Tire lateral properties 2

[sierra4000]

Hello ,
is possible (helpful) to measure and compare the tire lateral stiffness in garage conditions?

Thank You for ideas and opinions

Radek

 

[sierra4000]

"Rim increase normally does increase tire Mz stiffness but that's all gone near max lat."
that seems like good news for me
because just when car approach max lat. then transitioned to strong understeer ....................but ONLY when steering angles are low (vice versa in slow corners with more steering angles seems correct)

on low lat.g range and transitionals is perfect
is almost parallel steering
stiffer front bar or springs create more understeer
56% front weight
cannot get front more camber gain with McPherson
maybe again more caster will help (is 8,5 now)

 

[gruntguru]

Extra caster won't help much when steering angles are low. More static neg camber will probably help but at some detriment to tyre life and braking performance.

je suis charlie

 

[NormPeterson]

cannot get front more camber gain with McPherson

What about taller ball joints or relocated chassis-side pickup points (if class-legal)? Either way would involve a bumpsteer kit.


Norm

 

[cibachrome]

Even if the car rolled 10 degrees per g and you jacked the front 'bump steer' (i.e. steer by roll) up to 10%, the lateral mu-slip curve for both tires as a pair is, or should be, already flatten out. Shoving another degree of steer angle at the front end won't/can't build any more force.

A video of the INSIDE tire at max lat may show you that it's actually working against you. That's a static toe or Ackermann issue. But you'll pay for it in other ways.

Remove some heavy stuff from the chassis, motor, bumper, or driver. At max lat, the tires only listen to Fz, camber, and temperature for a fixed pressure and wheel size family. [ Bleeders allowed ??? ].

If your trick book is already used up to change the front, you'll have to loosen the rear and live with the extra pain that goes along with it. It WILL re-balance the car but introduce some other bad habits (like lessening 'social distancing' from other cars, walls, and barriers.)!

 

[sierra4000]

Is done Norm,
other front camber gain is limited with RCH i think
probably i can get little more from damper inclination
so maybe more KPI can help but this works against caster
for example extra+4 Dg KPI and extra+2 Dg caster

Yes Cibachrome,
probably almost all front ideas already are exhausted,
(bumpsteer,ackerman,static toe,toe stiffness,camber stiffness)
and car always was little better
remains "tire tuning"

in the past i tried get balance with loosen rear end with rear toe tuning, but although car feeld balanced over steady state in fact car slown down due to braking instabilty and corner entrys
(car is hill climber so stability seems like priority before steady state)

 

[cibachrome]

Get some hydraulic rear bushings and plum them into your rear brake line. They will solidify when pressure loads them up. A little pedal hysteresis, but get used to it. In the front, use steering assist pressure (need hydraulic power steering) to do the same thing. Solid rack mounts, lower steer ratio (less compliance from lower rack tooth separation forces, better wheel bearings, less caster (lower net tierod force), if your steering wheel returnability is then poor, put a pair of coil springs on the column to force the steering wheel to zero heading. Then get used to steering wheel torque falling off. A glass of wine on the dash will help you gauge the g-level better. Only drink 1/2 of it. (Is the glass half full or half empty ?) !

 

[sierra4000]

Excelent!!
you may not believe me, but I am had quite similar idea, but you gave me determination and direction in it!
I have to think how to get on semitrailing arm..........maybe extra toe link?
yes, steering rack is solid mount already, new wheel bearing units, stiff alloy knuckles (compliance improvement is significant improvement and measurable half value)
but really i love my high caster feel, although i know you reasons

if you are interest here my compliance tool in action

https://www.youtube.com/watch?v=3LPVXFKCMkk

" if your steering wheel returnability is then poor, put a pair of coil springs on the column to force the steering wheel to zero heading."
sorry i do not understand

 

[sierra4000]

oh,i understand,selfcentering system

i think i can get some extra toe due to rear crossmember controlled bending by brake line
(1mm crossmember center deflection = cca +2mm extra total toe-in)
in fact i think this part deforms anyway when laterally loaded

 

[cibachrome]

What is the caster setting of the rear suspension. You might have to disconnect some parts to let toe free to steer to do this on an aligner, but changing it from negative caster to positive will add some lateral force (oversteer) to you rear axle mechanism. Better than any aligning moment compliance influence.

BTW: have you looked at shaved tires (on the front only)? That will increase your grip. Your best bet is still to get on a wheel alignment machine with a digital steer and camber readout Hunter makes one) and then apply a lateral force (in phase) couple and and aligning moment couple (anti-phase) inputs. Also you can pull the car down and read out the ride-toe (steer and camber) characteristics. Somebody around there must have a lawn mower with hydraulic taps coming out for attachment controls. Even a snow plow truck setup. Then you can use the hydraulic cylinder to active the inputs. A pressure gauge can give you a ballpark force level if you know the cylinder push and pull diameter(s). You can make use of this apparatus to tell "better" or "worse" for part changes without worrying about absolute force readings.

Necessity is the Mother of Invention !!!

 

[gruntguru]

I liked your video. Unfortunately everything is moving on that side of the car. Do you have a video of the other side?

Mounting those tyres to a wider rim might reduce lateral compliance. Might increase rear grip too which is a good thing but won't help your understeer problem.

I couldn't see a rear ARB in that video?

je suis charlie

 

[cibachrome]

Put 90 psi in the tire and repeat, but now with the camera on the control arm. You should then be able to see what the rear problem is here. Try to get a view from the front or rear.

 

[cibachrome]

And in case you are wondering, although not exactly the right evaluation loads, I probably would not use Kz to gauge Ky. THis is jest a few (350) Continental brand, a lot of constructions and pressures.

 

[GregLocock]

What's a negative lateral compliance?

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[sierra4000]

Cibachrome
Unfortunatelly no exist rear caster............ is semitrailing axle
traing arms with low angle axis rotation (low camber gain and toe changes,low RCH)
How way shaved tires increase grip?
With my tool i know loaded contact patch quite accurate and repetable,deflection read at rim against body movement

on Video is not my car , is BMW E30 my friend car (almost stock setup)
noticable bearing deflection, found arm and crossmember significant bending(camber loss and toe-out under loading)

Gruntguru
no exist video of the other side, for reading is body movement subtracted
on my car is fully adjustable ARB blade style...............this BMW suffers sudden oversteer

Greg
this BMW have +0,5Dg/kN camber and -0,1Dg/kN toe loss
now after arm and crossmember reinorcement +0,3 camber and -0,05 toe

my car (Ford Sierra) currently +0,2 camber and -0,015 toe

 

[sierra4000]

some cars rear camber lateral stiffness
new tool = load to contact patch, old tool=load to wheelcenter

 

[cibachrome]

Plus and minus slip angle effect. Sorry, this calculation was a kluge/hack on my regular tire sniffer. But you can see an asymmetry. These are all tested as right front tires (rotation and dressed sidewall facing out.

 

[GregLocock]

I just checked. Our lateral compliance model is talking about the change of overturning moment with lateral force, and it does go negative for 4 tires that I have plots for. That is, the effective centre of vertical force at the CP can move outwards as an inward force is applied, at low vertical force loadings. At higher loads it goes the way that a simple structural model would expect, ie positive. maybe the lateral force is pushing the sidewall under the rim, so more of the vertical load is taken in the sidewall rather than pneumatically at low loads.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[cibachrome]

Well we both learned something today. Good investigation Greg. Nice to see somebody able to check out these details. I actually never gave it a thought. Sorry I can no longer call up the F.E.M. guys and ask what the deformation pattern sequence is.!

As for the compliance, All the Porsche's I've ever seen were at the 0.12 --> 0.14 range at 1 kN. Other cars in the Honda class bucket (0.15) are Opel Zafira, Ferrari F-430, Lexus IS-350, and Suzuki Grand Vitara.

If I were you I'g go for a 911. Guys are falling all over the results from these Porsches on a few forums. They use a bunch or roll steer at each end (15% FR, 6% RR) apparently to give the cars some manners, but you know what the cars are notorious for when the tires go blind to steer: "Another Brick in the Wall". (BTW The Brit Floyd Live version is so much better than the OEM version. And NOT the REM version). If they did computer modeling, would they be the OEM FEM REM gems ?

Ya know, Claude bought a (used but cared for) BLACK 911. If you could latch on to a RED one, you'd be faster. Red cars are ALWAYS faster.

 

[gruntguru]

I just checked. Our lateral compliance model is talking about the change of overturning moment with lateral force, and it does go negative for 4 tires that I have plots for. That is, the effective centre of vertical force at the CP can move outwards as an inward force is applied, at low vertical force loadings. . . . . maybe the lateral force is pushing the sidewall under the rim, so more of the vertical load is taken in the sidewall rather than pneumatically at low loads.

I would be interested to know if that behaviour reduces with increasing rim width.

je suis charlie

 

[cibachrome]

Judge for yourself. I believe its all construction based. When tires are designed or 'created', a specific rim width and pressure are the basis parameters for which tire stiffness, traction, grip, noise, rolling resistance, and noise are optimized.

Rare to find a single construction on 4 rim widths. Mz vs. Fy stiffness comparison on multiple rims is more 'analytic'.