I have been working my way through the book "Race Car Vehicle Dynamics"
(which I got out of our company library - it's great working for an auto
company) and I noticed that in the examples around pgs. 580 - 600 or so
it is said that you want the front ride frequency to be higher than the
rear. I did not see any explanation for this. Why would this be true?
Thanks,
Hal
Why front ride frequency should exceed the rear
Haqsau asked why Race Car Vehicle Dynamics recommends a higher front ride frequency than rear. Jonny Hodgson explained that road cars do the opposite for pitch cancellation over bumps, and Doug Milliken explained that rear-drive cars need most of their roll stiffness at the front to put power down, which in practice leads to stiffer front springs. The discussion clarified the roles of sprung mass, wheel rate and anti-roll bars.
physics/realism debatesetup/driving technique
Automated summary.
This is a completely **Uneducated** guess, I just want to see if I'm
right or not.
While accelerating, the weight of the car is shifted rearward. With the
rear being higher in the first place, while accelerating the car will be
level. Of course while breaking, that theory is shot out the window(?) Now
I'd like to find out how close I am to being right.
"Haqsau" <[email hidden]> wrote in message
news:ursk7.45822$[email hidden]...
Quoted text (10 lines)
> I have been working my way through the book "Race Car Vehicle Dynamics" > (which I got out of our company library - it's great working for an auto > company) and I noticed that in the examples around pgs. 580 - 600 or so > it is said that you want the front ride frequency to be higher than the > rear. I did not see any explanation for this. Why would this be true? > > Thanks, > Hal > >I think you are thinking of ride height. I'm referring to the natural
frequency of the suspension, i.e. at what frequency would it bounce up
and down if it had no dampers. The Millikens' say the front frequency
should be higher than the rear, and I think I have heard this elsewhere
too, but I would like to know why.
"Michael Barlow" <[email hidden]> wrote in message
news:rUsk7.407252$[email hidden]...
> This is a completely **Uneducated** guess, I just want to see ifI'm
> right or not.
>
> While accelerating, the weight of the car is shifted rearward.With the
> rear being higher in the first place, while accelerating the car willbe
> level. Of course while breaking, that theory is shot out thewindow(?) Now
> I'd like to find out how close I am to being right.
>
>
> "Haqsau" <[email hidden]> wrote in message
> news:ursk7.45822$[email hidden]...
> > I have been working my way through the book "Race Car VehicleDynamics"
> > (which I got out of our company library - it's great working for anauto
> > company) and I noticed that in the examples around pgs. 580 - 600 orso
> > it is said that you want the front ride frequency to be higher thanthe
> > rear. I did not see any explanation for this. Why would this betrue?
Quoted text (7 lines)
> > > > Thanks, > > Hal > > > > > >Maybe because the weight of the engine in the front???
"Haqsau" <[email hidden]> wrote in message
news:ursk7.45822$[email hidden]...
Quoted text (10 lines)
> I have been working my way through the book "Race Car Vehicle Dynamics" > (which I got out of our company library - it's great working for an auto > company) and I noticed that in the examples around pgs. 580 - 600 or so > it is said that you want the front ride frequency to be higher than the > rear. I did not see any explanation for this. Why would this be true? > > Thanks, > Hal > >
Billy Hutson <[email hidden]> wrote in message
news:j0yk7.21418$[email hidden]...
> Maybe because the weight of the engine in the front???
Well no, since the axle weight is accounted for in the nat freq
calculations.
It's the other way round for road cars, to achieve "pitch
cancellation" over bumps - the rear 'catches up' the front so that it
rapidly becomes a bounce mode, rather than pitch, which is more
comfortable.
Jonny
> "Haqsau" <[email hidden]> wrote in message
> news:ursk7.45822$[email hidden]...
> > I have been working my way through the book "Race Car VehicleDynamics"
> > (which I got out of our company library - it's great working foran auto
> > company) and I noticed that in the examples around pgs. 580 - 600or so
> > it is said that you want the front ride frequency to be higherthan the
> > rear. I did not see any explanation for this. Why would this betrue?
Quoted text (7 lines)
> > > > Thanks, > > Hal > > > > > >LOL OMG......
Dude, I just drive the race car, I don't squabble in here over all this
technical crap......
Go fast...... turn left.
"Jonny Hodgson" <[email hidden]> wrote in message
news:DpPk7.35981$[email hidden]...
Quoted text (36 lines)
> > Billy Hutson <[email hidden]> wrote in message > news:j0yk7.21418$[email hidden]... > > Maybe because the weight of the engine in the front??? > > Well no, since the axle weight is accounted for in the nat freq > calculations. > > It's the other way round for road cars, to achieve "pitch > cancellation" over bumps - the rear 'catches up' the front so that it > rapidly becomes a bounce mode, rather than pitch, which is more > comfortable. > > Jonny > > > "Haqsau" <[email hidden]> wrote in message > > news:ursk7.45822$[email hidden]... > > > I have been working my way through the book "Race Car Vehicle > Dynamics" > > > (which I got out of our company library - it's great working for > an auto > > > company) and I noticed that in the examples around pgs. 580 - 600 > or so > > > it is said that you want the front ride frequency to be higher > than the > > > rear. I did not see any explanation for this. Why would this be > true? > > > > > > Thanks, > > > Hal > > > > > > > > > > > >Okay the pitch cancellation part makes sense, but I'm still not sure why
the front should have the higher frequency. Can you explain that?
Thanks,
Hal
"Jonny Hodgson" <[email hidden]> wrote in message
news:DpPk7.35981$[email hidden]...
Quoted text (36 lines)
> > Billy Hutson <[email hidden]> wrote in message > news:j0yk7.21418$[email hidden]... > > Maybe because the weight of the engine in the front??? > > Well no, since the axle weight is accounted for in the nat freq > calculations. > > It's the other way round for road cars, to achieve "pitch > cancellation" over bumps - the rear 'catches up' the front so that it > rapidly becomes a bounce mode, rather than pitch, which is more > comfortable. > > Jonny > > > "Haqsau" <[email hidden]> wrote in message > > news:ursk7.45822$[email hidden]... > > > I have been working my way through the book "Race Car Vehicle > Dynamics" > > > (which I got out of our company library - it's great working for > an auto > > > company) and I noticed that in the examples around pgs. 580 - 600 > or so > > > it is said that you want the front ride frequency to be higher > than the > > > rear. I did not see any explanation for this. Why would this be > true? > > > > > > Thanks, > > > Hal > > > > > > > > > > > >
Haqsau <[email hidden]> wrote in message
news:NdSk7.13622$[email hidden]...
> Okay the pitch cancellation part makes sense, but I'm still not surewhy
> the front should have the higher frequency. Can you explain that?
Not really! Does that section refer specifically to, say,
front-engined, rear-drive cars? IIRC there was some general
theorising that the roll couple distribution should reflect the weight
distribution... but then there are anti-roll bars to do that without
upsetting ride frequencies.
Jonny
it
for
> > an auto
> > > > company) and I noticed that in the examples around pgs. 580 -600
> > or so
> > > > it is said that you want the front ride frequency to be higher
> > than the
> > > > rear. I did not see any explanation for this. Why would thisbe
Quoted text (12 lines)
> "Jonny Hodgson" <[email hidden]> wrote in message > news:DpPk7.35981$[email hidden]... > > > > Billy Hutson <[email hidden]> wrote in message > > news:j0yk7.21418$[email hidden]... > > > Maybe because the weight of the engine in the front??? > > > > Well no, since the axle weight is accounted for in the nat freq > > calculations. > > > > It's the other way round for road cars, to achieve "pitch > > cancellation" over bumps - the rear 'catches up' the front so thatQuoted text (10 lines)
> > rapidly becomes a bounce mode, rather than pitch, which is more > > comfortable. > > > > Jonny > > > > > "Haqsau" <[email hidden]> wrote in message > > > news:ursk7.45822$[email hidden]... > > > > I have been working my way through the book "Race Car Vehicle > > Dynamics" > > > > (which I got out of our company library - it's great workingQuoted text (11 lines)
> > true? > > > > > > > > Thanks, > > > > Hal > > > > > > > > > > > > > > > > > > >Can you got right too?? :) Sorry I couldnt resist!!
"Billy Hutson" wrote:
| LOL OMG......
|
| Dude, I just drive the race car, I don't squabble in here over all this
| technical crap......
|
| Go fast...... turn left.
---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
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The example that I was looking at was referring to a rear drive, solid
axle car that oddly enough was heavier in the rear. But the statement
on front ride frequency being higher than rear seemed to be a general
statement that did not apply only to this vehicle. And yes, it was
treated independently of roll couple. The examples calculate the spring
rate first, based on ride frequency, and then look at the resulting roll
couple and roll gain and adjust it with ARB's. The interesting thing is
that undamped ride frequency is a function of the static deflection of
the suspension, so that a vehicle with higher front ride frequency would
have lower static deflection in front. While this seems to agree with
some real life setups I have seen, some prominent on-line setup guides
advise spring rates that give equal static deflection front and rear.
That's part of why I'm trying to figure this out. The other reason is
that the examples pretty much give you an easy first cut at how to set
spring rates (or really wheel rates). If you go through the formulae
and charts, it implies that for race cars you want somewhere around 2 to
4 inches of static deflection (wheel deflection due to the car's
weight), with slightly less in front, and maybe about half of that if
you are setting up a car with significant downforce devices. That's a
very handy rule of thumb to have if we can just make sense of it!
"Jonny Hodgson" <[email hidden]> wrote in message
news:vuSk7.37467$[email hidden]...
Quoted text (7 lines)
> Not really! Does that section refer specifically to, say, > front-engined, rear-drive cars? IIRC there was some general > theorising that the roll couple distribution should reflect the weight > distribution... but then there are anti-roll bars to do that without > upsetting ride frequencies. > > Jonny
Haqsau <[email hidden]> wrote in message
news:nJUk7.13632$[email hidden]...
> on front ride frequency being higher than rear seemed to be ageneral
> statement that did not apply only to this vehicle. And yes, it was
Okay, just thought I'd check.
> have lower static deflection in front. While this seems to agreewith
> some real life setups I have seen, some prominent on-line setupguides
> advise spring rates that give equal static deflection front andrear.
When I went for a MonsterRank recently, I started with equal and ended
up with front slightly stiffer... My 4wd, off-road R/C car runs the
back somewhat stiffer I believe, but I also think that's due to the
way tyres are supplied out of their bags - once I've cut'n'shut them,
it moves closer to a sensible spring balance. Perhaps it's time I had
another look at those calculations...
> and charts, it implies that for race cars you want somewhere around2 to
> 4 inches of static deflection (wheel deflection due to the car's
> weight), with slightly less in front, and maybe about half of thatif
> you are setting up a car with significant downforce devices. That'sa
> very handy rule of thumb to have if we can just make sense of it!
Absolutely! Now, how does it translate to 1/10th scale? ;-)
Jonny
Stabbing in the dark a bit, but won't the frequency be related to the
unsprung weight? In which case you can change the spring rates to change
static deflection but the lighter spring is working on the same mass....or
something....and the mass of the drive train makes the rear frequency lower.
(?)
Ben
Haqsau wrote in message ...
Quoted text (33 lines)
>The example that I was looking at was referring to a rear drive, solid >axle car that oddly enough was heavier in the rear. But the statement >on front ride frequency being higher than rear seemed to be a general >statement that did not apply only to this vehicle. And yes, it was >treated independently of roll couple. The examples calculate the spring >rate first, based on ride frequency, and then look at the resulting roll >couple and roll gain and adjust it with ARB's. The interesting thing is >that undamped ride frequency is a function of the static deflection of >the suspension, so that a vehicle with higher front ride frequency would >have lower static deflection in front. While this seems to agree with >some real life setups I have seen, some prominent on-line setup guides >advise spring rates that give equal static deflection front and rear. >That's part of why I'm trying to figure this out. The other reason is >that the examples pretty much give you an easy first cut at how to set >spring rates (or really wheel rates). If you go through the formulae >and charts, it implies that for race cars you want somewhere around 2 to >4 inches of static deflection (wheel deflection due to the car's >weight), with slightly less in front, and maybe about half of that if >you are setting up a car with significant downforce devices. That's a >very handy rule of thumb to have if we can just make sense of it! > > >"Jonny Hodgson" <[email hidden]> wrote in message >news:vuSk7.37467$[email hidden]... >> Not really! Does that section refer specifically to, say, >> front-engined, rear-drive cars? IIRC there was some general >> theorising that the roll couple distribution should reflect the weight >> distribution... but then there are anti-roll bars to do that without >> upsetting ride frequencies. >> >> Jonny > >Yes, exactly, the frequency is a function of the wheel rate and the
unsprung mass supported by that wheel. But the question is not, "why is
the rear frequency lower?" but rather "why do the Millikens and other
advise that we set the chassis up so that the rear frequency is lower?".
It tends to work out that if the unsprung weight distribution is even
close to 50%, which it would be for many race cars, that you want
stiffer springs in front. This is apart from any roll rate
considerations, so it is not based on trying to get understeer. I'm
wondering if this is related to stability and control, or is it just a
carryover from passenger car design which is based on ride comfort.
"Ben Coleman" <[email hidden]> wrote in message
news:[email hidden]...
> Stabbing in the dark a bit, but won't the frequency be related to the
> unsprung weight? In which case you can change the spring rates tochange
> static deflection but the lighter spring is working on the samemass....or
> something....and the mass of the drive train makes the rear frequencylower.
> (?)
>
> Ben
Whoops, I mispoke there. In the previous message I meant to say sprung
mass and sprung weight, not unsprung. The unsprung mass does not affect
the ride frequency of the vehicle, although I suppose you could
calculate a natural frequency for the suspension based on the unsprung
mass and the tire rate. Also I suppose you could get into coupling
between the sprung and unsprung mass if their natural frequencies are
harmonics of each other, but that is way beyond the scope of what I am
trying to figure out here.
"Haqsau" <[email hidden]> wrote in message
news:fBfl7.56266$[email hidden]...
> Yes, exactly, the frequency is a function of the wheel rate and the
> unsprung mass supported by that wheel. But the question is not, "whyis
> the rear frequency lower?" but rather "why do the Millikens and other
> advise that we set the chassis up so that the rear frequency islower?".
the
> > unsprung weight? In which case you can change the spring rates to
> change
> > static deflection but the lighter spring is working on the same
> mass....or
> > something....and the mass of the drive train makes the rearfrequency
> lower.
> > (?)
> >
> > Ben
>
>
Quoted text (11 lines)
> It tends to work out that if the unsprung weight distribution is even > close to 50%, which it would be for many race cars, that you want > stiffer springs in front. This is apart from any roll rate > considerations, so it is not based on trying to get understeer. I'm > wondering if this is related to stability and control, or is it just a > carryover from passenger car design which is based on ride comfort. > > > "Ben Coleman" <[email hidden]> wrote in message > news:[email hidden]... > > Stabbing in the dark a bit, but won't the frequency be related toYeah, in the pits......
"Thom j." <[email hidden]> wrote in message
news:OTSk7.166856$[email hidden]...
Quoted text (18 lines)
> Can you got right too?? :) Sorry I couldnt resist!! > > "Billy Hutson" wrote: > | LOL OMG...... > | > | Dude, I just drive the race car, I don't squabble in here over all this > | technical crap...... > | > | Go fast...... turn left. > > > > --- > Outgoing mail is certified Virus Free. > Checked by AVG anti-virus system (http://www.grisoft.com). > Version: 6.0.276 / Virus Database: 145 - Release Date: 9/3/2001 > >which track? :)
"Billy Hutson" wrote:
this
| > | technical crap......
| > |
| > | Go fast...... turn left.
---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
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Quoted text (10 lines)
| Yeah, in the pits...... | | | "Thom j." wrote: | > Can you got right too?? :) Sorry I couldnt resist!! | > | > "Billy Hutson" wrote: | > | LOL OMG...... | > | | > | Dude, I just drive the race car, I don't squabble in here over allwhich track? :)
this
| > | technical crap......
| > |
| > | Go fast...... turn left.
---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com
Version: 6.0.276 / Virus Database: 145 - Release Date: 9/3/2001
Quoted text (9 lines)
| Yeah, in the pits...... | | | > Can you got right too?? :) Sorry I couldnt resist!! | > | > | LOL OMG...... | > | | > | Dude, I just drive the race car, I don't squabble in here over all
Well, tonight all of the ras posts I've been missing all arrived
at once...
The basic problem is that rear drive cars generally require that most of
the roll torque be taken at the front--if you don't do this, then it will
be difficult to accelerate out of corners. It's not usually practical to
get a large roll stiffness entirely from the anti-roll bar on a real car
(things bend!) -- so the engineering solution is to increase the front ride
stiffness (and accept the crappy ride) so that the ride springs provide
part of the desired roll stiffness.
On Sun, 2 Sep 2001, Haqsau wrote:
Quoted text (12 lines)
> I have been working my way through the book "Race Car Vehicle Dynamics" > (which I got out of our company library - it's great working for an auto > company) and I noticed that in the examples around pgs. 580 - 600 or so > it is said that you want the front ride frequency to be higher than the > rear. I did not see any explanation for this. Why would this be true? > > Thanks, > Hal > > > >Doug,
Thanks for that answer. I was confused because I saw the roll stiffness
being handled separately, after the calculation of ride frequency. But
based on what you and Jonny Hodgson have said it seems that you need to
have the ride frequencies be slightly different front and rear (and
presumably with no common modes) in order to prevent cyclic pitching,
and you arbitrarily choose the front as the higher one since you know
that you will need it to be higher when you get to working out the roll
stiffness and roll couple. Does that sound correct?
Thanks,
Hal
"Doug Milliken" <[email hidden]> wrote in message
news:[email hidden]...
>
> Well, tonight all of the ras posts I've been missing all arrived
> at once...
>
> The basic problem is that rear drive cars generally require that mostof
> the roll torque be taken at the front--if you don't do this, then itwill
> be difficult to accelerate out of corners. It's not usually practicalto
> get a large roll stiffness entirely from the anti-roll bar on a realcar
> (things bend!) -- so the engineering solution is to increase the frontride
> stiffness (and accept the crappy ride) so that the ride springsprovide
> part of the desired roll stiffness.
>
>
> On Sun, 2 Sep 2001, Haqsau wrote:
>
> > I have been working my way through the book "Race Car VehicleDynamics"
> > (which I got out of our company library - it's great working for anauto
> > company) and I noticed that in the examples around pgs. 580 - 600 orso
> > it is said that you want the front ride frequency to be higher thanthe
> > rear. I did not see any explanation for this. Why would this betrue?
Quoted text (8 lines)
> > > > Thanks, > > Hal > > > > > > > > >
Haqsau <[email hidden]> wrote in message
news:eFOm7.28795$[email hidden]...
> Doug,
>
> Thanks for that answer. I was confused because I saw the rollstiffness
> being handled separately, after the calculation of ride frequency.But
> based on what you and Jonny Hodgson have said it seems that you needto
> have the ride frequencies be slightly different front and rear (and
> presumably with no common modes) in order to prevent cyclicpitching,
> and you arbitrarily choose the front as the higher one since youknow
> that you will need it to be higher when you get to working out theroll
> stiffness and roll couple. Does that sound correct?
1) For road cars, the rear needs to be the stiffer end so that it
'catches up' after a bump (1.0 / 1.1 Hz are the target frequencies we
were given). The exception would be if you wanted to drive in reverse
lots... ;-)
2) Further thought suggests that the car will suffer greater pitching
moments under braking, so stiffer springs at the front allow ride
height to be better controlled as well as permitting the front to be
lower than the rear, as it's usually preferred to be.
Jonny
