A
2) the load capacity of the tire: increased pressure = more adhesion = B
But because A + B > 0, we increase the tire pressure?

PDX_Racer

iTrader: (2)

Initially increase tire pressure to prevent roll-over (sidewalls have LOUSY grip compared to tread). That gets you in the ballpark for tire pressures.

tmak26b

You lose grip as you increase in pressure, also tires don't build up as much heat with a tire that is high on pressure. You go with feel and what the pyrometer tells you. This is not 1980, you don't just go by how much you roll a tire

chishifu

iTrader: (5)

Hi, I posted that question above and the original poster explained he simply made a typo between under and oversteer. So he was agreeing with what I had made in the post.

Very interesting proposition that you made in your post between the spring effect and load capacity effect. I've read about load capacity in quite a few posts and have been wondering how it affects adhesion exactly. All it is, is how much load a tire can 'handle' from what i've read. But what happens when you exceed that load capacity and how does it affect handling? Thanks man.

black06z

iTrader: (1)

Wow!!!! you guys are having understeer problems on a RWD????weird stuff.
Anyways, to increase more oversteer, get a suspension to where you adjust dampening: Softer in the front, and harder in the rear.
Add negative camber at the front, reduce toe-in/add some toe-out, increase front tire pressures.

NGZ

iTrader: (1)

You are my hero.

NGZ

iTrader: (1)

I tried looking around for some kind of graph to see how all these various factors play together, but couldn't find anything.

What I did find though, is a similar discussion at HybridZ.org:
http://forums.hybridz.org/showthread.php?t=88751

chishifu

iTrader: (5)

again with the increase front tire pressure to get more oversteer... why????

PedalFaster

I think the mistake you're making is assuming that everyone on this thread (or any other internet forum thread) knows what they're talking about. They don't.

03BrickyardZ

+1

PDX_Racer

iTrader: (2)

+1,000,000

dovla

Because higher (within range) tire pressure reduces the slip angle (see http://en.wikipedia.org/wiki/Slip_angle) and lower pressure increases it.

Btw, I like your persistence.

Kolia

iTrader: (2)

Why would a change in slip angle be desirable?

chishifu

iTrader: (5)

Heh... yeah well, I'm not entirely sure of my own opinions about how tire pressure affects handling, which is why i've asked for an explanation of the physics so I can judge for myself how correct the information I've read is. However, what really threw me off is the prior post linking information from tirerack and rsr stating that increasing front tire pressure will reduce understeer... I still would like that explained.

Interesting... I have never heard of this type of explanation before, but again, a problem I see is that tires in general have maximum adhesion when they are at a slip angle of between 5-10 degrees. Thus at the limit, if the rears are at this maximum traction, but the fronts are at a smaller slip angle, the rears will have more grip and you will continue to have understeer.

BTW, how did you come up with this information about slip angle and tire pressures?

Thanks guys.

Kolia

iTrader: (2)

It has been explained more than once in this topic already. It's related to tire spring rate. And it assumes the tire's pressure is optimized for the setup to begin with.


Tires do generate the most grip at their slip angle limit. But I doubt in means the more slip angle you get, the more grips the tire will generate in turn...