G2IntegraGS

Hey guys,

I'm in this argument with a ricer about wider rear tires on a Scion TC. He claims that handling will be improved by using wider rear tires and he can make it around a autocross track faster with wider tires.

My arguement is that more grip in the rear will negitivly affect handling.

His argument is that more grip anywhere is faster.

Is he right?

And would someone mind explaining the physics behind this?

I would really appreciate it.

SMHero

Personally if I put wider rear tires on my car it would make me slower, however I like the car to rotate and I drive in a manner to make that happen. I'm going to say the guy is wrong though only because I've never seen that on a front drive car and I feel like the set up would lend itself to understeer.

G2IntegraGS

Yeah I'm thinking it would cause more under steer.

I will paypal someone $5 if they provide a valid explaination on this.

That how determined I am.

beanbag

The answer is right here:

If he can make it around the course faster with wider rear tires, then that is a fact, and any theory has to work around this fact.

I am happy to make up all kinds of reasons in order to support empirical observations.

In general, more grip doesn't hurt, unless you are driving in a style that tries to reduce grip on purpose, e.g. drifting or getting the back of the car to swing around for a sharp corner.

JBrettHowell

There are diminishing returns.

My first rule of thumb is to avoid taking grip out of the car. So by that standard one would run the wheels/tires with the most grip front and rear.

On the other hand, weight is horsepower for the corners. Removing the rotating mass of larger rear wheels/tires, plus shedding the unsprung weight from the rear corners could make the rear of the car work better (make the existing suspension components more efficient).

Additionally it could be beneficial in terms of suspension setup to have rear tires that can tuck rather than bottoming out fender to tire (especially on Civics where the unibody has to be chopped up to significantly clearance the rear fenders).

The typical justification for running a staggered setup (wider front wheels than rear wheels) is to allow the car to rotate more. That in and of itself is not something I would consider to be a complete justification for running smaller rear tires, but if it allows one to not have to run a rear swaybar (for instance) for less weight or get the car balanced on spring rates that allow the front tires to grip better, then it is a good thing.

Simplified example: if a car can corner at 1.4 g's and is limited by front grip, then wider rear tires will not improve anything. Obviously if the car is limited by rear grip, then wider rear tires will improve total grip.

I would say your friend is not right all the time - there are circumstances where narrower rear tires can work better, but all things being equal (if one can fit wider rear tires without adversely impacting weight or compromising setup) then wider rear tires will offer more total grip.

Another consideration is the ability to rotate tires front to back.

And finally, there is a potential course dependency - if you run in a region where course speeds are usually slower, then having a car that can rotate and dance around may be the better setup than one where the rear end always stays planted no matter what the driver does. On a faster transition heavy course it could be advantageous to have a car that is inherently more stable in the rear.

To help you with your argument, have your friend consider a car with narrower rear tires that allows the car to rotate more at slower speeds, but with a rear wing that adds rear downforce at higher speeds. In this way the car would feel more nimble in the tight stuff yet still feel stable at higher speeds.

G2IntegraGS

Dude, he CLAIMS it will make hime faster. He never actually autocrossed it and determined that the wider rear tires are faster. If he had I would agree with him.

Stinkycheezmonky

A person can't just look at the front or rear of a car and say: "This will be faster." There is a relationship between the front and the rear that dictates the overall behavior of the vehicle.

If the rear was already too narrow and lacking in grip, then making it wider will most likely help things. Conversely, if the rear already had enough grip, adding more won't do anything but increase weight and potentially increase understeer on the front (as an analogy, consider fresh rear Hoosier A6s and bald 10-year old front all-seasons; it would be an understeering bitch mother, BECAUSE you increased rear grip...I mean, it would understeer anyway, but you just made it so much worse).

nsxtasy

What a doofus your friend is!

The best handling usually occurs when a car is well balanced, with good grip in the front as well as the rear. Sometimes a little wider at one end or the other will help, depending on the car, but if the difference is dramatic, chances are it's only going to make the car handle worse, not better.

His "more grip anywhere is always faster" theory implies that it's great if you add a ton of grip in the front (even though it will cause handling to suffer through oversteer), and it's equally great if you add a ton of grip in the rear (even though it will cause handling to suffer through understeer). Yeah, sure, buddy...

Davidss

I used to have a beater 2000 model civic sedan that I bought out to a few time trial events for fun. It was around 4 seconds a lap slower with good tires (kuhmo xs) all around than with good tires on the front and crappy 400 treadwear all seasons on the rear.

JBrettHowell

Ah, one more advantage to narrower rear tires in an autocross setting - assuming similar backspacing, the narrower tires will be less likely to knock over those pesky cones.

ronald55555

Just by coincidence I was reading a Mark Ortiz newsletter on the the same subject, although his article uses rwd as examples, some of the ideas still apply to a fwd. Below is a copy and paste of the article taken from November 2003 issue:

WEIGHT DISTRIBUTION AND TIRE SIZE
At what point is it worthwhile to install wider wheels and/or tires on the back of a rear-wheel-drive
car – specifically if the car has close to 50/50 front/rear weight distribution? Directly related to this
point, I often wonder: why is it that an F1 car can brake so much more rapidly than it can
accelerate, even though the front tire contact patches are smaller?
The basic rule of thumb is that tire size should be roughly proportional to tire loading, assuming we
are talking about a car that has to corner well. So if the rear percentage is close to 50%, the tires
should be equal size, and if the weight distribution is 40/60, the rear tires should be half again as
wide as the fronts. Ordinarily, we go by tread width for this, rather than overall width at the
sidewalls. Of course, the rule is only an approximation in any case.
The rule gives us roughly optimal values for steady-state cornering. That may not be the only thing
we’re trying to get the car to do well, but it’s certainly important in most cases.
In many cases, we do not have a free choice of tires or tire sizes. Often, our task is to optimize the
car for the tires, rather than the other way around. With race cars, our tire sizes are usually limited by
the rules. For street cars and some race cars, we may be constrained by the fenders. In many
production cars, if we just put the biggest tire at each corner that will fit without hitting the fenders,
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we end up with the rears bigger than the fronts. This is partly because the rears don’t have to steer,
and partly because it is usual to allow for snow chains on the rears, with equal size tires.
Other practical constraints may intrude as well. I have an old Chevy Impala station wagon that
serves as both my transportation and my bedroom. When I first got the car, I set it up with 8” wheels
with ½” offset all around, a rear anti-roll bar, and a much stiffer front anti-roll bar than stock. The
car was fun to drive, cornered quite flat, and was well-balanced. Only trouble was that it kept
breaking front a/r bar links and other a/r bar hardware, and I couldn’t keep front wheel bearings in it
for more than 10,000 miles. I could have put a full racing front end in, but this is an old, beat-up car
that I don’t want to put huge amounts of money and time into. So I went to 7” wheels with zero
offset in front, a much softer front a/r bar from a sedan, and 8 ½” rear wheels with the same stiff rear
bar as before. The car is balanced this way too. It rolls more, but less than stock. The ultimate lateral
grip is less, but acceptable. And I don’t have to fix the front end all the time.
I help out a Formula SAE team. Sometimes they take my advice, and sometimes they don’t. They
have had a policy of doing their car in a single year, and making minimal changes from the previous
year’s car. One consequence of this is that many design elements get adopted simply as carryovers.
The team has been using wider tires in back, and the 2003 car had the same feature, despite my
urging the team to use the widest and biggest tires possible all around. The car has only 52% rear.
The rear tires have 58% of the tread width. We still managed to get balanced handling, by using
stiffer springs and anti-roll bar at the rear.
My point here is that in many cases tire sizes are chosen based on factors other than vehicle
dynamics theory – sometimes rationally, sometimes irrationally. And because cornering balance
depends on suspension as well as tires, a surprisingly wide range of tire size combinations can be
made to work acceptably on any given car.
Well, okay – but limiting ourselves to considerations of vehicle dynamics, why might we want
bigger tires on the rear, when the car is not markedly tail-heavy?
Depending on aerodynamic balance, higher speeds may argue for bigger rear tires, or alternatively
for more nose-heavy weight distribution. We know that a tire has limited capability for combined
lateral and longitudinal force. To get more longitudinal force from a tire, we sacrifice some ability to
generate lateral force. We speak of the traction circle, traction ellipse, traction perimeter, or traction
envelope, as a representation of the limiting values for the vector sum of lateral and longitudinal
force.
When somebody mentions steady-state cornering, we may think of a typical skidpad test, with
speeds somewhere in the 60 mph (100 kph) range. Throttle application to maintain this speed will
probably be fairly moderate, meaning the rear tires have a large percentage of their traction
envelope available for cornering. But we can also have steady-state cornering at, say, 150 mph (250
kph). Just to run that fast in a straight line requires a fair amount of power. Add the drag of four tires
operating near peak slip angle, and the car may need full power, or something close, just to maintain
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constant speed. And powerful cars can sometimes spin the wheels in top gear, in a straight line. So in
this situation, how much of our rear tire traction envelope do we have left for cornering? Not a lot,
unless the traction envelope was generous to start with (big tire). Or maybe quite a lot, if the car
generates sufficient rear downforce at high speed to compensate for the other effects. In a case such
as a NASCAR Cup car, both the tires and the aerodynamics (except for details) are dictated by the
rules, and we pretty much tune the suspension and the ballast placement around the tires and aero
package. The tires are required to be equal size at both ends, and for medium to high speed tracks,
the car likes around 52% front. To run more rear percentage, wider rear tires would be helpful. We
could run more rear, with equal tires, but we would be making less use of the left front tire, and midturn
speeds would be lower.
We may want to run larger rear tires in search of greater forward acceleration. Any tire has an
optimum inflation pressure for making lateral force, and another, lower, optimum pressure for
longitudinal force. Consequently, if we have a car that’s balanced with equal size tires front and rear,
and then we install larger rears but run them somewhat underinflated for cornering, we still have a
balanced car, but it puts power down better.
We can take this a step further, and add roll resistance at the rear, reduce roll resistance at the front,
and further increase the tire size disparity. If we take this to an extreme, we have a car that is
optimized for drag racing, but also has acceptable cornering balance – although it isn’t really
optimized for cornering. The inside rear tire will be very lightly loaded when cornering, but with a
limited-slip diff, this may be acceptable. With a live axle rear, we improve the car’s launch at the
drag strip if we provide a very stiff wheel rate in roll at the rear and a very soft wheel rate in roll at
the front. This helps because driveshaft torque produces less change in diagonal percentage when the
car is stiff at the rear and soft at the front. We may disregard cornering completely in a car we only
race in a straight line, but even if we are concerned with balanced cornering behavior on the street, a
drag racing suspension setup will often call for larger tires in back. Note that this reasoning
regarding reduction of driveshaft torque effects does not apply with independent rear suspension,
where these effects are absent regardless.
Looking at the opposite end of the spectrum, IMCA-style modifieds, as raced in the US, may have as
much as 59% rear, or even 60% with a full fuel load, and they are required to run equal size tires
front and rear. In this case, we have a car that often runs on very slick dirt tracks, with tires that don’t
give much grip, and has lots of power. It needs the rear percentage to put power down. Even in the
turns, a large percentage of the cornering force is car-longitudinal drive force from the rear tires,
applied at an angle to the car’s direction of travel because the car is powersliding. To get decent
cornering balance in such a car, we have to make it corner on three wheels, or very nearly so. We
under-utilize the left front tire, but we accept this to get forward traction.
If we run the same car on pavement, we need to move ballast, and perhaps even the engine, forward
in the car.
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In answer to your question of why F1 cars achieve greater accelerations rearward when braking than
forward under power, even though the front tires are smaller than the rears, actually almost all
vehicles exhibit this property. The main reason is that we have brakes on all four wheels, but
propulsion on only two. A secondary reason is that drag, aerodynamic and mechanical, acts
rearward, so it assists braking but opposes propulsion. About the only way we could produce a
vehicle that accelerates faster forward than rearward would be to have no front brakes.

dirty19

iTrader: (1)

ricer, wide tires, never raced.. for lack of better word..
Pendejo!
Gum Flapper!

The Destroyer

Sounds a bit backwards no?

Stinkycheezmonky

Unless you are a national autox champion, there are hundreds of stock class guys that would prove your argument wrong.

Edit: That doesn't mean I'm supporting the ricer's argument, only that significantly decreasing grip like that should not be the fastest way around a course.

AfghnKid

I ran a couple laps to test this hahahah, i had to! well we made a Make shift track in this abandoned naval base in alameda. We used 205 50 15 falken's in for 10 laps we made a time of 45sec. Then we threw some 185 50 15 falkens on and ran 42sec averge. it seemed the smaller tires in the back allowed for a faster entrance to the turn and my friend said it felt like the rear tires lost grip on the back a lil so he could kinda drift the turn and come out a lil harder. Car used. 98 gsr b18c1 ITR tranny. Headers,cams,b16 pistons. Tokieco blues. strut/ sway bars. over and under engine. under back. and that X pattern bars in the trunk. Hit me for a lil video lol.

Stinkycheezmonky

I was going to ask you wtf an "over and under engine" was, then realized you meant you have strut bars at the top and bottom of the engine bay, and on the rear subframe.

Just FYI, strut bars are WAY different from sway bars

AfghnKid

Lol thanks, i was kinda rushing to finish my post, IM on THAT COD!

AfghnKid

He has a sway in the back, nd strut's on the top and bottom of the motor lol.

The Destroyer

Jesus christ.

Egezzy

iTrader: (2)

also too..that back to back test isnt within the subject of the question asked and also the rt615s only come in 205/50 so if that is the tire you used for one and some crappy falken 185 on the second then that test doesnt mean much.

and g2teg why dont you invite scion guy out for some local autox. im sure your local scca will be thrilled to have him.

migs

Wider Rear Tires = better traction

HAHAHAHAAHAHAHAHA

CivicSiRacer

Also with testing you have to go back to the original setup in order to see if running the course multiple times had an impact on your runs. The more runs you get the better you get at the course, so your times should drop.

Should have done RT615s, then the crappy tires, then back to the RT615 to make sure everything stayed consistent. I would highly doubt RT615s would be slower than the crappy tires unless the RT615s were rock hard from incorrect storage, heat cycled, too much heat, or wrong pressures.

JBrettHowell

One other slightly OT data point...when the Snot Rocket's battery box decided it had enough this weekend, I was able to jump into a VERY competitive ST car for my runs (jumped from a turbocharged EF with 275-15 A6's to a NA EF with 195-15 R1Rs) and the contrast was very enlightening. The ST car just turned in so much more crisply (as one would expect given the difference in front tire widths). The Snot Rocket is a very good car, but if we can figure out how to get the front end to work more like the ST car, it will be ridiculous.

The Destroyer

Im sure the 275's are on a wider wheel, and the combo makes it a good bit heavier. Im guessing the increase in unsprung mass makes the car feel less nimble.

Are 275's really the only option? See if someone will let you try out some skinnier tires next event and see how it reacts.

I am interested in the turbo ef, if you could pm me some info or have any pics that would be awesome. Thinking about doing a sub 250 turbo sm car next season.

.brad

iTrader: (1)

Many of the courses we see around here are pretty small and have tight corners. Being able to whip the rear of the car around them will improve your times so thinner tires would aid in oversteer.

On the flip side, if I were on an actual track with higher top speeds and larger turns I would prefer wider rear tires to prevent oversteer.