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Yes ... measured with calipers symmetric to the center pin.
And the stock ones are also symmetrical?
Sorry to be a pain, but I'm just surprised that the DRMs actually don't change anti-squat, and aren't made like Randy said they were in the post that I linked.
Last edited by MatthewMiller; 03-04-2018 at 01:21 PM.
Sorry to be a pain, but I'm just surprised that the DRMs actually don't change anti-squat, and aren't made like Randy said they were in the post that I linked.
I mentioned that in a previous post.
Using the dimensions I and 'SuperL98' have provided the spread is 8.61mm +/- I suppose both up and down from center. Do the squat calculations yourself.
All that's needed is dimensions from the early brackets so that a person could do their own calculations.
Using the dimensions I and 'SuperL98' have provided the spread is 8.61mm +/- I suppose both up and down from center. Do the squat calculations yourself.
Just to be clear, I fully understand that if both brackets have symmetrical pivot hole spacing from the center frame mount, then they both have the same anti-squat value. And I understand that this is the case in the pics above (thanks, SuperL98). I just took a look at my DRM brackets on the car. Without removing from the car to measure, my DRMs appear to be the same way too. I'm just surprised that the DRM brackets aren't actually made as advertised.
Are you using an early c4 rear end in a c3 ? I'm using '84 parts in the rear suspension to help with packaging and trackwidth requirements (I retained the stock C3 suspension width so I can use 15" wheels on the street). (FWIW, I'm still using the stock C3 differential for strength and packaging reasons.)
The c4 (and the early c4 in particular) have lots of anti squat, about 115%. Reduced slightly in the 88+ with the lower bolt hole.
I have done some research on the subject recently as I have replaced my whole rear suspension prior to our new season that starts this month.
Apparently the high anti squat values built into the c4 (keeping in mind this car has only about 2 inches of compression at the shock) are to prevent the production built sportscar from bottoming out with 2 people, a full load of gear and a full load of fuel , particularly when it hits a bump or pothole at speed, and turning sideways into a tree resulting in nasty and expensive court cases !
Higher anti squat values are ok for drag racing, but not so good if your type of racing involves accelerating out of corners. this can also depend on grip and track temperature.
Have you mapped out your set up so you know where your 4 link intersecting points are at ? No. When I modified the frame several years ago I just copied the '84 angles as I didn't want to reinvent the wheel before I had any time on the track to see what needed to be changed or adjusted. I dont know how the c3 chassis compares with the c4. The C3 has a two inch longer wheelbase, and I suspect my c/g is a touch higher and more forward than a C4.
Here is a diagram I did of my intersecting points and with a 1" drop of both trailing arms at the bracket (I only ended up achieving 3/4" drop on the trailing arm bracket)
you can see how the intersecting point (IC) goes from above the neutral line to below the neutral line reducing anti squat.
If you want to know what effect this change has, I can tell you after March 18 - if I dont spear off sideways into a tree !
Please post any thoughts or conclusions you arrive at from your track testing.
Just to be clear, I fully understand that if both brackets have symmetrical pivot hole spacing from the center frame mount, then they both have the same anti-squat value. And I understand that this is the case in the pics above (thanks, SuperL98). I just took a look at my DRM brackets on the car. Without removing from the car to measure, my DRMs appear to be the same way too. I'm just surprised that the DRM brackets aren't actually made as advertised.
Remembering the old discussions, Richard Newton was convinced that these brackets help his C4 bite sooner applying gas out of the corners. He also claimed to have talked with Corvette Challenge racers that welded, re-drilled and painted their brackets to hide a similar change.
I thank Randy and appreciate his support of the C4, but his facts on these brackets where all over the place. Possibly, DRM copied the change the challenge racers made without quite understanding why.
I found Richard’s old post…
I have the DRM brackets. I wasn't sure about them at first. Then I talked to Dave McLellan and said he felt there might be some appropriate uses for them. That sold me.
Then I talked to some of the old time Corvette Challenge guys. It turns out they were all doing something similar to the DRM brackets. The problem was they were cheating and had to cover all of it up. They all felt the benefits were worth the work that went into making the modifications in such a way that no one could see the changes.
I put them on my car and the biggest difference seemed to be how much quicker I could put the power down coming out of the slower corners. Grip was much improved on exit.
I couldn't feel any difference at all on the street.
I didn't do any real hard data comparison but after running Sebring for the past decade I have a pretty good idea about what actually works.
Remembering the old discussions, Richard Newton was convinced that these brackets help his C4 bite sooner applying gas out of the corners. He also claimed to have talked with Corvette Challenge racers that welded, re-drilled and painted their brackets to hide a similar change.
I thank Randy and appreciate his support of the C4, but his facts on these brackets where all over the place. Possibly, DRM copied the change the challenge racers made without quite understanding why.
Yeah, that's not really the improvement you should feel from either: 1) reduced anti-squat, or 2) a longer SVSA. Sort of.
The DRM brackets appear to only lengthen the SVSA, without changing anti-squat. That should make the anti-squat percentage more stable as the car moves around. As the suspension moves in rebound, it should mean that anti-squat rises less for every inch of suspension movement. And for compression, it loses less anti-squat for every inch of travel.*
If you really lowered anti-squat percentage at rest, you should have more linear handling on and off the throttle. Coming out of corners, you'd have a little lower overall ride height, which should mean less rear weight transfer from left to right, and you'd have more compliance over bumps without losing traction. In those senses, you'd feel better and especially more consistent traction on corner exit. All of this assumes your rear suspension doesn't end up on the bumpstops under acceleration, of course. If it does, then all sorts of hell could break loose. Conversely, it would be easier to spin the tires from a standing start. So this isn't what drag racers would be looking for.
*So for a lowered C4, it will have less anti-squat even with stock brackets. If I'm thinking about this correctly, it's possible that with DRM brackets a lowered C4 might actually have more anti-squat than with stock brackets, since SVSA angle doesn't change angle as much. I'll have to measure and do some calculations, I guess.
I'll have to measure and do some calculations, I guess.
A longer pivot arm would also lesson roll oversteer-understeer from the body roll cornering, because the rear wheels would move less fore and aft with suspension movement, and maybe also help with the rear suspension binding as well.
When I was younger and into this stuff, the more I read about suspension design (and listened to people argue about it), I came to realized it may be more of an art than a science
A longer pivot arm would also lesson roll oversteer-understeer from the body roll cornering, because the rear wheels would move less fore and aft with suspension movement, and maybe also help with the rear suspension binding as well.
On a solid-axle car it would improve roll steer every time, but on an IRS-equipped car we're looking at bumpsteer. The toe link should generally be able to maintain proper toe even as the uprights move forward or backward through suspension travel. At least I think so, anyway. I agree about bind, although with stock rubber bushings that shouldn't add a lot of wheel rate. I think bind becomes a real issue when people switch to urethane bushings in the rear suspension, though. Rod ends are the best answer, except of course they hurt ride quality and noise.
When I was younger and into this stuff, the more I read about suspension design (and listened to people argue about it), I came to realized it may be more of an art than a science
There are an awful lot of variables to factor in. This is one area where computers can really help zero in on the desired geometry. And the C4 is a relatively simple design - they can get considerably more complicated!
I have a set of DRM brackets in a box someplace ... will see if I can find them.
Those dimensions came from an article written by the late Richard Newton, of 101 Projects fame. I never verified them & it would be good to do it.
We had a long post on these DRM brackets and anti squat years back, and it looked like they just moved the holes out an equal amount. If true that doesn't change the anti squat angle or percent anti squat, just the length of the virtual pivot arm.
In relation to that diagram, I showed that to an engineer I know personally and he actually started swearing out loudly !
it has been circulated on car forums that the neutral line starts at the middle of the hub on an independent rear, well apparently that is a fallacy, the acceleration force is always where the tyre meets the ground, you cant apply acceleration force 10" in the air (unless its a hover board !)
and that means that the neutral line is in the wrong position, its lower, and that means any calculation based on it is wrong.
when you widen the arms the intersecting point moves forward, that does 2 things, if it pushes the IC forward enough it goes UNDER the neutral line (that goes from the bottom of the rear tyre to the CG height at the front tyre) and reduces anti squat. Secondly it creates a longer lever arm that produces more force at the rear suspension.
heres a pic where the neutral line is at stock height.
(keep in mind that most cars are lowered an inch or so and all measurements and CG height is lowered accordingly)
here is the diagram of where my rear end is at, the CG is down to 14" and the neutral line drops 1" also, you can see the green lines are standard angle, the red lines are dropping both bracket holes 1", you can see that it immediately drops the anti squat under the neutral line (disregard the 15" CG written next to it, should be 14")
Also the line from the CG down to the ground is considered the 'sweet spot' and suspension engineers will talk about this. For road racing try and hit the CG line with the IC. The sweet spot can differ between a road racing and drag racing setup.
You can see from the IC of the green lines that the GM engineers like that sweet spot too.
the lowering of the bottom 4 link bracket hole any amount WILL result in a lowering of anti squat.
That is a nice image, but unfortunately it's wrong.
I know that because I created it.
I come from a drag racing background of four links and live axles and didn't understand how IRS anti squat works ... so I asked if it was right on the racing forum here ... It's not ... It was scaled from the engineering drawing that I believe was drawn with zero anti-squat and anti-dive and a 17 inch CG height. The real dimensions, measured from my 1988 are in the cad drawing below.
Here is the right image and some other information that may help you...
BTW the neutral line is not shown in the above image. The 10 deg is just CAD math 17/96.2. The neutral line would be 10 deg from the rear axle center.
Data from Mike Antonick’s “Corvette Specs” book.
1984 AntiSquat 51 % AntiDive 46 %
1985 AntiSquat 51 % AntiDive 46 %
1986 AntiSquat 62 % AntiDive 46 %
1987 AntiSquat 62 % AntiDive 46 %
1988 AntiSquat 62 % AntiDive 46 %
No data from later years.
Google ( IRS Anti Squat ) all kinds of good information around.
That is a nice image, but unfortunately it's wrong.
I know that because I created it.
I come from a drag racing background of four links and live axles and didn't understand how IRS anti squat works ... so I asked if it was right on the racing forum here ... It's not ... It was scaled from the engineering drawing that I believe was drawn with zero anti-squat and anti-dive and a 17 inch CG height.
This is correct. The reason is that in a live-axle car the torque reaction (the force that tries to spin the differential around the axle) has to be reacted through the suspension arms; whereas in a car with IRS the diff is mounted solidly to the frame and the suspension doesn't have to deal with the torque reaction. That last image that SuperL98 posted (the one that says "Suspension Geometry - Section 17.3" at the top) is from the Millken bible Race Care Vehicle Dynamics. It is largely considered the definitive book on the subject.
ETA: By the way, the same thing is true of brakes and anti-lift. We are used to brakes being outboard, so that the suspension arms have to react the brake torque. That means that in most cars (including a C4), the force line originates at the tire's contact patch and the length of the SVSA changes the magnitude of anti-lift. However, if the brakes are inboard and mounted to the frame (i.e., the frame reacts the brake torque and the suspension is not involved), then the force line originates at the axle/hub center.
Originally Posted by blackozvet
when you widen the arms the intersecting point moves forward, that does 2 things, if it pushes the IC forward enough it goes UNDER the neutral line (that goes from the bottom of the rear tyre to the CG height at the front tyre) and reduces anti squat.
This is true of solid-axle cars, but not of IRS cars.
Secondly it creates a longer lever arm that produces more force at the rear suspension.
This is backward for a solid-axle car. The longer the SVSA, the less force it can impart to the sprung mass of the car. That's why what you wrote just above this - that the longer the SVSA the lower the anti-squat - is true.
PS - Back in my post #18, where I deleted a whole paragraph that was wrong, the reason I had to do that was because I was figuring the anti-squat force from the contact patch like on a solid-axle car. I'm glad I'm not the only one who slips back into "live-axle brain" every once in a while.
Last edited by MatthewMiller; 03-05-2018 at 09:05 AM.
not sure about these numbers tho - it is accepted that the neutral line is regarded as being 100 % anti squat - and the c4 intersecting lines are just above that neutral line (exact position depending on chassis height and bolt holes in trailing arm bracket)
The early c4 in stock configuration has about 115 % anti squat.
Are they using a different measuring method ?
Data from Mike Antonick’s “Corvette Specs” book.
1984 AntiSquat 51 % AntiDive 46 %
1985 AntiSquat 51 % AntiDive 46 %
1986 AntiSquat 62 % AntiDive 46 %
1987 AntiSquat 62 % AntiDive 46 %
1988 AntiSquat 62 % AntiDive 46 %
No data from later years.
Remember ... that drawing with the square grid is what marketing used to show off the C4, not what is in the actual cars. And again, I came to believe that the engineers drew the suspension with 100% anti's (neutral) to start ... that's what I would do.
I measured my 1988 and posted the real dimensions ... I Photo Shopped the neutral line (100% anti squat for an IRS) in for you.
The anti squat, for an IRS, is just the ratio of the two angles 6.3 and 10 degrees.
Regarding the brackets, I'm quite sure a 'local' used M12 'heavy washers' and moved his holes to where he wanted them. He reinforced the holes by spot-welding the 'heavy washers' at all four locations on each bracket.
That certainly seems to be a better idea than buying the DRM brackets, I don't recall if his was a choice after seeing the issues or if he just did it 'his way'. He's quite creative so I expect it was a 'his way' thing.
And again, I came to believe that the engineers drew the suspension with 100% anti's (neutral) to start ... that's what I would do.
It's interesting to me that people refer to the 100% angle as the "neutral line." It's worth pointing out that this is anything but neutral! 100% anti-squat means that the suspension is applying a vertical force vector to the sprung mass equivalent to the acceleration's force through the moment arm across the wheelbase. Overall, it will raise the CG of the car during acceleration: during acceleration the rear ride height stays the same, but the front rises, so the sprung mass is higher overall. To me, that is anything but neutral! I know the nomenclature is what it is, but "neutral" should be when the suspension imparts no vertical force to the sprung mass and has no effect on ride height or suspension compliance: i.e., zero anti-squat.
Originally Posted by WVZR-1
Regarding the brackets, I'm quite sure a 'local' used M12 'heavy washers' and moved his holes to where he wanted them. He reinforced the holes by spot-welding the 'heavy washers' at all four locations on each bracket.
Just looking at the brackets on the car, it seems that the major limitation to how much the pivots can be lowered would be interference between the bolt heads that hold the brackets to the frame with the ends of the control arms.
It's interesting to me that people refer to the 100% angle as the "neutral line." It's worth pointing out that this is anything but neutral! 100% anti-squat means that the suspension is applying a vertical force vector to the sprung mass equivalent to the acceleration's force through the moment arm across the wheelbase. Overall, it will raise the CG of the car during acceleration: during acceleration the rear ride height stays the same, but the front rises, so the sprung mass is higher overall. To me, that is anything but neutral! I know the nomenclature is what it is, but "neutral" should be when the suspension imparts no vertical force to the sprung mass and has no effect on ride height or suspension compliance: i.e., zero anti-squat.
Just looking at the brackets on the car, it seems that the major limitation to how much the pivots can be lowered would be interference between the bolt heads that hold the brackets to the frame with the ends of the control arms.
Historically anti squat was used to be called 'pitch control' but over time the term 'anti-squat' was coined and it was obviously much more catchy and engineers have had to get on board so that everybody knows what is being talked about.
Also it appears that there are 2 different ways of measuring it, firstly the neutral line being 100 % and whether the IC is above it (more than 100%), on it (100%) or below it (less than 100%),
secondly measuring the IC point and how far up the CG line it is (e.g. if the CG is 14" and the IC is 7" up off the ground its 50%) This would explain the percentage numbers listed in some Corvette documentation.
I have re drilled my early brackets 3/4" down on both holes (this is the most the stock brackets can sustain) to get around 90% (this figure is a guestimate)
03-04-2018, 01:21 PM
