Different Diameter Coil Springs Front & Rear

[RidingOnRailz]

Different Diameter Coil Springs Front & Rear


Just on a hunch last year, I grabbed my set of metric wrenches and measured the diameters of the coil springs on my 2010 Honda and my wife’s 2004 Toyota.

On both vehicles, the front coil diameters(the thickness of the actual spring metal) were 1-2mm thicker than the diameters of the rear springs.

Does this mean that the engineers have already compensated for the front-to-rear weight bias(typically 60/40 in these cars)?

And what about rear-wheel or all-wheel drive cars: Are all the coil springs, if so equipped, the same diameters on those?

[RidingOnRailz]

It’s so nice that someone found my post useful. But would anyone care to answer my questions?

[jeffmorris]

I think that the front springs have thicker spring diameter than rear springs because the engine and transmission/transaxle are heavier in front than in rear.

[RidingOnRailz]

Originally Posted by jeffmorris

I think that the front springs have thicker spring
diameter than rear springs because the
engine and transmission/transaxle are
heavier in front than in rear.

That was what I was implying, above.

Could it also have anything to do with varying spring rate at each axle?

[RidingOnRailz]

One reason I’m bringing up coil spring thickness is that I’m experimenting with using different tire pressures on the front vs rear tires.

My Honda specifices 32psi cold in all four tires. Since the front coils are thicker than the rears, has that compensation for weight already been factored in by the builders?

[CapriRacer]

Spring rates for coiled springs depend on many things, wire diameter being one of them. The engineers that designed the spring/shock system probably didn't design the size of the wire strict on one parameter - weight - as you've alluded to. They probably took into account the handling behavior of the vehicle - and maybe THAT is even more of a factor.

On my race car, I used MUCH stiffer springs, but I sized the spring for spring rate - and in particular, spring rate relative to the rear springs in order to get the handling characteristic I wanted. While the load on a corner is a factor, the relative size is a function of the handling.

Don't forget, the springs usually aren't compressed at the same rate. There is leveraging action taking place and for some vehicles, the mechanical set up front to rear can be quite different.

And in your experimentation, be aware that sway bars play a role in the handling balance. It has been known that the spring and the sway bar work together in the handling balance. In other words, I could stiffen up the front spring OR the front sway bar and get similar results.

Then there is the issue of the tires. Inflation pressure changes the spring rate - adding another variable to the mix. You can change the front to rear balance by changing inflation pressure.

Top say this is complex is an understatement. That's why the car manufacturers have a whole department dedicated to driving vehicles and judging how well balanced they are - and if not well balanced, what needs to be changed so that it is.

[RidingOnRailz]

Originally Posted by CapriRacer

Spring rates for coiled springs depend on many things, wire diameter being one of them. The engineers that designed the spring/shock system probably didn't design the size of the wire strict on one parameter - weight - as you've alluded to. They probably took into account the handling behavior of the vehicle - and maybe THAT is even more of a factor.

On my race car, I used MUCH stiffer springs, but I sized the spring for spring rate - and in particular, spring rate relative to the rear springs in order to get the handling characteristic I wanted. While the load on a corner is a factor, the relative size is a function of the handling.

Don't forget, the springs usually aren't compressed at the same rate. There is leveraging action taking place and for some vehicles, the mechanical set up front to rear can be quite different.

And in your experimentation, be aware that sway bars play a role in the handling balance. It has been known that the spring and the sway bar work together in the handling balance. In other words, I could stiffen up the front spring OR the front sway bar and get similar results.

Then there is the issue of the tires. Inflation pressure changes the spring rate - adding another variable to the mix. You can change the front to rear balance by changing inflation pressure.

Top say this is complex is an understatement. That's why the car manufacturers have a whole department dedicated to driving vehicles and judging how well balanced they are - and if not well balanced, what needs to be changed so that it is.

So, if the carmaker specifies 32psi(or 30, or 36, etc) in all four tires, then would it be prudent to maintain the same pressure in the fronts and rears, whether it be 32, or another pressure of my choice? And to not experiment with different front and rear pressures based on things like maximum GAWR(Gross Axle Weight)?

[CapriRacer]

Originally Posted by RidingOnRailz

So, if the carmaker specifies 32psi(or 30, or 36, etc) in all four tires, then would it be prudent to maintain the same pressure in the fronts and rears, whether it be 32, or another pressure of my choice? And to not experiment with different front and rear pressures based on things like maximum GAWR(Gross Axle Weight)?

Oh, you can experiment, but the suspension was designed using the same pressure front to rear. If you use a different pressure, what you get out would be a change in the front to rear balance.

And I strongly recommend not going below the placard pressure.

[RidingOnRailz]

Originally Posted by CapriRacer

Oh, you can experiment, but the suspension was designed using the same pressure front to rear. If you use a different pressure, what you get out would be a change in the front to rear balance.

And I strongly recommend not going below the placard pressure.

Thanks for that clarification, on the design and not going below placard recommendation.


I have a really odd question for you:

Has the area, in square-inches or square-cm, of the inside cavity created by a tire mated to a rim, ever been calculated? That is, the inside surface area of the tire plus the facing surface of the wheel.

Is there an average of such measurement out there?

[CapriRacer]

Originally Posted by RidingOnRailz

Thanks for that clarification, on the design and not going below placard recommendation.

I have a really odd question for you:

Has the area, in square-inches or square-cm, of the inside cavity created by a tire mated to a rim, ever been calculated? That is, the inside surface area of the tire plus the facing surface of the wheel.

Is there an average of such measurement out there?

I am not sure what you are asking, but tires get distorted ahead of and behind the footprint as well as the sidewalls bulging out.

Yes, there are some measurements of tire footprints. I'll bet that not every tire has such data.

No, I don't think tire manufacturers will give that data.

Also, the size of the footprint can not be calculated using inflation pressure and load on the tire. But if you want an ESTIMATE, the inflation pressure is likely your best bet.

[RidingOnRailz]

Originally Posted by CapriRacer

I am not sure what you are asking, but tires get distorted ahead of and behind the footprint as well as the sidewalls bulging out.

Yes, there are some measurements of tire footprints. I'll bet that not every tire has such data.

No, I don't think tire manufacturers will give that data.

Also, the size of the footprint can not be calculated using inflation pressure and load on the tire. But if you want an ESTIMATE, the inflation pressure is likely your best bet.

I’m sorry if I was vague. What I was asking for was an approximation of the surface area, in square inches, of the inside of the tire, not of the contact patch with the road

[CapriRacer]

Originally Posted by RidingOnRailz

I’m sorry if I was vague. What I was asking for was an approximation of the surface area, in square inches, of the inside of the tire, not of the contact patch with the road

You could estimate it. Making the assumption that the tread and casing are a certain thickness and the sidewalls a different thickness, then construct toroidal shaped object that closely fit those dimensions but is truncated at the nominal diameter.

I did part of the calculation once, many years ago, using an actual drawing of a cross section of a tire with a reiterative process that actually gave me the inside perimeter. I did that to calculate the strength of the tire needed to contain the inflation pressure. I didn't take it to the next step of sweeping that perimeter into a volume shape, but the math is rather trivial once you have the perimeter. The perimeter is the hard math, and I am sure you'd have to derive it yourself. I'll bet no one has published such a formula on the 'net.

[RidingOnRailz]

Originally Posted by CapriRacer

You could estimate it. Making the assumption that the tread and casing
are a certain thickness and the sidewalls a different thickness, then
construct toroidal shaped object that closely fit those dimensions
but is truncated at the nominal diameter.

I did part of the calculation once, many years ago, using an actual
drawing of a cross section of a tire with a reiterative process that
actually gave me the inside perimeter. I did that to calculate the
strength of the tire needed to contain the inflation pressure. I didn't
take it to the next step of sweeping that perimeter into a volume
shape, but the math is rather trivial once you have the perimeter.
The perimeter is the hard math, and I am sure you'd have to derive
it yourself. I'll bet no one has published such a formula on the 'net.

Hypothetically though, let's assume the area, in square inches, of the inside of a typical production tire size - 215-60R16 - is 900sq/in.

On a particular vehicle, the recommended cold tire pressure = 30psi. That implies that, at 30 * 900, the air in that tire, at that pressure, is placing 27,000lbs of force on the inside of that tire.

Now for the multiplyng effect: Say Joe or Jane consumer "adds a pound" of air to that tire, to bring it up to a cold pressure of 31psi. He's not adding one lb. of air to that tire! He's adding 1psi times 900 of pressure to the inside of that tire. The air in that tire now applies 27,900lbs of pressure to the inside of it.

So just that additional 1psi of cold air pressure applies 900 additional lbs of pressure to the inside of our hypothetical typical automobile tire.

[CapriRacer]

Originally Posted by RidingOnRailz

Hypothetically though, let's assume the area, in square inches, of the inside of a typical production tire size - 215-60R16 - is 900sq/in.

On a particular vehicle, the recommended cold tire pressure = 30psi. That implies that, at 30 * 900, the air in that tire, at that pressure, is placing 27,000lbs of force on the inside of that tire.

Now for the multiplying effect: Say Joe or Jane consumer "adds a pound" of air to that tire, to bring it up to a cold pressure of 31psi. He's not adding one lb. of air to that tire! He's adding 1psi times 900 of pressure to the inside of that tire. The air in that tire now applies 27,900lbs of pressure to the inside of it.

So just that additional 1psi of cold air pressure applies 900 additional lbs of pressure to the inside of our hypothetical typical automobile tire.

I've always been fascinated by the fact that large numbers are frequently quoted - like you did here - but it is common to neglect to mention that the increase is a small percentage - in this case 3%.

And don't forget all that force is being contained by THOUSANDS of cords. And that it is fatigue that causes a tire to fail, so the casing is made many times the breaking strength to accommodate.

[RidingOnRailz]

Originally Posted by CapriRacer

I've always been fascinated by the fact that large numbers are frequently quoted - like you did here - but it is common to neglect to mention that the increase is a small percentage - in this case 3%.

And don't forget all that force is being contained by THOUSANDS of cords. And that it is fatigue that causes a tire to fail, so the casing is made many times the breaking strength to accommodate.

What I was really questioning was, not the durability of the tire, but the impact, of those 900 additional lbs of force in our scenario, on handling, ride, etc. The average person assumes 1psi more or less pressure is not that big a deal.

I, for one, can feel a difference in driveability and ride with a 1psi difference in pressure, let alone 4-10psi difference.