Has anyone selected their tyres for their car based on the F/R weight distribution?

A surprising number do not, though since a Locost is pretty near 50/50 already, using the same size makes since. However, since we don't have any rules to meet, there's no reason why we shouldn't vary them. I used larger wheels and tires on Kimini to help offset the rear weight bias.

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Originally posted by kb58
A surprising number do not, though since a Locost is pretty near 50/50 already, using the same size makes since.

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Originally posted by Mr Whippythis seems strange as my front springs are much stronger than the back, indeed I can lift the back right off the ground but can't do much with the front other than put my back out.

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my car is shaping up to be 1000kg, with a 60R 40F weight bias.
Tyres rear were to be 255/40/17 and front were to be 225/45/17....though 215 may be a better choice.

60/40 means you'd theoretically want the tires at the heavy end to be 60/40 times wider than the other end, or 50% wider. In practice that's usually not realistic, as using 215s at one end would mean using 325s at the other! OTOH, anything you can fit would help.

Of course that's just part of the picture, as spring rates, tire pressure, shock valving, and anti-roll bars all have a hand in the overall balance. You just have to ask yourself just how much you really care. After all, most of us are building expensive toys, not anything that's going to win us an F1 championship. At least that's true for me...

What surprised me was that my 40/60 f/r Mini was VERY hard to spin on the skid pad, and that was with equal size tires, 215/50/-13s.

[Edited on 12/27/07 by kb58]

At the moment I'm arriving at a 40 - 60 split also.
Currently the plan is for 235's on the front and 275's on the rear which on the face of it looks like the fronts are too big.
What I also plan to do though is buy front wheels where 235's are the largest tyre they'll take and at the rear 275 is the smallest they'll take so hopefully there's a tyre combo available in the range that'll fit the wheels that'll work well on the car.

Truth be told though without the load - grip - slip angle graphs this is all just guess work without real testing (and a dozen sets of tyres to go through).

MGF
185 and 205
MR2 mk3
185 and 205
Mk2 Elise
175 and 225
Boxster
205 and 255
Ferrari 360
225 and 285
Zonda
255 and 345
Ultima
245 and 335

So that's a difference of:
20mm for the least powerful cars (under 2 litres) except for the Elise which has a 50mm difference.
50mm for 2.5 to 3.2 litres
60mm for 3.6 litres
90mm for circa 5.7 to 7.5 litres

Another noteworthy point is how close the tyre wall depth is for those cars. For example a 185/55 tyre (MR2/MGF) and a 335/30 tyre (Ultima) both have very close to 4 inches from the rim to the road.

So I'd suggest:
185/55 - 205/50 15 inch
195/50 - 225/45 16 inch
205/50 - 235/45 16 inch
205/45 - 235/40 17 inch
205/50 - 255/40 17 inch
205/45 - 265/35 18 inch
225/45 - 285/35 18 inch
245/40 - 335/35 18 inch
255/40 - 345/35 18 inch

Doug, I'd decided to not go wider than 255 for a couple of reasons...cost was a biggie and tyre/compound availability too.
cymtriks interesting data and ideas, but why the 205 fronts covering such a wide band?

As you go up in tyre size the rim width also goes up.

If the power or displacement indicates a tyre size at the rear that only comes in rim sizes over 16 inches then, assuming you want a level look, you are restricted to the smallest tyre size available in the larger rims which tends to be 205.

A quick look at production cars also shows a lot of combinations around this size.

The MR2 and MGF, with circa 115 to 160 bhp have 185 fronts while the Ferrari 360 has 225 fronts and circa 400 bhp. That's an enormous range of performance that can be covered by 195, 205 and 215 fronts.

Of these 195 is found up to 16 inches in rim size. Add 50 mm to this you're looking for a 245 in 16 inches. Possibly a 195/50 and 245/40 combination which might be harder to find in 16 inch rims.

I suppose you might use 195/50 with 235/45 but I can't think of another 16 inch combination.

Once you go up to 17 inch you are probably looking at a minimum of 205 and 235, or possibly the common Porsche Boxster combination of 205 and 255.

Once you go up another power level you are probably looking at 60mm difference so the smallest sizes are 205 and 265 followed by the Ferrari 360 combination of 225 and 285.

It's all about juggling available sizes based on a level look!

Adding the 195 and 235 combination to my earlier list gives:

185/55 - 205/50 15 inch
195/50 - 225/45 16 inch
195/50 - 234/45 16 inch
205/50 - 235/45 16 inch
205/45 - 235/40 17 inch
205/50 - 255/40 17 inch
205/45 - 265/35 18 inch
225/45 - 285/35 18 inch
245/40 - 335/35 18 inch
255/40 - 345/35 18 inch

ah, I could see rim diameter but not width.
'Fit for purpose' is another issue. 16" in my local market is a bit of a dead end as it would limit tyre availability, particularly in 'competition' tyres.
In addition I need a certain rolling diameter to keep the desired gearing. In this diameter a 255/17/40 is lighter than the 16/18" equivalent as a tyre and wheel combination.

Looks like, I am planning to be be "over-tyred" at the front, maybe I should reconsider that.

I'm not planning to use the car in competion too much so don't plan to go through too many sets of tyres.

One thing I find interesting is that in every major championship where wheel size is 'free' as far as I can tell 18" wheels dominate on tarmac.

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Originally posted by kb58

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Originally posted by Mr Whippythis seems strange as my front springs are much stronger than the back, indeed I can lift the back right off the ground but can't do much with the front other than put my back out.

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What's your weight distribution? I doubt your car is balanced 50/50, or you are one strong guy! A typical Locost weighs, oh, 1300lbs, so that's 650lbs at each end. Even if it's 60/40, that's still 520lbs minimum.

[Edited on 12/27/07 by kb58]

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Originally posted by Doug68
Looks like, I am planning to be be "over-tyred" at the front, maybe I should reconsider that.

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Doug there is a good reason, the 'good' cars tend to be modern, relatively heavy i.e. 1400kg and thus need big rotors (350mm- Jim Richards has siad that he has hit 280kph on TT stage) and calipers...so you need space. In addition these guys are not price sensitive when it comes to setup costs, suspension component costs and tyre costs...e.g. the cost for one front runner for the Targa Tas was $45,000 for the event.
With respect to tyre width, there is a trade-off.
More grip is better up to a point and that point is transition behavior post limit, I have the feeling that a lower ultimate limit with more progressive transition would be ideal for me and my car. Familiarity and experience will assist in upping the grip limits over time.
I REALLY want to avoid the mid-rear enigma: a seemingly docile 'fast' car at 90% effort and then a car that bites your arse off at 91% effort!

[Edited on 28/12/07 by rpmagazine]

I'm going to have to use at least 18" wheels due to the brakes I've bought already.

I think that there's an interesting parallel with motorcyles where 17" wheels are more or less universal. On the front of bikes the fronts of all sports bikes 400cc or bigger will be a 110 or a 120 where as the rears will be anywhere between 140 and 200. Basically the 120/70 is the universal default size, go any bigger than that and it won't steer right.
The larger and larger rears being required to cope with more power from the larger bikes, but they do have a detrimental effect on the steering.

So my thoughts are that there will be some detrimental effect on the handling if the front tyres are too big, maybe wandering under brakes or some such, but I've no practical experience of this, just a hunch.

I think the lower the profile of the tyre and the lesser amount of compliance the more critical the suspension design and tune. Personally I want to keep compliance at this point, but added rotating mass is bad and it is worse the further from the hub for obvious gyroscopic reasons.
I also thing the gyscopic effects are less critical/noticed when you have skilled engineers setting the cars up ('ne porsche) and also when your car is relatively heavy as opposed to 'our' cars or bikes in your example.
BTW: http://cgi.ebay.com.au/19-Buddy-Club-Wheels-Rims-for-RX7-RX8-JZA80-R32-R33_W0QQitemZ320198456949QQihZ011QQcategoryZ6782QQssPageNameZWDVWQQrdZ1QQcmdZVi ewItem

[Edited on 28/12/07 by rpmagazine]

Can I take a three yearolds view on this discussion (which is very interesting by the way)...

If, lets say, you take your engine and gearbox from an Audi A3......and also use the wheels and tyres from the Audi as a good place to start...

You may increase the power you are putting down by tuning the engine a bit, but the overall weight of the car will be much less....So would that not be a sound basis for sticking with wheels similar in size and profile to that of the Audi as they obviously work Ok for that car...(Although it is front wheel drive, so my theory might not work)

Obviously you may need to go bigger to fit your brakes.....

Or have I over simplified things to much?

if you are keeping the dash/box etc etc then keeping the same diameter is wise. But the ability to widen the tyre is a good start as the tyres will no longer have to steer and you can get more traction...assuming the transmission can stand the loads...which it 'should'.

A good starting point is what are the widest tyres available to your specification/ budget/availability and then choose the smaller front size to suit.

For my purposes the road legal Kumho V700 was available in medium compound at 275 wide at a very reasonable price. With a 65/35 weight distribution I chose a 205 wide front tyre.

I do not adhere to the size pro rata to weight distribution theory because the static weight distribution is very different to the dynamic "weight distribution".

In braking even with a long wheelbase and a low center of gravity 100 to 200kg will be transferred to the front wheels and although the reverse is true of acceleration the acceleration g forces can't match braking g forces. Hence the front tyre needs to be bigger. With the higher CofG that road cars have this effect is accentuated to the extent that the tyres need to be the same size(as a sweeping generalisation but thereabouts).

In cornering most people trail brake into the apex either intentionally or otherwise the main exceptions are British advanced drivers who follow the outdated Roadcraft principles. Same arguement applies the fronts have added dynamic load

Also remember that a lot of the production cars have the tyres chosen by the styling dept even the famous ones - don't believe me? read J Daniels "car suspension at work" - it's an eye opener for sure.

A further consideration is litigation. Set the car up so that even if the silly so and so gets the tiye pressures completely wrong it will always understeer ( or never oversteer to be more accurate). This means smaller front tyres regardless of best grip.

All the above is assuming dry grip. With loads of water about wider tyres willl aquaplane easier but why are you going so fast on the public road in a thunderstorm anyhow? Racers will just change their tyres.

Tyres are a fundamental consideration in designing a sports car and I reckon its the most important one we can make. I spent months agonising, reading, theorising and I'm not convinced its 100% right yet - good luck with your choice.

Interesting discussion.

Putting aside the grip issues for one moment: Let's not forget that your front wheel choices may, and I say should, also be limited by scrub radius. Obviously changing the width and height of the rim and tyre from the dimensions originally designed for the upright will change this geometric point. Scrub radius plays an important part in how the car provides steering feedback and shouldn't be outboard of the wheel centerline at the contact patch. So you need to keep that in mind as you choose.

Happy New Year everyone!

I am aiming for a 45/55 front/rear split but have varied my tyres front to rear acording to the grip.... but I also have little choice due to the size of the rear tyres.

Mine are 345 rear and 255 front.

Mark Ortiz - November 2003
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, 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, beatup
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 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 mid-turn 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. 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.
WIDTH VERSUS DRAG
I'm preparing an SCCA GT2 car (production car shape fiberglass body, tube frame).
I have the opportunity to increase the car's width up to 4 inches, either by splitting
the body or extending the flares. I have always assumed that going with the widest
width is best chassis-wise, to minimize load transfer and maximize grip. However, I
would think that at some point you lose more due to drag than you gain from
improved cornering. These cars don't make much downforce, so I don't think there's
much gain there from building the car wider. Is there any way to calculate an ideal
width?
As long as cornering is an important part of the game, the best approach will usually
be to go for greatest allowable width. This is particularly true when power is ample.
Certainly, the drag does cost you some speed on a long straight. But if you come out
of the preceding turn faster, and you also enter the following turn faster, you are
faster not only through those turns, but also over the first and last portions of the
straight.
Width is a mixed blessing though, even for cornering. A narrow vehicle lets you take
a better line, especially if the turns are tight and the road is narrow. This is one of the
main reasons motorcycles are as fast as they are. Narrowness can be very important
in autocross, especially for slaloms. A narrow vehicle also can have an easier time
passing other cars, which was the reason the FIA narrowed the width limit for F1
cars a few years ago. But on most road courses, the cornering power gained from a
wide track width is worth more than the improved line and reduced drag with a
narrow track width.

Phew! I'll need a couple of days to digest that.

The examples cover a wide range but typically of Mark makes it quite difficult to understand by obscuring the central point - either that or I'm really Dumb.

I think most cars, including F1, or the american equilivent, can brake at a much higher g force than they can accelerate, I recall Mark making this very point in another of his musings, and add weight transfer to the front requiring proportionally larger front wheels when braking into the corner.

Coming out of a medium to slow corner we require the biggest wheels we can get for the rear traction circle- power permitting, but to even up the lateral weight transfer at the back we want the roll resistance to be more at the front - requiring proportionally larger wheels on the pointy end.

Mid corner, high speed, I'm with Mark it just does not last as long as the other phases and other than on ovals its not as important ( thats my opinion by the way not necessaraly others opinions).

So proportionally bigger front tyres on sub 150mph projectiles going round medium to tight corners and the reverse for over 150mph on sweeping bends.

I guess there is no right answer only the right answer for you and your intended use which is almost always the opposite from the guy giving advice as I at least have found out to my cost.

Try and see its quicker and dearer but then life is short ( sorry I've been reading Bransons biography recently!!!!!)

Cheers

PS

If you fit the biggest tyres you can get within your budget/regs/availability it is easy to reduce the grip of the front tyres but damn difficult to get more than the optimum out of the back

[Edited on 3/1/08 by v8kid]