Front axle preparation

[tiagosantos]

I think he meant to ask if John thought the increased spring rate was beneficial to the car's handling hehe..

[jstoezel]

Spot on

I'm pretty sure, that all Jean was asking is how the increased spring rate affected the car's handling.

[brian]

There's been a lot of discussion on the forum as to preferences, but in very simple terms, higher spring rate in front will be less compliant in the bumps and tend to control roll on zero roll cars.

[hardingfv32-1]

Yes, we have had that discussion before.

I am not sure the average FV driver can detect a 20% change in front spring rate. At the front that would mean a "wheel rate" change of 6 lb/in. One normal full spring is about 30 lb/in. You think you can feel a 6 lb/in change?

Brian

[tiagosantos]

Isn't it relative though? 30 to 36lbs should be as noticeable as 300 to 360, shouldn't it? Not saying I'd notice the difference, but..

Anyway, I had a look at a car with the two adjusters recently and my ultra scientific spring rate tester (jumping up and down on the front beam..) seemed to indicate a much bigger difference than that.. There might have been something else going on, though!

[hardingfv32-1]

Like the shocks were in place?

Brian

[smsazzy]

Yes, we have had that discussion before.

I am not sure the average FV driver can detect a 20% change in front spring rate. At the front that would mean a "wheel rate" change of 6 lb/in. One normal full spring is about 30 lb/in. You think you can feel a 6 lb/in change?

Brian

You are incinuating that you would feel it?

Most changes are not felt consciously, but look at lap times to see if it made a difference. Most drivers can also not tell the difference between going a 1/2 second faster or slower on that lap vs the other either.

90% of driving is subconscious. So "noticing" the change is irrelevant. Make the change, do 4 laps and look at the data/lap times.

[tiagosantos]

Like the shocks were in place?

Brian

No, like it was stiffer than 20% over stock..

[hardingfv32-1]

So the shocks were not mounted? In that case was the front end built correctly with no binding? Plain vs roller bearings?

The 20% was a simple calculation, the front spring was 20% shorter per side. The 30 lb/in wheel rate is a measure number, +-2 lb/in.

Brian

[jstoezel]

I would say you should be able to start feeling a difference, at least road feeback going on a bumpy straight (like the landing strip at GMP).

At least I can feel the difference in pressure on tires that are 1.5 lbs different from one session to another. Could be in my head too, and in a FV it's not as obvious as it was in the gokarting days (where you really feel any tire pressure difference, since the gokart tires are also the springs/shocks). At GMP the track is very rough, it's concrete and some sections are rough, with a higher tire pressure you almost feel each and every irregularity.

Yes, we have had that discussion before.

I am not sure the average FV driver can detect a 20% change in front spring rate. At the front that would mean a "wheel rate" change of 6 lb/in. One normal full spring is about 30 lb/in. You think you can feel a 6 lb/in change?

Brian

[jpetillo]

Hi John:
I meant: "how do you like the increase in spring rate, at the track".
I enjoyed the explanation though.
Jean

Jean, That's too funny - sorry about that!

However, I don't have an answer to your real question. I drove the car two-three times during one race weekend with the original stock spring pack when I first got the car, before changing over to the two adjusters. Even if I could have felt the difference, that was not enough time to get to know the car.

With the shocks off and pushing down with my foot, I can't say I could feel the difference - not that I had the before and after adjusters there to try at the same time.

With regard to the push down discussion with the shocks on, on my two cars, the shocks are by far the biggest difference in how the push down test feels.

John

[hardingfv32-1]

"At least I can feel the difference in pressure on tires that are 1.5 lbs different from one session to another. Could be in my head too..."

Yes, it is in your head! I assume feel is ride harshness. You could feel a change in balance. You should be more precise when using the word feel.

1) The tire spring rate is 1450 #/in @ 18 # and 1540 #/in @ 20 #. That is a about a 6% difference.
2) If the front wheel rate is about 30 #/in, then the suspension is doing almost all the work or movement. The tires are simply not seeing the track's irregularities. In a kart the spring rate of the chassis is much higher than the tires, so the tire spring rates become useful.

The only performance change you get from a tire pressure changes is a change in the shape of the tire's contact patch.

Brian

[jstoezel]

Hi Brian:

Ahaha, I see your point it's science vs feelings I guess. I totally agree with being able to feel the change in the contact patch, if this means tire roll on the side wall. This can clearly be felt on the radial tires we run in F1200, in "s" sections of the track where you change direction quickly, or even coming off a sharp corner if the exit is not smooth. If you're rolling too much, the car will wiggle side to side with sharp changes in direction.

Without willing to be deliberately confrontational I maintain it's possible to feel ride harshness too. In the rain for example, we use the same radial tires as in the dry. However we do increase the tire pressure (even more as the track gets more water) to "cut" through water and limit aquaplanning. I find the difference in ride at the higher pressure to be very real on the straight.

Next time I'm at the track I will make a point to increase the tire pressure on purpose, but I guess this won't convince anybody other than me.

"At least I can feel the difference in pressure on tires that are 1.5 lbs different from one session to another. Could be in my head too..."

Yes, it is in your head! I assume feel is ride harshness. You could feel a change in balance. You should be more precise when using the word feel.

1) The tire spring rate is 1450 #/in @ 18 # and 1540 #/in @ 20 #. That is a about a 6% difference.
2) If the front wheel rate is about 30 #/in, then the suspension is doing almost all the work or movement. The tires are simply not seeing the track's irregularities. In a kart the spring rate of the chassis is much higher than the tires, so the tire spring rates become useful.

The only performance change you get from a tire pressure changes is a change in the shape of the tire's contact patch.

Brian

[hardingfv32-1]

Not arguing.... but

1) How can you feel the tire's harshness if the much softer suspension gets first crack at any bump? What is going to be left for the tire to respond too? The very soft suspension is isolating the tires from the driver.

2) I would not say that a pressure change has that much effect on tire side roll. You do not see it when examining tire slip angle/ side force tests. The contact patch change I am talking about is an area issue. More pressure, rounder shape, would mean a smaller area of contact patch. Could also be some effect on tire temps with the change in same work being done by a small area (higher pressures), etc.

3) Too really learn you must do your tests blind. Have someone else controlling the changes and you just evaluate. It takes a very special driver to sense most these small changes, amateur or professional.

4) Could your perception of your tires in the rain be one of increased concentration? In the rain more of your concentration is focused on how the tires are interfacing with the road surface than in dry conditions.

Brian

[jstoezel]

Hi Brian:

Got the email with the tire data, thanks for sharing the information... and crushing my beliefs.

Jean

Not arguing.... but

1) How can you feel the tire's harshness if the much softer suspension gets first crack at any bump? What is going to be left for the tire to respond too? The very soft suspension is isolating the tires from the driver.

2) I would not say that a pressure change has that much effect on tire side roll. You do not see it when examining tire slip angle/ side force tests. The contact patch change I am talking about is an area issue. More pressure, rounder shape, would mean a smaller area of contact patch. Could also be some effect on tire temps with the change in same work being done by a small area (higher pressures), etc.

3) Too really learn you must do your tests blind. Have someone else controlling the changes and you just evaluate. It takes a very special driver to sense most these small changes, amateur or professional.

4) Could your perception of your tires in the rain be one of increased concentration? In the rain more of your concentration is focused on how the tires are interfacing with the road surface than in dry conditions.

Brian

[brian]

Much has been written about the effective spring rate inherent in tires and the effects of air pressure on that rate. I can't honestly say I can tell the difference in one or two psi, but the NASCAR guys really work with pressures to change handling. Maybe because ovals require very different set ups. I can grasp the idea of a very soft suspension being insensitive to air pressure but the tires are the primary transmitter of force and if the spring rate of the tire is changed via air pressure, it has to an effect on the overall spring rate and suspension.

Brian, these dialogs are informative but I don't know how much effect this stuff has on our ol crude vees. Hope to see you next week.

[hardingfv32-1]

Most race cars and stock cars have very high wheel rates. So the tire rate take on more importance.

To be totally accurate, our tire rates do have an effect. The influence of the tires is on the order of 3% at the front and 9% at the rear. So if you make a 10% (2 psi) tire pressure change, you have effected the total system by .6% at the front. I think this is an accurate description.

With winged cars and stock cars the wheel rates are similar to the tire spring rates, so the influence of the tire rates is in the 50% range. The tire spring rate is so important in NASCAR the teams use to test the spring rate of every newly mounted tire in the pits. The practice (equipment) has been outlawed.

This discussion highlights what to expect from a tire pressure change on a FV. Having correct expectations of changes helps when setting up the car.

Brian

[tiagosantos]

Also look at that, 5 pages and zero fighting! I guess winter's officially over

[problemchild]

If you want to accept Brian's grotesque over-simplification as fact ..... to avoid arguing .... then I guess winter is over. There is far more involved in changing tire pressure than the difference it makes to the wheel rate. Tire engineers could spend their lives writing thesis on the effects of changing tire pressure 2 psi on a tire that only operates at 20 psi. That is 10% of the whole or about 50% of the normal operating range.

For example, Much of Goodyear's problems with their tires of a decade ago were that people tried to run them at Hoosier pressures ... 16-18. The fast guys (like Stout, etc) were running 19-21 or 20-22. Brad was kicking ass while many people consider the GY undrivable and went running back to Hoosier. Tire pressure adjustment of 2-3 psi is a drastic tuning tool on a FV and Brian's comments are just meant to mess with people who don't know better.

[tiagosantos]

That's not how I took it at all.. I believe that changing tire pressures has a dramatic effect on the car's handling and tire temps/durability, etc. What I thought Brian was saying is that you would not feel a difference in spring rate (i.e., the car wouldn't necessarily feel any different over bumps or on a rough section). I tend to agree with that.

[cendiv37]

I haven't read what Brian sent, but from observing this post, I think for just the "feel" of the car on the track, the tire still is critical. In my perception, a cold tire "feels" stiffer over bumps than a warmed up, softer tire even though the pressure has gone up a few pounds. The rubber is stiffer when it's cold and sidewall stiffness changes as the tire heats up.

Also, car "feel" is not just what the care does over larger undulations where the suspension stiffness (or lack of) is dominant. It is also what it "feels" like over tar strips and smaller but more sudden changes in track surface. I propose that in this case, the inertia of the suspension keeps it from following the road, but the compliance within the tire still does follow the road. Here, tire pressure (and sidewall stiffness, etc.) will be important, and one could "feel" the difference. The amplitude and frequency of the input to the "system" is important, not just the net spring rate that results from adding up the component pieces.

[SR Racing]

Tire pressure adjustment of 2-3 psi is a drastic tuning tool on a FV and Brian's comments are just meant to mess with people who don't know better.

I don't think you understood the thread. Brian was talking total spring rate. And I would not argue with those numbers. Tire compliance is a separate issue and Brian stated that. Changing the contact patch and the rate of heat (from that change) of course has significant compliance effect.

[hardingfv32-1]

Bruce,

1) I did state that there was about a 1% change in total system spring rate with a 10% tire pressure change. I do not know how sensitive the average human is to amplitude and frequency change.

2) While I appreciate the theory of the tire adsorbing track irregularities do to inertia issues, I have never seen this mentioned in my research or tires and tire models. I will do more research with this idea in mind, but as is often the case I first have to figure out the proper description for the search. Frankly, research the tire contact patch compliance topic SEEMS almost non existence. Again I might not be describing the issue correctly.

Brian