The Effects of CG Location on Steady-state cornering

[Red coupe]

"A neutral car is best for steady-state cornering. by tuning cars with a range of CG postions near the center of the car (and same tires front and rear) can be made to be neutral. If the CG is forward, the Lighly loaded read end will stick better then the front(because of tire load sesitivity)..."from race car vehicle dynamics by Milliken and Milliken

can some one please explain this to me? doent a tires ability to grip increase with load?

[maxnix]

"A neutral car is best for steady-state cornering. by tuning cars with a range of CG postions near the center of the car (and same tires front and rear) can be made to be neutral. If the CG is forward, the Lighly loaded read end will stick better then the front(because of tire load sesitivity)..."from race car vehicle dynamics by Milliken and Milliken

Can some one please explain this to me? doesn't a tire's ability to grip increase with load?

Not in an overloaded situation. The overloaded tires will wash out (lose "grip").

In the example provided above, assuming the rear tires are within their dynamic load design parameters, the rear tires will have more grip.

[Q45tech]

The entire slope of the tires friction [stickson vs slip angle] vs load is not linear at the bottom 15% nor above some point say 73% of max load.

Also friction is speed sensitive and of course temperature sensitive.

Vehicle manufacturers usually use tires whose static load is 75-80% [90 Q 76%] of max load so front tires having the greatest static load are always in the non linear region whereas the lighter static loaded rears are at 65% fully in the linear region. The rear 170 pound lower weight per tire [depending on fuel level] is 15%.

The proper solution is to find front tires with a higher load rating than the rear yet not decrease the rear reserve................a 99-100V front is ideal yet even the 235/60/15 [or 255/50/16] Michelin is only 1620 [98H] [71.5% static to reserve].

Big BMW and Mercedes use 100-101 [1700-1800 + pound] front tires.

Having the front wash out first in a curve is part of the self protective understeer design. Natural for untrained driver to just keep turning steering wheel more in a curve as the front doesn't track.

Now since a front tires can get uploaded by 350-400 extra pounds in a turn and no tire in existance can fit and accept a 1600 pound load in its linear region 1600 x 1.3= ~~2080 pounds max load in a V rated.

Think how the problem get so much worse on 5800 pound SUV where static load maybe 1500-1700 pounds on each front tire and due to high C of G and track limitations the weight shift might be 700 pounds needing a 3000 pound plus max load tire.

On a car every pound of weight lost up front could yield 1.0 to 1.35 pounds of increased traction in a serious curve at least till the tire gets down to linear range loads.

[Red coupe]

Thanks, makes a little more sence. And people wonder why good tires cost so much...

[Q45tech]

I have seen a 27" Michelin on a 16" wheel used on a 90Q front........just a tiny bit of scrubbing at full lock.......this opens up another higher load index range for the Q.

A 235/60/16 ~~27" has a 99 load index or 1709 pounds ......188 pounds or 12.3% more reserve or a 255/50/16 Pilot Sport 100Y 1764 pounds ~~ 243 or 16%.

Put the 255 PS on the front and it will be a totally different vehicle.

Sorry but I only think about Q45 tires but you can extrapolate

Red coupe.....G35 has 950 static on front.........needs a ~~1600 pound load rated front tire to stay in linear range under max front cornering.

[Nismo_Freak]

2780 lbs. w/ driver, full fuel

94 (1477) - Front98 (1653) - Rear

99.4" wheelbase~65" track3.5" - 4" road clearance400% greater spring rate front300% greater spring rate rearHigh Grip Street Compound

Given your peak of 400# upload to the loaded outer tire I get 1095 lbs. of overall tire load (1078 lbs. being the 73% boundry).

However with the much stiffer suspension I'd more than likely not see 400 lbs. of weight shift? I mean 400 lbs. in a 4000 lb. Q45 is 10% of the vehicles weight, whereas 400 lbs. in my 240 is 14% of my vehicles weight. Granted weight shift is all relative to the rate of accel/deaccel, wheelbase, CoG, etc. Under braking I could easily transverse that much weight to the front provided enough traction, but I can't see it under a cornering condition.

This is using the following tires:

255/40/17275/40/17

OEM: 205/55/16 - 91 load rating.

Am I correct in my tire choice?

[Q45tech]

Spring stiffness doesn't affect weight transfer: only CG/track width = transfer ratio x sprung weight. Where did you get ~~65" of track width.

But in a way you are correct, that a 2,000 lb car with the same CG and track will transfer only half the weight amount ~~200 pounds.

Stock these car have downsized tires also why a 3000 pound load car out handles a 4,000 pound one.

I calculate 1340# as the minimum front tire load index to stay in linear region at 1.0 G...........in theory you should be able to do ~~1.1 G if everything is set up correctly with your current front tires if they have enough friction [soft compound].........

You have a 57-61%* [plus sway bars?] spring ratio, is 57-61% of the total weight on the front. Match the spring ratio exactly with the weight ratio then use sway bars to tweek the front for just a tiny under steer.

300/400% leaves me to guess real spring rates. What is the sidewall lateral stiffness of the front tires you chose? As with your stiffer springs you are now closer to 40% of the tires stiffness instead of 10% so special calculations required............two springs in series!

[Nismo_Freak]

Spring stiffness doesn't affect weight transfer: only CG/track width = transfer ratio x sprung weight. Where did you get ~~65" of track width.

In a way springs can affect the amount of weight distribution by the dive angle of the car. If you roll the car over further then gravity will do it's part with anything that isn't tied down, like the 60 lbs. of fuel in the back, the 200 lb. driver, and a number of other factors.

65" was just a guess, I don't know the actual track of the vehicle so I took 4" off the width lol. More than likely something like 60" track between the two centerlines of the tires. Either way it's at least 3" wider in the front than stock.

But in a way you are correct, that a 2,000 lb car with the same CG and track will transfer only half the weight amount ~~200 pounds.

Good to know I'm on the same page there.

I calculate 1340# as the minimum front tire load index to stay in linear region at 1.0 G...........in theory you should be able to do ~~1.1 G if everything is set up correctly with your current front tires if they have enough friction [soft compound].........

220 treadwear low silica content compound, it might be good for 1.1G but that's pushing it in my opinion. I need to figure in tread deflection and operational temp into the dynamic scheme of things.

You have a 57-61%* [plus sway bars?] spring ratio, is 57-61% of the total weight on the front. Match the spring ratio exactly with the weight ratio then use sway bars to tweek the front for just a tiny under steer.

Front:(6 kgf/mm) 336 lb. linear rate175mm spring length60mm spring diameter (I believe)

Rear:(5 kgf/mm) 280 lb. linear rate250mm spring length60mm spring diameter (I believe)

OEM is ballparked around 2.0 kgf/mm F - 2.0 kgf/mm R ... I think.

1 kgf/mm = 56 lbs. / in.

Sway Bars:Front 27.2mm Rear 15.9mm

Both are hollow bars I believe... they are OEM 240SX SE bars.

Weight distribution is about 55% : 45% if you take load placement... static resting is 50% : 50% via Longacre Scales by cornerweighting the car.

300/400% leaves me to guess real spring rates. What is the sidewall lateral stiffness of the front tires you chose? As with your stiffer springs you are now closer to 40% of the tires stiffness instead of 10% so special calculations required............two springs in series!

The tires don't have a sidewall insert like the Falken Azenis ST215 tire... this shaves about 3-4 lbs. of weight off the tire, but as a result has a softer sidewall. I'd rate them moderatly hard as far as street tires go, but I have no hard numbers on them.

[Q45tech]

336 pound vs 112 oem front springs means that for an equal G loading the new spring would compress 1/3rd. Since the front sway bar was ideal with oem springs we would guess at something like 100-120 lb/in after all the bushing slack is taken up..........in the first inch or two.Since the car now rolls 2/3 rds LESS the front bar probably adds little! [336 from springs maybe 40 from bar]As I said the new springs are doing almost all the roll restriction workSame with the rear, the oem 15.9 mm rear bar never added more than 12-15 pounds per inch due to rubber bushings. Stock it was 10% now less than 5% of total rear stiffness.

You need a 20-21-22mm rear bar [40-60-80 lb/inch] to increase the rear roll stiffness so that the ratio is 55% front 45% rear when the front bar is considered [it is 100-120 so rear needs 50-60 not 12-15 as you have now].

Obviously bar coupling in front and rear can vary why they make adjustable bars unfortunately they come in too BROAD an adjustment range...........find a 21 mm and try it.

Notice that most aftermarket rear bars are in this range because the right numbers are the right numbers.

My 20mm rear bar only adds 40 to the 120>200 progressive rear springs.......25% under easy manuevers and 16% in the extreme.......ideally I would want a 22 mm [custom made] providing 40% >29%..........but the rear would never roll enough so really 40%> 33%.

Now some racers use a rear bar equal the the spring rates but this causes problems since shocks are never designed to cover the bar varying rate and whip saw side to side transitions are not slowed down enough by shocks and going straight is way way over damped. Smart people limit sway bars to 50% of springs on the rear and add only 20% to the calculated amount need by new springs!

You have stiff springs which need stiff shocks to control the rebound!

[Nismo_Freak]

Indeed... my next purchase in suspension was going to be sway bars. We just got a bunch of the Whiteline bars in and they are nice, and adequately sized (no 30mm rear bar diameter crap ).

Multiway blade adjustable, which I will use to fine tune the inherant lack of rear pivot that my current suspension setup should net me.

BTW, my endlinks are 100+k rubber... so we can just factor out the rear sway bar due to slop ahhaha. However spy on these beauties that will adorn my S14 once I get around to buying the sways.