How the rear end of the DC2 chassis works against us...
08-30-2003, 09:37 PM
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How the rear end of the DC2 chassis works against us...
The thread on Bushings - https://honda-tech.com/zerothread?id=493789 - got me thinking that a closer look at the rear suspension on a DC2 would be interesting.
Given that for most of us our biggest problem is understeer, any part of the car that works against us deserves particular attention. It's very easy to overlook the dynamic toe behavior at the rear - but it's worth the time, even if only for the sake of understanding.
Our general rule is get the most work out of the poor overworked front as possible, and then shed enough rear grip with our camber setting to get a driveable balance.
But what about the direction the outside rear wheel is pointing? Toe-in in roll at the rear of an FWD car - a Race Car - is precisely NOT what we need. But that is exactly what we get with a DC2, the origins of which, let us not forget, are in safety philosophy as it pertains to transportation vehicle design.
Let's start at the usual beginning. Here's the basic bump steer I did some time back, where the zero datum is at 5.5 inches to the rockers on 23 inch tires.
Bump---2---0.085 IN
xxxxx---1---0.060 IN
xxxxx---0---0
xxxxx---1---0
Droop---2---0.040 OUT
*Measurements are Total (L + R). Note that 1/16 = 0.0625
If you've played around with your rear toe settings much you know how sensitive these cars are, and how significant 1/16 or so of static toe change can be. As it happens the dynamic toe change is likely greater than that.
The dynamic toe curve is controlled by the geometry of the trailing arm, lateral control arms, and the toe link.
In rear view the virtual swing arm formed by the lateral control arms is about as long as the track, and is of variable length thru the range of wheel travel. There is a disproportionality between the relationship of the lengths of the rear view virtual swing arm to the segment of the trailing arm from axle to bushing, and the length of the toe link to the nose of the trailing arm that invites attention - none of this being an accident.
In top view the whole thing is a skew quadrilateral in warp (non-planar). Lateral forces are seen to be resolved between the lateral control arms and the toe link - NOT the main trailing arm bushing.
If the trailing arm bushing is replaced by a spherical bearing the suspension design is transformed into something more akin to a wide based semi-trailing arm with an undesirably acute angle. The toe link then does nothing - nothing, aside from try to rip the nose off the trailing arm. Only if the spherical bearing is free to move laterally on the pivot shaft is such an application sensible at all - but the shaft and bearing will wear quickly.
Looking at the assembly in side view shows the axle position forward of the axis created by the outboard lateral control arm pivots. Lateral load (outside wheel) compresses the toe link. And because the toe link is splayed forward this pushes the trailing arm rearward, generating even more toe-in. Think of this as a mildly self amplifying response to lateral load. And don't forget about the toe-in from the bump portion of the bump steer curve (here as roll steer).
Under braking there are several concurrent reactions. There is rearward deflection at the trailing arm bushing (toe-in factor). There is the moment about the outboard lateral pivots axis (toe-out factor). And there is the toe out in droop from the bump steer curve. Keep in mind how squirelly under brakes these cars are with much beyond zero static toe, and you've got an idea how these factors add up.
IT ADDS UP TO ALOT OF DYNAMIC TOE-IN UNDER EVERY OPERATING CONDITION EXCEPT BRAKING.
Oh, yeah, trailing brake and mid corner left foot braking? Trailing brake yes...maybe...depends. Mid corner left foot braking...not for me...the fix is worse than the problem.
So, there it is - the outside rear is being dynamically steered inward toward the cg and contributing to our basic understeer problem.
What can we do?
The stiffer Mugen trailing arm bushings reduce the rearward deflection under braking. This reduces some of the induced toe-in factor under braking. In conjunction with the narrower envelope associated with race spring rates and use of harder bushings elsewhere in the system it's a useful item.
Use of toe-links made up out of spherical rod ends will reduce toe change. The loss of compliance will cost in the life expectancy of the structure at both ends, but it's certainly the easiest partial remedy.
Relocation of the toe link inner pivot location about an inch lower would re-zero the bump steer curve at the racing ride height and address the toe out in droop within the envelope allowing a little more static toe out without braking worries.
They get harder from here on.
Re-engineer both the location and length of the toe links.
Reduce the offset of the rear axle with respect to the axis between the outboard ends of the lateral control arms. This would require major surgery on the trailing arms or fabrication of replacements.
Most of these are illegal modifications in the most common racing classes, but that's the scoop.
Scott, who thinks it's a nice design overall, and not that unfriendly to improvement...you just gotta be careful to not create any new problems...
Modified by RR98ITR at 10:57 AM 8/31/2003
Given that for most of us our biggest problem is understeer, any part of the car that works against us deserves particular attention. It's very easy to overlook the dynamic toe behavior at the rear - but it's worth the time, even if only for the sake of understanding.
Our general rule is get the most work out of the poor overworked front as possible, and then shed enough rear grip with our camber setting to get a driveable balance.
But what about the direction the outside rear wheel is pointing? Toe-in in roll at the rear of an FWD car - a Race Car - is precisely NOT what we need. But that is exactly what we get with a DC2, the origins of which, let us not forget, are in safety philosophy as it pertains to transportation vehicle design.
Let's start at the usual beginning. Here's the basic bump steer I did some time back, where the zero datum is at 5.5 inches to the rockers on 23 inch tires.
Bump---2---0.085 IN
xxxxx---1---0.060 IN
xxxxx---0---0
xxxxx---1---0
Droop---2---0.040 OUT
*Measurements are Total (L + R). Note that 1/16 = 0.0625
If you've played around with your rear toe settings much you know how sensitive these cars are, and how significant 1/16 or so of static toe change can be. As it happens the dynamic toe change is likely greater than that.
The dynamic toe curve is controlled by the geometry of the trailing arm, lateral control arms, and the toe link.
In rear view the virtual swing arm formed by the lateral control arms is about as long as the track, and is of variable length thru the range of wheel travel. There is a disproportionality between the relationship of the lengths of the rear view virtual swing arm to the segment of the trailing arm from axle to bushing, and the length of the toe link to the nose of the trailing arm that invites attention - none of this being an accident.
In top view the whole thing is a skew quadrilateral in warp (non-planar). Lateral forces are seen to be resolved between the lateral control arms and the toe link - NOT the main trailing arm bushing.
If the trailing arm bushing is replaced by a spherical bearing the suspension design is transformed into something more akin to a wide based semi-trailing arm with an undesirably acute angle. The toe link then does nothing - nothing, aside from try to rip the nose off the trailing arm. Only if the spherical bearing is free to move laterally on the pivot shaft is such an application sensible at all - but the shaft and bearing will wear quickly.
Looking at the assembly in side view shows the axle position forward of the axis created by the outboard lateral control arm pivots. Lateral load (outside wheel) compresses the toe link. And because the toe link is splayed forward this pushes the trailing arm rearward, generating even more toe-in. Think of this as a mildly self amplifying response to lateral load. And don't forget about the toe-in from the bump portion of the bump steer curve (here as roll steer).
Under braking there are several concurrent reactions. There is rearward deflection at the trailing arm bushing (toe-in factor). There is the moment about the outboard lateral pivots axis (toe-out factor). And there is the toe out in droop from the bump steer curve. Keep in mind how squirelly under brakes these cars are with much beyond zero static toe, and you've got an idea how these factors add up.
IT ADDS UP TO ALOT OF DYNAMIC TOE-IN UNDER EVERY OPERATING CONDITION EXCEPT BRAKING.
Oh, yeah, trailing brake and mid corner left foot braking? Trailing brake yes...maybe...depends. Mid corner left foot braking...not for me...the fix is worse than the problem.
So, there it is - the outside rear is being dynamically steered inward toward the cg and contributing to our basic understeer problem.
What can we do?
The stiffer Mugen trailing arm bushings reduce the rearward deflection under braking. This reduces some of the induced toe-in factor under braking. In conjunction with the narrower envelope associated with race spring rates and use of harder bushings elsewhere in the system it's a useful item.
Use of toe-links made up out of spherical rod ends will reduce toe change. The loss of compliance will cost in the life expectancy of the structure at both ends, but it's certainly the easiest partial remedy.
Relocation of the toe link inner pivot location about an inch lower would re-zero the bump steer curve at the racing ride height and address the toe out in droop within the envelope allowing a little more static toe out without braking worries.
They get harder from here on.
Re-engineer both the location and length of the toe links.
Reduce the offset of the rear axle with respect to the axis between the outboard ends of the lateral control arms. This would require major surgery on the trailing arms or fabrication of replacements.
Most of these are illegal modifications in the most common racing classes, but that's the scoop.
Scott, who thinks it's a nice design overall, and not that unfriendly to improvement...you just gotta be careful to not create any new problems...
Modified by RR98ITR at 10:57 AM 8/31/2003
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I'll have to re-read it in the morning...
08-31-2003, 06:14 AM
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Re: How the rear end of the DC2 chassis works against us... (RR98ITR)
What can we do? How about raising the rear end?
Since most of the weight is up front, that shouldn't raise the CG by that much. And it would, like you said, re-zero the toe curve.
Since most of the weight is up front, that shouldn't raise the CG by that much. And it would, like you said, re-zero the toe curve.
08-31-2003, 12:21 PM
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Re: How the rear end of the DC2 chassis works against us... (RR98ITR)
Does the rear toe become more of an issue when you go from stock to more agressive setups? IE race rate springs and suspension? Or did I miss something or just not understand it all? (most likely the latter.)
I've read and reread it, of course a diagram would be phenomenal.
My position pertains to stock setup but, to learn more with the car in stock form. Will I become more aware of the toe issue when I make the jump to a race suspension?
Or is this whole topic something that can be written off as "thats just how the car is, and just another of it's handling traits/ quirks?"
Good interesting stuff though
Thanks for your time and effort.
I've read and reread it, of course a diagram would be phenomenal.
My position pertains to stock setup but, to learn more with the car in stock form. Will I become more aware of the toe issue when I make the jump to a race suspension?
Or is this whole topic something that can be written off as "thats just how the car is, and just another of it's handling traits/ quirks?"
Good interesting stuff though
Thanks for your time and effort.
08-31-2003, 03:44 PM
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Re: How the rear end of the DC2 chassis works against us... (1GreyTeg)
gah! i'd imagine that the lower your ride height is, the larger the rear toe change per inch of travel. so the question is, will the lower ride height benefits pay off more then the "better" roll steer number?
nate-can only imagine how bad the roll steer is at his 4.25" ride height.
nate-can only imagine how bad the roll steer is at his 4.25" ride height.
08-31-2003, 06:28 PM
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Re: How the rear end of the DC2 chassis works against us... (solo-x)
Hey RR98ITR, How about the role of a stiff rear sway bar on toe? Greater roll stiffness should control toe-in on the loaded side...right?
As for slightly higher rear ride height, the ITR came from the factory with about 3/8" positive rake; perhaps this helps the toe curve? Whaddaya think?
As for slightly higher rear ride height, the ITR came from the factory with about 3/8" positive rake; perhaps this helps the toe curve? Whaddaya think?
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09-01-2003, 10:54 AM
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Re: How the rear end of the DC2 chassis works against us... (RR98ITR)
a camera session focusing on the rear wheel for a session... shooting before -and- after Mugen bushing upgrade would be cool....
09-01-2003, 10:56 AM
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Re: How the rear end of the DC2 chassis works against us... (GhettoRacer)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by GhettoRacer »</TD></TR><TR><TD CLASS="quote">a camera session focusing on the rear wheel for a session... shooting before -and- after Mugen bushing upgrade would be cool....
</TD></TR></TABLE>
Camera? That wont do anything for you other than give you some nifty footage. What would really be cool is some position sensors on the wheel wheel and wheels to get an accurate measurement of toe change through the corner.
</TD></TR></TABLE>
Camera? That wont do anything for you other than give you some nifty footage. What would really be cool is some position sensors on the wheel wheel and wheels to get an accurate measurement of toe change through the corner.
09-01-2003, 11:19 AM
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Re: How the rear end of the DC2 chassis works against us...
Actual hard data would certainly be cooler however, that is a lot more complex to setup than a camera (which could tell us a lot of the movement probably). Just trying to be realistic. How about I setup something with a cam, you figure out how to collect some real data for us.
This is what's on the mega dollar Foss-Tech Integra that Tom Lepper has. Although I'm not sure if this piece was from Foss-Tech, or from US aftermarket...
rest of Foss-Tech car at:
http://www.ghettoracer.com/car...x.htm
This is what's on the mega dollar Foss-Tech Integra that Tom Lepper has. Although I'm not sure if this piece was from Foss-Tech, or from US aftermarket...
rest of Foss-Tech car at:
http://www.ghettoracer.com/car...x.htm
09-01-2003, 07:52 PM
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Re: How the rear end of the DC2 chassis works against us... (GhettoRacer)
This is the best diagram if the ITR rear suspension I could find. Anyone care to try and illustrate some of the scenarios described above?
09-02-2003, 08:26 PM
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Re: How the rear end of the DC2 chassis works against us... (Floyd)
I don't mean to be unresponsive, but I don't have the time or inclination to provide drawings to illustrate my points. If you've had coursework in Dynamics and/or read the classics (Puhn, Smith, etc) then you should be able to picture what I've discussed pretty easily - and that drawing in Floyd's last post is sufficient.
The thing that this suspension design is meant to achieve is what Porsche popularized in the 928's design - dynamic toe control in response to force not position. It's quite brilliant, but it's just not optimal for some applications.
For other reasons we want to run lots of rear spring rate - and can get away with it. A happy side effect is reduced toe change. That helps.
There are no simple fixes beyond what I've mentioned...that I'm aware of.
Scott, who says beware of Road Runner Physics...
The thing that this suspension design is meant to achieve is what Porsche popularized in the 928's design - dynamic toe control in response to force not position. It's quite brilliant, but it's just not optimal for some applications.
For other reasons we want to run lots of rear spring rate - and can get away with it. A happy side effect is reduced toe change. That helps.
There are no simple fixes beyond what I've mentioned...that I'm aware of.
Scott, who says beware of Road Runner Physics...
09-03-2003, 12:09 AM
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Re: How the rear end of the DC2 chassis works against us... (RR98ITR)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by RR98ITR »</TD></TR><TR><TD CLASS="quote">Relocation of the toe link inner pivot location about an inch lower would re-zero the bump steer curve at the racing ride height and address the toe out in droop within the envelope allowing a little more static toe out without braking worries.</TD></TR></TABLE>
Would the trailing arm bushing attachment points have to be spaced as well?
Would the trailing arm bushing attachment points have to be spaced as well?
09-05-2003, 04:02 AM
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Re: How the rear end of the DC2 chassis works against us... (RR98ITR)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by RR98ITR »</TD></TR><TR><TD CLASS="quote">I don't mean to be unresponsive, but I don't have the time or inclination to provide drawings to illustrate my points. If you've had coursework in Dynamics and/or read the classics (Puhn, Smith, etc) then you should be able to picture what I've discussed pretty easily - and that drawing in Floyd's last post is sufficient..</TD></TR></TABLE>
I think that one might be hard pressed to learn about the intricacies of design inherent in the Honda/Acura rear suspension by reading books from Puhn and Smith. The Honda suspension is a road-car suspension with intentionally built-in flex or give (the field of elastokinematics), and has very many excellent characteristics, one of which is the control of noise, vibration and harshness, and another is excellent handling due to the control of track change in bump and rebound which gives excellent control of the roll-center height.
Road cars are made to understeer to be safe, and effects such as toe change under braking or roll or bump negociation should not upset the average driver's control of the car. Of course we all want to race road cars, but just have a look at a Formula Ford or Formula 2000 suspension and you will see how a double A-arm should be done, if you want low compliance and a well controlled chassis. However no one would buy a road car with spherical joints, as it would rattle your teeth out.
There is an excellent text which does describe the design of road car suspensions, and just happens to have many pages dedicated to the analysis of the Honda/Acura suspension, since it is such a fine example of design. The book is called "The Automotive Chassis" by J.Reimpell. It is an SAE book. I have scanned a few pages which show what Scott is talking about, and I think you will find the analysis enlightening.
Cover page:
Honda/Acura rear suspension:
Honda camber change in bump:
Toe-in during braking example:
I think these might help illustrate the effect.
I think that one might be hard pressed to learn about the intricacies of design inherent in the Honda/Acura rear suspension by reading books from Puhn and Smith. The Honda suspension is a road-car suspension with intentionally built-in flex or give (the field of elastokinematics), and has very many excellent characteristics, one of which is the control of noise, vibration and harshness, and another is excellent handling due to the control of track change in bump and rebound which gives excellent control of the roll-center height.
Road cars are made to understeer to be safe, and effects such as toe change under braking or roll or bump negociation should not upset the average driver's control of the car. Of course we all want to race road cars, but just have a look at a Formula Ford or Formula 2000 suspension and you will see how a double A-arm should be done, if you want low compliance and a well controlled chassis. However no one would buy a road car with spherical joints, as it would rattle your teeth out.
There is an excellent text which does describe the design of road car suspensions, and just happens to have many pages dedicated to the analysis of the Honda/Acura suspension, since it is such a fine example of design. The book is called "The Automotive Chassis" by J.Reimpell. It is an SAE book. I have scanned a few pages which show what Scott is talking about, and I think you will find the analysis enlightening.
Cover page:
Honda/Acura rear suspension:
Honda camber change in bump:
Toe-in during braking example:
I think these might help illustrate the effect.
09-05-2003, 02:33 PM
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Re: How the rear end of the DC2 chassis works against us... (turbohappy)
This is really good stuff (everything above!). it's been so long since I've seen a really technical Honda Tech article like this one!!
09-05-2003, 08:53 PM
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Re: How the rear end of the DC2 chassis works against us...
I met Lee N. @ ITR Expo, he is a chassis engineer with Honda. Lets see what he has to say... He was suppose to send me some data on ITR chassis reenforcements...
09-06-2003, 03:24 AM
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Re: How the rear end of the DC2 chassis works against us... (GhettoRacer)
Sorry about the mini scans, but they weren't really. The Honda-Tech web site removes the .orig.jpg at the end of the link to the image, such that only the thumbnail is linked to. They are posted in an album on Sony's imagestation site. The links are to the thumbnails Sony's site uses, and they show up as mini scans. You can just right click on the pictures to get the properties. Copy the URL link from the properties box and paste it into the URL bar on a browser window. Now add the following text .orig.jpg after the .jpg in the URL and this will take you to the full size image. The URL should end in .jpg.orig.jpg
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09-06-2003, 09:32 AM
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Re: How the rear end of the DC2 chassis works against us... (descartesfool)
these scans are awesome!! thanks a bunch for sharing.
09-07-2003, 09:27 AM
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Re: How the rear end of the DC2 chassis works against us... (GhettoRacer)
so without fabricating new trailing arms, lca's, uca's, toe links, and completely re-engineering the geometry and pivot points of the rear suspension...
basically the only fixes would be sphereical bearings for the toe and camber links to limit the actual effect in toe these forces are causing?
Modified by euclid at 3:28 PM 9/7/2003
basically the only fixes would be sphereical bearings for the toe and camber links to limit the actual effect in toe these forces are causing?
Modified by euclid at 3:28 PM 9/7/2003
09-07-2003, 10:48 AM
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Re: How the rear end of the DC2 chassis works against us... (RR98ITR)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote »</TD></TR><TR><TD CLASS="quote">But what about the direction the outside rear wheel is pointing? Toe-in in roll at the rear of an FWD car - a Race Car - is precisely NOT what we need. But that is exactly what we get with a DC2, the origins of which, let us not forget, are in safety philosophy as it pertains to transportation vehicle design.</TD></TR></TABLE>
What do the race car really want? Zero toe changes?
I don't think I'm in agreement with you on the dynamic toe in change is all that bad.
If the toe link, camber link, and lower LCA is replaced with spherical bearings with the TA bushing being the only rubber left, I think the improvements will be significant.
Also, in real world performance, these type of mods improvements the car feel to the driver, but I think the actual corner speed improvement is very very minor.
Just my 2 cents.
What do the race car really want? Zero toe changes?
I don't think I'm in agreement with you on the dynamic toe in change is all that bad.
If the toe link, camber link, and lower LCA is replaced with spherical bearings with the TA bushing being the only rubber left, I think the improvements will be significant.
Also, in real world performance, these type of mods improvements the car feel to the driver, but I think the actual corner speed improvement is very very minor.
Just my 2 cents.
09-07-2003, 05:32 PM
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Re: How the rear end of the DC2 chassis works against us... (GhettoRacer)
Road car suspensions are made to go towards understeer as the car goes into bump to make it stable, by having for example the rear toe in under bump. This is called roll steer and it is intentionally built in. They are also designed to understeer by having the rear roll center height lower under load which reduces the dynamic wheel load transfer in a curve, allowing the vehicle to understeer more as load is increased. These effects depend on load in the car as they are made to be driven with either just a single driver all the way up to the fully loaded condition. None of this occurs in a road car that has been optimized to be driven on the track.
It is indeed quite amazing how much the static rear toe setting affects the handling of an ITR on the track. Just 1/16 total toe change at the rear can make the car unstable under braking, which makes it hard to predict exactly what the car will do when you release the brake to turn in at the end of a long straight. But then changing the rear toe can positively affect how the car rotates. An alignment that is good for autocrossing with fast rotation at the low speeds in autocrossing is not a good choice on the race track, since you need some driver confidence to push the car to its limits. When I was at Road America, and RealTime was there testing, I asked one of their crew what they did to tune their ITR's at the various tracks. He told me the thing they adjusted the most was the alignment. Shocks, sway bars and springs did not need to be changed much.
I checked to see if the book by Reimpell in my post above was available from amazon, and it did not seem to be. I checked on SAE's web site and it appears to be available. It is a very good text that discusses many items that I have not found in any other book. As many of the SAE books go out of print, if you are interested in an engineering level text, I would highly recommend you buy it. It will not teach you how to tune your car for the track, but it will give you a great deal of understanding on how a chassis is designed and why. It even shows the pitfalls of lowering a car such as a Golf GTi by 30mm, which puts the suspension outside of its proper operating range, and it illustrates this with a couple of charts. Here is where you can get it:
http://www.sae.org
Click on the bookstore link and then on the transportation books section. You can just search under the authors name: Reimpell
Also check out their other books. There is a newly published work book and CD for Milliken's text. I have ordered it but haven't seen it yet. It replaces the previous work book. If you take the time, you can learn a lot.
Modified by descartesfool at 10:02 PM 9/7/2003
It is indeed quite amazing how much the static rear toe setting affects the handling of an ITR on the track. Just 1/16 total toe change at the rear can make the car unstable under braking, which makes it hard to predict exactly what the car will do when you release the brake to turn in at the end of a long straight. But then changing the rear toe can positively affect how the car rotates. An alignment that is good for autocrossing with fast rotation at the low speeds in autocrossing is not a good choice on the race track, since you need some driver confidence to push the car to its limits. When I was at Road America, and RealTime was there testing, I asked one of their crew what they did to tune their ITR's at the various tracks. He told me the thing they adjusted the most was the alignment. Shocks, sway bars and springs did not need to be changed much.
I checked to see if the book by Reimpell in my post above was available from amazon, and it did not seem to be. I checked on SAE's web site and it appears to be available. It is a very good text that discusses many items that I have not found in any other book. As many of the SAE books go out of print, if you are interested in an engineering level text, I would highly recommend you buy it. It will not teach you how to tune your car for the track, but it will give you a great deal of understanding on how a chassis is designed and why. It even shows the pitfalls of lowering a car such as a Golf GTi by 30mm, which puts the suspension outside of its proper operating range, and it illustrates this with a couple of charts. Here is where you can get it:
http://www.sae.org
Click on the bookstore link and then on the transportation books section. You can just search under the authors name: Reimpell
Also check out their other books. There is a newly published work book and CD for Milliken's text. I have ordered it but haven't seen it yet. It replaces the previous work book. If you take the time, you can learn a lot.
Modified by descartesfool at 10:02 PM 9/7/2003
09-10-2003, 04:36 PM
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Re: How the rear end of the DC2 chassis works against us... (descartesfool)
1/2" total rear toe out+70psi rear tire pressure+-.7* rear camber=counter steering on highway on ramps.
nate-off to change his shorts
nate-off to change his shorts
09-10-2003, 06:05 PM
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Re: How the rear end of the DC2 chassis works against us... (KC)
ya know, it sucks when you borrow someone else's laptop and forget to log them out of h-t.....
nate
nate
