Wheel frequencies VS track surface

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DaveW
239
Joined: 14 Apr 2009, 12:27

Re: Wheel frequencies VS track surface

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Greg Locock wrote:
21 Feb 2018, 20:20
I don't get involved with 4 posters.
I've said this before, but in my humble view, Greg, you should get involved. You would learn much about the dynamics of the cars you work on. It's not about the vehicle you think you know, its more about what you didn't know (or what the designers didn't intend).

If I recall, a perfectly respectable MTS rig is installed at Geelong, which, with a decent measurement system, should work well.

Greg Locock
233
Joined: 30 Jun 2012, 00:48

Re: Wheel frequencies VS track surface

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I'd characterise primary ride as that part of the motion of the sprung body where springs, jounce bumpers , tire rates, sta bars and shocks are the dominant tunable parts, and typically fall in a range of 1 to 6-9 Hz, at fairly low wheel velocities.

This overlaps with secondary ride, which I would perhaps claim starts at 6 Hz and runs out in the 20-30 Hz or so area. SR is tunable by altering many things, not least seats.

Neither of these is definitive.

Engine mounts can affect PR, but not much (my guess is the mass ratio is small, and the accelerations are very small), whereas SR is all about engine mounts and wheelhop and structural modes if you've got them wrong, which was not uncommon in the 80s. My models have the engine in as a single inertia with more or less correct engine mounts. I wouldn't swear to their accuracy but the only time I needed to look at engine motion it worked pretty well.

I agree, developing ride on a 4 poster seems like a natural idea, I've just thought of a likely victim. Same guy as did the 9 test matrix above in fact. Sadly we don't get as much play time as I'd like to develop new techniques.

DaveW
239
Joined: 14 Apr 2009, 12:27

Re: Wheel frequencies VS track surface

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Broadly, your definitions follow conventional wisdom. I kind of agree with that Greg, but not necessarily with your arguments. You are right about seats, however. But that is another topic.

Suspension, as you know, has to work against the sprung mass. The higher the effective sprung mass that better will (or can) be the ride (all things being equal).

For one high end production road vehicle I tested, the effective sprung mass fell to 18% of actual over a limited frequency range in the region of 10Hz. Just to emphasise that, generating a damper force at that frequency would require over 5 times the sprung mass acceleration of a monolith. It often pays to know that, because damper adjustments won’t cut it (will just make matters worse).

Interestingly, at that frequency the engine mounts on my example actually dissipated as much energy introduced by road inputs as the dampers & tyres combined. I wonder how long they will survive.

Rustem 1988
0
Joined: 05 Sep 2017, 11:38

Re: Wheel frequencies VS track surface

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How many percent changes the load on the wheel on the road bump?

Rustem 1988
0
Joined: 05 Sep 2017, 11:38

Re: Wheel frequencies VS track surface

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I looked at the load for compression and rebound.

http://www.theoryinpracticeengineering. ... damper.pdf
These forces can be different in both directions. Generally the
rebound forces are greater than the compressive forces. This can be attributed to the idea
that the compression damping controls the unsprung mass while the rebound damping
controls the movement of the sprung mass. Because of the lower mass of the unsprung
mass, less force is required to control this motion. Another contributing factor is the fact
that during compression the forces of the spring and damper are in the same direction
while during rebound the spring and damper forces act in opposite directions.

WilO
4
Joined: 01 Jan 2010, 15:09

Re: Wheel frequencies VS track surface

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Rustem,

While we wait for someone who really knows what they're talking about to respond, you might consider the content of the text you'd quoted, and whether or not it makes sense to you. Particularly the last bit about the direction of spring and damper forces in compression and rebound.
One of the fundamental parameters affecting 'grip' is the normal load on the tire. As such, minimizing the variation of this load about some mean value would be a good thing, if our primary concern were optimizing grip (and we had a driver that could make use of it).
So you might wonder why it would advantageous to have a wheel (tire and rim assembly) working against a high damper force in rebound.

johnny comelately
110
Joined: 10 Apr 2015, 00:55
Location: Australia

Re: Wheel frequencies VS track surface

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spacer wrote:
24 Jul 2016, 14:03
Hi guys,

Regarding wheel rates, or actually, wheel frequencies; I've always tried to set up my car with the filosophy "get the softest springs you can get away with without running into problems somewhere along the track or limiting your setup flexibility". But recently I started having doubts over that assumption. Oh and we don't get any testing sessions so I'm curious about your thoughts and am interested in the theoretical side before throwing away some of the actual race results ;).

So please enlighten me :) .

Say we have the following situation:
-FWD car
-no aero whatsoever, all low speed <80km/h corners, mechanical grip is focus
-sharp reaction in transient conditions isn't of much concern, tracks don't have chicanes (mostly ovals) and tracks can often be driven fastest with smooth driving
-car doesn't bottom out anywhere
-coilovers don't end up hitting bumpstops
-I'm quite happy with curent suspension geometry and changes in wheel travel (double wishbone), so body roll is not something I necessarily want to reduce
-car is driven on offroad/gravel/dirttrack type surfaces
-we have enough setup range/flexibility to set up the car well balanced
-front unequal length double wishbone, no ARB. rear trailing arm, with additional ARB

Despite the above, are there any reasons why the softer sprung car is worse than having it stiffer sprung?

Now the reason I'm asking is this: our car is (by some margin) the softest sprung car of all the paddock. At some events we've got the fastest car and are in front, often lapping 0.5sec faster at a 20sec lap than the rest of the field. However, once we get to tracks with a lot smoother surfaces we tend to drop back and fight for P5, struggling for midcorner grip (U/S) and some corner-exit traction. And I mean dirt tracks that are black from begin rubbered in, having the tires squeeling.

This has me searching for what fundamental issue we have at these events, compared to our collegues.
Tyres are the same btw, car weight and distribution nearly identical, and we should have an drivetrain advantage. So this has me looking at the spring rates.

Wheel frequencies our car:
~100 CPM front, ~110 CPM rear
Wheel frequencies competition:
~115 CPM front, ~115 CPM rear

Many thanks for your insights!

-Tom
My two bob's worth (from a 1970's superstar :wink: before formulas were invented)
You race a fwd car at low speeds requiring only good mechanical grip on a variety of tracks some being bumpy where you go well and at the smooth tracks you dont. What are the anti roll bar specs? What tyre pressure changes between tracks?
The soft spring you have allows good compliance with the road bumps and overall corner speed is reduced compared to equivalent smooth track due to these bumps (compliant suspension or not) and the damper rates seem to be working.
Then going to smooth tracks with the same set up you are slower than the competition: because of smooth track corner speed is up and the spring rate is not up to it, giving more body roll where load is all on outside tyres.
therefore wouldnt you go up in spring rate and adjust damping accordingly?

DaveW
239
Joined: 14 Apr 2009, 12:27

Re: Wheel frequencies VS track surface

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WilO wrote:
25 Feb 2018, 23:36
...
I thought the diagram was good but, after reading the text twice, I was as confused as the next person. One thing I am sure about, however, rebound-biased damping is not necessarily a good idea for lap time.
.
johnny comelately wrote:
26 Feb 2018, 09:45
Then going to smooth tracks with the same set up you are slower than the competition: because of smooth track corner speed is up and the spring rate is not up to it, giving more body roll where load is all on outside tyres.
therefore wouldnt you go up in spring rate and adjust damping accordingly?
I think the advice is a little late (2016), but good if all else fails.

I have experienced two issues running "soft" springs, one is tyre temperature, the other a font suspension stability problem. The first is fairly obvious, the other not so.

Tyres "work" well in a relatively narrow band of temperatures, ouside of that "bucket", performance drops off. Track surface affects work done by the tyres and hence temperature. Drivers can help, but often don't. I first encountered the stability problem playing with active suspension. It is best described as lateral tyre slip coupling with vertical suspension response. One of my BTCC customers lost performance when they encountered it after reducing springs - the solution was as you suggested.

Rustem 1988
0
Joined: 05 Sep 2017, 11:38

Re: Wheel frequencies VS track surface

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I read an article that says race car will require the damping ratio of 0.5-0.7 to control the heave, pitch and roll resonances of the sprung mass, and a damping ratio of 0.3-0.5 to control the unsprung mass.
I think something like the ratio might be for other racing cars.
Why is the damping ratio low for the wheels? What problems can occur for wheels without sufficient damping? Are these problems mainly at resonance frequencies?

gambler
1
Joined: 12 Dec 2009, 19:29
Location: Virginia USA
Contact:

Re: Wheel frequencies VS track surface

Post

DaveW wrote:
22 Feb 2018, 23:53
Broadly, your definitions follow conventional wisdom. I kind of agree with that Greg, but not necessarily with your arguments. You are right about seats, however. But that is another topic.

Suspension, as you know, has to work against the sprung mass. The higher the effective sprung mass that better will (or can) be the ride (all things being equal).

For one high end production road vehicle I tested, the effective sprung mass fell to 18% of actual over a limited frequency range in the region of 10Hz. Just to emphasise that, generating a damper force at that frequency would require over 5 times the sprung mass acceleration of a monolith. It often pays to know that, because damper adjustments won’t cut it (will just make matters worse).

Interestingly, at that frequency the engine mounts on my example actually dissipated as much energy introduced by road inputs as the dampers & tyres combined. I wonder how long they will survive.
I appreciate the mathmatical explinations, way over my head. So just a quick question, is it possible to hit a bad frequency that will "heat" the tire carcas to the point it misleads the tire engineer with wrong tire temps in regard to how the chassis is actually working? I can also see this as an advantage to build heat during qualifying if it is possible? P.S. No doubt your shock man has a terminal headache ! haha

DaveW
239
Joined: 14 Apr 2009, 12:27

Re: Wheel frequencies VS track surface

Post

Rustem 1988 wrote:
26 Feb 2018, 15:09
I read an article ....
Rustem: A reference (citation) might help....
.
gambler wrote:
26 Feb 2018, 15:33
.. So just a quick question, is it possible to hit a bad frequency that will "heat" the tire carcas to the point it misleads the tire engineer with wrong tire temps in regard to how the chassis is actually working? I can also see this as an advantage to build heat during qualifying if it is possible?
I suppose it is possible, but usually only in the event of an instability.

Heating tyres for qualifying was (& probably still is) the key to qualifying well. In the past damper and bar settings were adjusted to work the tyres harder, but that is not now an option in F1, so the driver must do it with driving strategy. Tyre pressures would also help, but now that is frowned upon...

Rustem 1988
0
Joined: 05 Sep 2017, 11:38

Re: Wheel frequencies VS track surface

Post

WilO wrote:
25 Feb 2018, 23:36
Rustem,

While we wait for someone who really knows what they're talking about to respond, you might consider the content of the text you'd quoted, and whether or not it makes sense to you. Particularly the last bit about the direction of spring and damper forces in compression and rebound.
One of the fundamental parameters affecting 'grip' is the normal load on the tire. As such, minimizing the variation of this load about some mean value would be a good thing, if our primary concern were optimizing grip (and we had a driver that could make use of it).
So you might wonder why it would advantageous to have a wheel (tire and rim assembly) working against a high damper force in rebound.
Pitch and roll also have a lot of influence on the ratio of the values of the shock absorber and compression?

Rustem 1988
0
Joined: 05 Sep 2017, 11:38

Re: Wheel frequencies VS track surface

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The bounce of the wheel on the road surface may be related to the fact that the movement of the sprung mass becomes more than the movement of the road input? For this, we probably need to know the complete picture of the movement and acceleration of the wheel on the road surface.

gambler
1
Joined: 12 Dec 2009, 19:29
Location: Virginia USA
Contact:

Re: Wheel frequencies VS track surface

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10-4 Thanks, Dave.

Greg Locock
233
Joined: 30 Jun 2012, 00:48

Re: Wheel frequencies VS track surface

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"The bounce of the wheel on the road surface may be related to the fact that the movement of the sprung mass becomes more than the movement of the road input? For this, we probably need to know the complete picture of the movement and acceleration of the wheel on the road surface."

Yes. the usual way is to measure distance to the ground from the body with a laser, and a stringpot to measure the wheel's motion relative to the body. The difference between the two is the tire deflection+airgap. If you add a suitable accelerometer or differential GPS to the car you can then map its absolute height and so work back to the road profile. That, more or less, is how our lot measure road profiles.

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