Ratu dydis

[spade]

There is a common misconception afloat that a taller rim will always provide increased lateral grip, but typically result in a weight penalty that hurts acceleration and deceleration.

Well, this is partially true, but it also has to do with the height of the sidewall and its ability to flex enough to hook up with the pavement.

A 17" wheel with a tire sized to fit within the 5G Civic wheel well (for instance) is going to have such a short sidewall that the lack of material to flex is going to have a negative impact on aceleration and deceleration. It could have a positive effect on cornering at steady state on a smooth surface, but in the real world one rarely encounters these conditions. Typically the car is in transition 95% of the time, and road surfaces are rarely smooth. As a result the lack of sidewall will have a negative impact on cornering more than 95% of the time.

Then consider the distance from the rotational axis of a 17" vs. a 16" wheel...even if the 17" wheel was the same weight (which would be unusual, but possible if one was willing to shell out the bucks for an extremely well engineered wheel) then you still have to deal with the fact that the actual outer shell of the wheel was a half inch further from the axis of rotation all the way around. This means it requires more energy to change the velocity of the 17" wheel (either braking or accelerating).

But what about the belief that a shorter sidewall will flex less and provide better handling?

Well that is just not always true. There is a limit. It is just another iteration of a well-travelled myth people who don't know any better recite over and over again.

Yes, a 50 series tire will provide less sidewall flex than a 75 series tire. But that does not necessarily provide better handling...just a different feel, and depending on the construction of the tire carcass, the compound, and the road surface, the 75 series tire could provide better traction. Ever notice that WRC cars use a taller tire on the rough surfaces than they do on tarmac? Do you think that is because they can't afford to buy taller wheels? Or maybe they just don't have the resources to tune the suspensions to handle the rough stuff? Yeah right.

The truth is that one quickly approaches the fine point where the tire's lack of compliance actually results in less lateral grip than a taller sidewalled tire would. I contend that a 215/35-17 (the size tire that would be closest in outer diameter to the stock 175/70-13 tire - on a 5G - and would fit under the fenders of Civic) while providing less flex and a firmer ride than the 215/45-15, will not provide superior predictability or enhanced control. Firmer ride, maybe, but firmer doesn't always mean better.

What does improve handling is increasing the size of the contact patch and/or increasing the coefficient of friction between the tire and the asphalt. The latter can be done with softer tire compounds and things that happen at a molecular level as a particular tread compound breaks down due to friction at the tire/aslphalt interface...but that's not what we are talking about here.

What I am primarily talking about here is maximizing the size of the contact patch and keeping it in contact with the road.

A contact patch's shape is not a constant...it varies depending on the forces (latitudinal and longitudinal) applied to the contact patch due to the amount of torque applied to the wheel by the motor (accel and decel) and the difference between the direction the tire is pointing vs. the direction that corner of the car is travelling (a.k.a., the slip angle). Based on the compliance of the tire (and the tread blocks - this is sometimes referred to as tread squirm), these forces can allow the contact patch to deform in a number of ways that either increase or decrease the total area of tire tread that is in contact with the road surface. We seek to find the way that maximizes the amount of tread that is coming in contact with the road surface.

As a car corners, the suspension compresses on the outside and droops on the inside, which results in all sorts of variations in the orientation of the tire surface to the road (note, we are not focusing on the camber angle which is the angle of the tire relative to the chassis, but rather just the orientation of the tread relative to the road surface).

In a perfect world the outside suspension would compress and add just the right amount of negative camber to that wheel so that it remained perfectly oriented to the road surface such that the contact patch size was maximized (neither the inside nor outside edge would pull up from the road). Likewise the inside wheel would do the same thing but in the opposite direction. Unfortunately since the forces that cause the body of the car to roll, thus creating the compression and droop actions at each side of the car, are actually being applied slightly in advance of the body's ability to adjust and roll as the car is entering a corner and the suspension is yet to reach its steady state, or on corner exit in the opposite degree of compression/droop, we rarely find ourselves in that perfect world.

If one were to take the theory that less sidewall flex resulted in better handling to its extreme, then we would be seeking zero sidewall flex, right? Well, if the sidewall doesn't flex, and the suspension is not able to keep the tire tread perfectly oriented relative to the asphalt, guess what...the tread will lift and only the shoulder of the tire will remain in contact with the road surface. So now the contact patch area has decreased dramatically and you are losing traction.

If you reach and remain in steady state, then a stiffer sidewall can translate into enhanced lateral grip assuming the suspension does utilize spring/damping rates that work with the spring rate of the tire and that it provides sufficient camber correction as well as ackerman to deal with the body roll and the difference in diameters of the circles described by the inside and outside tires (BTW, there is currently no chassis that does that perfectly...certainly not our beloved Civcs which - let's face it - were designed for fetching groceries first).

But in the real world you do not remain in steady state but for a fraction of the time. Even around a skidpad one is typically making adjustments with the wheel or the throttle in an effort to keep the circumference of the circle constant, and these adjustments will cause shifts in the degree of body roll.

Now throw in a bump...let's say there is a .5"x.5" strip of wood lying across the surface in the corner. As your tire encounters this piece of wood, the whole tire lifts off the asphalt and is now resting on the wood...the contact patch has been reduced to whatever the approximate width of the contact patch was before, but only half an inch long. This will produce less lateral grip.

If you've ever experienced a "jittering" sensation when your car was cornering, then you know what it feels like when the tire and suspension is unable to comply quickly enough with variations in the cornering forces or road surface, and the jittering is the result of the tires hooking up momentarily, then losing traction, then hooking up again, etc.

Alright, so let's say we stay away from extremes...let's think about the two sizes I mentioned above (215/45-15 vs. 215/35-17). The 15" tire will have nearly an additional inch of sidewall than the 17. Take weight out of the equation if you wish, and assume that both wheels and tires are the same width filled to pressures where they have the same contact patch at rest.

Once you start applying the forces I described that are present in transitions, and once you throw a few bumps or surface irregularities into the equation, and then factor in the imperfections in the camber curve and ackerman present in the Civic suspension (basically factor in the "real world") you will find that the additional compliance present in the 15" tire will result in improved lateral grip over the 17" tire. At the very least it will provide an enhanced level of predicatbility and control.

So why do all those touring cars have tall rims and short sidewalls? I believe it is primarily due to the desire to run huge friggin' brake rotors (which are free under most touring car rules). These cars make tremendous amounts of horsepower and the drivers do the majority of their passing in late-braking maneuvers going into corners. Nearly every corner entry is a battle for who can break the latest and hardest. Larger brake rotors give them more leverage against the rotational inertia of the wheel, more pad surface, and a greater heat sink. This allows them to charge down the straight, balls to the wall, wait until they see the other guy brake out of the corner of their eye, and then jump on the binders as quickly as possible.

Ever notice how easily a touring car spins? A touch of nerfing and they break loose like tops, and its nearly impossible to catch the cars once they start to break loose. Know why? Because the sidewalls on the tires they run are so damn short that they have very little tolerance for being upset...very difficult to control and can be somewhat unpredicatable (which IIRC was your contention that short sidewalls provided). Its not because the suspensions are set up too stiff...the teams we are talking about - especially the works teams - have development resources coming out the yin-yang. Surely they could design a suspension that was capable of taking a wack from a competitor without causing the car to irreversably lose traction - hell, even NASCAR teams have figured out how to do that some of the time. Nope...the touring car builders are faced with a compromise - since the maximum rim diameters and outer tire diameters are constrained, they have to figure out how to get the most braking capacity built into the cars as possible. In exchange they sacrifice some stability and predictability.

So in conclusion, when selecting new wheels and tires for your car, keep in mind that there is a point at which a taller rim and a shorter sidewall begin to be a detriment to your performance...and not just the ability to put down the power or the extra rotational inertia or the extra unsprung weight either, but rather in the predicability of the car in transitions. Civics and CRXs are especially prone to being "over plus-sized" because of the constraints on the total size of the wheels. Don't be a lemming.

J. Brett Howell
Pebble MotorSports

[romkasponka]

nuo kada amerikonai pradejo suprasti kazka apie pakabas?

[spade]

Poziurek i Civico rotukus: http://www.pebblemotorsports.com/

[eddie_gt4]

spade'ai, ale ir iskapstei straipsni...

kaip apie didelius ratus gatvinese masinose yra pasakes vienas garsus lenku ralistas (K. Holowczyc):
"Dideli ratai yra kaip XL dydzio prezervatyvai - trumpa geros autoprezentacijos akimirka neverta velesniu problemu"

t.y. jos verta detis tik tada, kai jie is tiesu butini

[romkasponka]

Poziurek i Civico rotukus: http://www.pebblemotorsports.com/

Nu ir ka bugniniai nudazyti stabdziai

Shiaip pasiziurek su kuo berg cup vaziuoja geriau

[kwlw]

t.y. jos verta detis tik tada, kai jie is tiesu butini

Nori pasakyti fizikos desniai skiriasi gatveje ir lenktynese?

Lygiai taip pat stabdzhiai netelpa ir gatvineje ir ralineje tochkeje :/.

Sau

[spade]

Shiaip pasiziurek su kuo berg cup vaziuoja geriau

Cia mers ka apie ilgi kalbam... Paziurek F1...

[romkasponka]

Cia mers ka apie ilgi kalbam... Paziurek F1...

F1 reglamentas kitokiu neleidzia.... Kas yra gerai, nes nes nebutu sukure geru compoundu kebulam

[eddie_gt4]

t.y. jos verta detis tik tada, kai jie is tiesu butini

Nori pasakyti fizikos desniai skiriasi gatveje ir lenktynese?

Lygiai taip pat stabdzhiai netelpa ir gatvineje ir ralineje tochkeje :/.

Sau

nori pasakyti, kad gatveje ir ralyje greiciai ir apkrovos identiski/palyginami

taip sveikai ukiskai mastant: a kam raliniai stabdziai/ratai/etc. gatvinej geldoj?
jokio racionalaus paaiskinimo tam nera, nebent trumpo pasiilginimas

[fivefivefive]

nori pasakyti, kad gatveje ir ralyje greiciai ir apkrovos identiski/palyginami

taip sveikai ukiskai mastant: a kam raliniai stabdziai/ratai/etc. gatvinej geldoj?
jokio racionalaus paaiskinimo tam nera, nebent trumpo pasiilginimas

nu manau nevienas paiilgintu jei butu galimybes

[kwlw]

nori pasakyti, kad gatveje ir ralyje greiciai ir apkrovos identiski/palyginami

Kuom raliniai 200kmh skiriasi nuo gatviniu 200kmh? Ypach prisiminus, kad gatvine mashina nebutinai lengvesne uzh raline. Pavirshius, beje, tas pats.

taip sveikai ukiskai mastant: a kam raliniai stabdziai/ratai/etc. gatvinej geldoj?

Tam, kad stotu kaip raline. Jei galingumo turi kiek raline, tai ka, stoti butinai turi blogiau? Per kuria vieta chia sveikas ukishkas mastymas?

Be to feel'as vairavimo procese yra toli grazhu ne paskutineje vietoje. Taipogi su mazesniu sidewalu - lengviau ishprovokuoti ratu slydima, kas manau idomu dreiferiam ir sheip norinties saugiai pasijausti lenktyninkais .

jokio racionalaus paaiskinimo tam nera, nebent trumpo pasiilginimas

Na kas kam rupi ir i galva ateina . Man tai norisi, kad stabdytu normaliai.

Lygiai kaip nera racionalaus paaishkinimo, kam pirkti gt4, ar wrx'sa vietoj normalios, ukishkos beigi racionalios 1.6 celikos ar imprezos. Gi ne ralyje vazhiuoji...

Kad vel nemalt shudo apie ta pati, galim palikti rice bojus shone, pas kuriuos didzhiuliam rate, mazhyte stabdzhiu shaibute gyvena. Na tokius kaip BG pvz .

Sau

[eddie_gt4]

kwlw, nu tik nepasakok, kad su savo gatvine gelda vazineji tokiais pat greiciais ir apkrovom, kokiom vazineja raliniai bolidai - vis vien nepatikesiu

o jei vis tik taip vazineji, tai imho apsimokejo iskart imt atitinkama raline masina, o ne is geldos lipdyt bolida - turbinant atmosferika, dedant big brake'us, etc.

lygiai taip pat apsimoka iskart jamt, pvz., WRX'a, o ne Tavo mineta 1.6 Impreza, ir po to verkt, kad ji nevalit beigi bandyt is jos isspaust ta pati, ka turi stock'inis WRX - va Tau ir racionalus paaiskinimas del pirkimo

[romkasponka]

tarp kitko raliniai bolidai labai greit nevaziuoja, tiesiog daznai ir daug stabdo..

[spade]

jokio racionalaus paaiskinimo tam nera, nebent trumpo pasiilginimas

Jei jau naudojantis sia tavo alegorija: tu tikras, kad size doesn't matter? As tai abejociau...

[eddie_gt4]

(...) tu tikras, kad size doesn't matter? As tai abejociau...

mes cia apie ratus, ar apie ... ?

kad butu i tema, stai trumpa istrauka, susijusi su sioje temoje daznai linksniuojama Impreza (konkreciai - GC8 Preza):


"People associate big wheels with sporty cars, but the key factor is low-profile tyres - the link is that to keep the rolling circumference the same (to avoid introducing speedometer errors) then as the wheels get bigger, the tyres must get thinner. Low-profile tyres reduce the amount of spring and flex in the tyre. Less spring means reduced body roll in corners, but will make the ride somewhat harsher as bumps will not be soaked up as before. Less flex means that there is less lag between turning the wheel and the tyre following, so steering response will be more precise ... but the tyres will not mask ruts and grooves in the road, feeding back through the steering. Also, thinner tyres mean that big potholes have more chance of damaging the wheels.

Prodrive suggest that a 16" wheel offers the best compromise between looks, ride comfort, performance and cost, and indeed from the '98 model year, Imprezas have 16" as standard. However, the fact that Prodrive's demonstrators have always been fitted with 17" wheels shows that they certainly give additional performance - or better looks! ( )

The 17" wheel and tyre combination increases the unsprung weight to about 21kg, up from around 16kg. This will have a noticeable effect on both ride quality and handling, although most owners say it's not too bad and well worth it. Uprated suspension becomes more desirable with heavier wheels."

[Hassan]

eddie - o tu cituaji ka? Nes vien todel, kad tekstas angliskas - jis nera nei butinai tiesa, nei kazkoks autritetas. Juolab, kad jame labiau samprotaujama, nei kazkas teigiama. Bet as sutinku, kad oversized wheels yra blogis, telieka susitart, kas yra oversized

[Minde_Kei]

realiai nieko naujo.

tik skirtuma realiai galima pajausti lietuvos salygomis ziema. imho

pabandyti reik vaziuoti kokioj rukloj su r17 ir su r15 padangom gerom...

aisku ant asfalto skirtumai daug mazesni, bet kitaip sunku pajausti...

[kwlw]

eddie - o tu cituaji ka? Nes vien todel, kad tekstas angliskas - jis nera nei butinai tiesa, nei kazkoks autritetas. Juolab, kad jame labiau samprotaujama, nei kazkas teigiama. Bet as sutinku, kad oversized wheels yra blogis, telieka susitart, kas yra oversized

Buvau parashes posta shita tema, bet jis deja neishliko. Susitarti kas yra size, ir kas yra oversize manau yra esme. Ir kas yra pagrindinis atrankos kriterijus.

Sau

[eddie_gt4]

eddie - o tu cituaji ka? Nes vien todel, kad tekstas angliskas - jis nera nei butinai tiesa, nei kazkoks autritetas. Juolab, kad jame labiau samprotaujama, nei kazkas teigiama. Bet as sutinku, kad oversized wheels yra blogis, telieka susitart, kas yra oversized

ta citata is Subaru Impreza FAQ (tai tokia a la GC8 Imprezistu Biblija, jei ka )

kas yra oversized?
sakyciau, kad daugumoj atveju* +2 (ir kokie +20mm plocio) nuo OEM dydziu (imho butent tai ir yra "pgr. atrankos kriterijus"/atskaitos taskas) jau yra oversize

________________________
*ypac jei kalbam apie +/- stock'ine pakaba ir galia

[romkasponka]

jo, idomu kodel tada stcc vaziavo 19" tekiniais?

[VAZz]

Na taigi tas pats spade'o straipsnis, pateikia galimas priezastis
So why do all those touring cars have tall rims and short sidewalls? I believe it is primarily due to the desire to run huge friggin' brake rotors (which are free under most touring car rules)
NORS - stcc- the maximum diameter of the brake disc is 332 mm

[eddie_gt4]

jo, idomu kodel tada stcc vaziavo 19" tekiniais?

nori pasakyt, jie tiesiog vietoj, pvz., OEM'iniu R-16 ar R-17, ant stock'ines masinos tiesiog uzsimaudavo R-19 ir varydavo i gonkes?
krutai

[romkasponka]

Ka tu skaitai oemine, ten kompleksas perdarymu padarytas...

[eddie_gt4]

jo, idomu kodel tada stcc vaziavo 19" tekiniais?

nori pasakyt, jie tiesiog vietoj, pvz., OEM'iniu R-16 ar R-17, ant stock'ines masinos tiesiog uzsimaudavo R-19 ir varydavo i gonkes?
krutai

nu tai apie tai ir eina kalba...
jei masina atitinkamai modifikuota, tai vaziuojant tam tikrais rezimais (pvz., ziedas) yra tolkas det bile kokius volus ir placias plonas gumas...

as sios temos message'a matyciau taip: isivaizdavimas, kad didesnio diametro (ir platesniu) ratu uzmetimas ant stock masinos, skirtos vazinet gatvem, pagerins handlinga ir per4mansa, yra klaidingas, nes dazniausiai efektas buna atvirkstinis - aisku su sia nuomone galima nesutikt

[romkasponka]

Bet faktas tame, kad zemo profilio padangos geriau priglunda prie asfalto, nes maziu deformuojasi posukiuose, o betkokia deformacija yra sugerta energija. Pvz wtcc kiek pamenu vaziuoja su R17J9 tekiniais, o stabdziai man regis stock. Dar vienas niuansas, kad su didesniais tekiniais stabdziai aushinasi geriau...