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 Post subject: Anti-Squat
PostPosted: April 28, 2020, 8:06 am 
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I'm running the C4 Corvette rear suspension. With the length and angles set to factory, in theory I would have 63% Anti-Squat. My CG is probably three or four inches lower than the stock C4, and I have pulled the engine back 14" from the C4 location with the same wheelbase. My question is not so much what my Anti-Squat would end up at using the factory dog bone angles, but do I need any anti-squat at all? Car should come in around 1,800 lbs. or so. Would I be better off setting up the dog bones as a parallel four link? I've seen some of the math used to calculate it, and it makes my head hurt. Plus I still don't have all of the numbers to calculate what it will be if I go with a particular set of angles and lengths. She's going to be a Highway Flyer, maybe a bit of Autocross or Track days, but basically a Street car.

I know that GM set up the C4 for that 63% Anti-Squat to avoid bottoming the suspension in bump as there is only about 1-1/2" of compression in the shocks. I have mine setup with 3" of compression, 2.5" or so of droop, so I am not too concerned with bottoming the suspension, only how the car behaves with a spirited launch from a standstill, or exiting a corner under power.

Found a copy of the factory setup online:


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 Post subject: Re: Anti-Squat
PostPosted: April 28, 2020, 8:51 am 
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I am still learning, but I was curious about your question and did a little rummaging around the web.

Came across this discussion on a Corvette forum. Maybe you've already seen it. Looks like it has some useful info and it's Corvette-specific.

https://www.corvetteforum.com/forums/au ... irs-2.html

Edit: looks like the pic you included was part of the thread I referenced so you've probably been there already.


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 Post subject: Re: Anti-Squat
PostPosted: April 28, 2020, 8:54 am 
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I have no idea what the numbers should be, but You will want some Anti-Squat for that "spirited launch from a standstill"! The A-S in your rear suspension will also act as "Lift-Bar" type traction bars! If you're running any decent amount of power in an 1800# car, you'll want that effect in a "SLFAS"! Just one man's opinion! :wink:

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 Post subject: Re: Anti-Squat
PostPosted: April 29, 2020, 7:00 am 
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I have an autocross car with an adjustable 3 link - my experience is --> Not enough is OK , too much causes rapid suspension unloading/loading when braking hard - feels like there is a jackhammer in the rear suspension So....some is good, more is not better.
Can you make the mounting for the forward links removable, so you could have an opportunity to adjust? (ie have several mounts with different hole locations.)

Dont use slotted mounts tho


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 Post subject: Re: Anti-Squat
PostPosted: April 29, 2020, 9:30 am 
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Sean in CT wrote:
I have an autocross car with an adjustable 3 link - my experience is --> Not enough is OK , too much causes rapid suspension unloading/loading when braking hard - feels like there is a jackhammer in the rear suspension So....some is good, more is not better.
Can you make the mounting for the forward links removable, so you could have an opportunity to adjust? (ie have several mounts with different hole locations.)

Dont use slotted mounts tho


That's what I was looking for, some real world stuff! All else being equal with the chart above, I have enough room to get two adjustment holes beneath the existing attaching point on the outboard end of the upper links. One 1" down, and one another 3/4" down. So I can significantly drop that number from 63% based on that. Thanks for the input! And don't worry, I would never use slots! Been there, done that with other equipment.

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 Post subject: Re: Anti-Squat
PostPosted: April 29, 2020, 3:16 pm 
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Light cars with low CGs tend to not benefit much from anti geometry. Add it if you'd like but it definitely isn't necessary.

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 Post subject: Re: Anti-Squat
PostPosted: April 29, 2020, 7:16 pm 
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To your original question, you don't "need" any anti-squat at all. However, as the weight transfers and the rear suspension compresses, you're going to lose anti-squat. So if you start with 0, once the weight shifts, you'll actually end up pro-squat.

However, contrary to how I read your question about going parallel, doing so won't give you 0 anti-squat either. The IC line just comes off at the angle of the parallel links.

I'm a little lost on your intention too though. On one hand you're asking about going parallel, which would mean lowering the lower or raising the upper link mounts on the chassis, but then mention adding holes to lower the anti-squat by dropping the upper link mount on the chassis?

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 Post subject: Re: Anti-Squat
PostPosted: April 29, 2020, 7:47 pm 
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Driven5 wrote:
To your original question, you don't "need" any anti-squat at all. However, as the weight transfers and the rear suspension compresses, you're going to lose anti-squat. So if you start with 0, once the weight shifts, you'll actually end up pro-squat.

However, contrary to how I read your question about going parallel, doing so won't give you 0 anti-squat either. The IC line just comes off at the angle of the parallel links.

I'm a little lost on your intention too though. On one hand you're asking about going parallel, which would mean lowering the lower or raising the upper link mounts on the chassis, but then mention adding holes to lower the anti-squat by dropping the upper link mount on the chassis?


After seeing the comments from NGPMIKE , Sean in CT, and a. moore, I saw that I probably did need some anti-squat, and that by adding a couple of extra holes to drop the upper dog bones down, I could basically cut it in half from the original 63% to about 30%, and from there to about 15%, so I have a pretty good range to choose from. I just hadn't seen anyone that was doing IRS talk about anti-squat at all, let alone how much they were dialing in.

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 Post subject: Re: Anti-Squat
PostPosted: April 29, 2020, 11:41 pm 
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That will reduce your anti-squat, but will also significantly reduce your instant center length. Do you have any ability to add reduced height chassis mounting holes for the lower links as well?...The again, I've been toying with a geometry that has an even shorter instant center than either of yours would. So even though it is a bit different, I really can't judge.

There hasn't been much talk about anti-squat for IRS Locosts because so few are running a trailing arm style 5 link, like the C4 has. It's unusual for double wishbones to run significant anti-squat.

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 Post subject: Re: Anti-Squat
PostPosted: April 29, 2020, 11:58 pm 
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I think I compensated for that instant center length, at least somewhat, by dropping the lower bar from 10* to 9* (The engine is 2" to 3" lower than stock), and I'm going to pop a couple of extra holes in the bracket for the lower arms as well. You hit the nail on the head about the other IRS guys. No mention of it for the most part. That's why I was questioning whether or not I need it, and if so what a reasonable % was. We're gonna find out, one way or another! I think I may just leave this portion just tacked until I have most of the rest of the equipment in, and I can get a better idea of the true CG and other dimensions.

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 Post subject: Re: Anti-Squat
PostPosted: April 30, 2020, 12:19 am 
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On Car9 I decided to use trailing arms that were level and about parallel. They are also longer. I don't understand your comment above about moving the link. Just like your other suspension arms you don't want short instant centers. Especially when you'are adding more travel to the suspension.

Another two ways to look at this. The most basic is that you can measure or double check this by seeing how much the hub moves forward and backward as it it move up and down. That's not really such a great thing. Imagine having to change the length of the wheelbase of the car when you tilt the car ( squat, dive, lift etc. ). This is why the suspension feels like it's not really working. With anti squat you need to move ( lift against the car against it's inertia ) the car forward a bit in order to get it to squat because the hubs move backwards as they rise upwards.

It's also similar to how you design for cornering, it's just a different direction. If you put your roll center at the center of gravity the car doesn't roll while cornering, but people don't do that. Having your roll centers on the ground and also zero anti is sort of the simplest and purest setup. In many areas more is not better.

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 Post subject: Re: Anti-Squat
PostPosted: April 30, 2020, 6:28 am 
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horizenjob wrote:
On Car9 I decided to use trailing arms that were level and about parallel. They are also longer. I don't understand your comment above about moving the link. Just like your other suspension arms you don't want short instant centers. Especially when you'are adding more travel to the suspension.

Another two ways to look at this. The most basic is that you can measure or double check this by seeing how much the hub moves forward and backward as it it move up and down. That's not really such a great thing. Imagine having to change the length of the wheelbase of the car when you tilt the car ( squat, dive, lift etc. ). This is why the suspension feels like it's not really working. With anti squat you need to move ( lift against the car against it's inertia ) the car forward a bit in order to get it to squat because the hubs move backwards as they rise upwards.

It's also similar to how you design for cornering, it's just a different direction. If you put your roll center at the center of gravity the car doesn't roll while cornering, but people don't do that. Having your roll centers on the ground and also zero anti is sort of the simplest and purest setup. In many areas more is not better.


"And the dim bulb began to glow"... Thank you Marcus, now I have a better understanding of how the C4 suspension actually works. I had read that they went to the 4 link to get a longer virtual swing arm over what they had with the C3, for exactly the reason you stated of fore and aft movement of the hubs. Much clearer now. So if I understand this correctly, without also being able to change the heights and locations of the HUB mounting points and the lengths of the dog bones, changing the angles of the dog bones won't really give me much adjustability at all without screwing up the virtual swing arm length and other factors. It would appear that the best you could accomplish is making the virtual swing arm longer, at the expense of increasing the anti-squat %.
So, since the attaching points at the hub are not conducive to being modified,and there is now way to make the dog bones longer, it would appear I'm best off leaving the setup as it came from the factory, and living with my choice of suspension, good or bad.

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 Post subject: Re: Anti-Squat
PostPosted: April 30, 2020, 10:28 am 
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Dismantalus wrote:
It would appear that the best you could accomplish is making the virtual swing arm longer, at the expense of increasing the anti-squat %.
Conversely to lowering the upper arm chassis mount reducing anti-squat but shortening the instant center distance, lowering the lower arm chassis mount will reduce anti-squat while lengthening the instant center distance.

With short arms especially, I would probably target an instant center that is equal to the wheel/hub center height when the suspension is compressed by 1.0-1.5 inches. This will allow for some initial anti-squat and then minimize wheelbase change on the harder working side of the car in the corners.

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 Post subject: Re: Anti-Squat
PostPosted: April 30, 2020, 11:50 am 
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Driven5 wrote:
Dismantalus wrote:
It would appear that the best you could accomplish is making the virtual swing arm longer, at the expense of increasing the anti-squat %.
Conversely to lowering the upper arm chassis mount reducing anti-squat but shortening the instant center distance, lowering the lower arm chassis mount will reduce anti-squat while lengthening the instant center distance.

With short arms especially, I would probably target an instant center that is equal to the wheel/hub center height when the suspension is compressed by 1.0-1.5 inches. This will allow for some initial anti-squat and then minimize wheelbase change on the harder working side of the car in the corners.


I told you this stuff made my head hurt....Geometry was 40 years ago... This is why I love this site. Lot of different folks with their niche of expertise. I was looking at lowering the lower arm just by the angle, and the print would suggest that that would increase the AS, ie drop the lower to 8*, then 6.3 / 8 = .78, but forgetting that as you move the IC further out, the upper number reduces as well.
I think I can get a fair amount of adjustability in the lower arms to accomplish what you are suggesting. Thanks!

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