I've been doing a lot of reading and learning lately - huge thanks to Chelsea for sharing a lot of his knowledge on the subject of making 36/46 rear grip. I have been searching for more rear grip for a long time now and have learned a few things. I would like for this thread to become a knowledge base so those that know more than me about rear suspension geometry can teach me things too.
Essentially I've been approaching the rear end incorrectly - I've always thought obvious rear squat = increased rear grip. That is not true at all. This introduces the concept of anti-squat, which is essentially a ratio of the total load transfer to the rear from acceleration that goes directly into the suspension linkage and bushings, rather than the spring/shock compression. 100% anti-squat means on acceleration, the rear does not squat at all. It will stay perfectly level, and the entire load transfer F-R on acceleration will be taken directly by the suspension linkage, pushing the tire harder into the pavement than with less anti-squat.
From what I understand, forces used to compress the rear suspension on acceleration are essentially "wasted" in doing so, as they could be doing a more effective job of pushing the tire into the pavement. That all sounds great, but how do we increase anti-squat?
-Raise RTAB front pocket mounts
-Raise ride height of the car
-Adjustable trailing arms that are not commonly available and would be custom race parts
Now, remembering Chelsea's E46... that thing was a monster truck. Really tall ride height, tons of anti-squat. Also if you watch old videos of him accelerating on the run up, there was essentially no rear squat. The front end would rise and the rear would stay where it was. Supposedly this is the key to everything in the back.
Now I had a higher ride height before with my old 16k rear springs. My 5" 10k springs are pretty soft and the car is very low with no adjusters in it or anything. I'm going to try raising the car substantially for Lonestar Bash next week and report if the grip levels change dramatically. If you guys have any input on this subject, I'd love to hear it. Studying drag racing suspension design, anti-squat is a huge deal and finely tuned for a perfect launch.
Thoughts?
Let's also see some ride height numbers if you guys have access to your cars to measure:
Front: jack point with no jack pad to ground
Rear: jack point with no jack pad to ground
I'll post mine tomorrow and we can compare heights for the sake of learning more.
Mike
IG: @mikevanshellenbeck
dude yes! i like as much rear grip as possible with my set-up. im on a stock 328is with basic tein coilovers that have no dampening adjustment. i basically lower my tire pressure in the rear as far as possible to get the grip i want. def gonna pay attention to this thread.
Also, ill see you at bash! got everything setup and ill be there. itll be my first time driving any Lone Star event. been putting in work doing my learning at the local San Antonio Day of Drifts and the Lost in the Sauce events. ill come say hi when im there and talk to you. look for my e36 with the texas flag hood.
This is GREAT! Thanks for making this thread Mike.
Awesome, see you there!
I know some specifics are industry secret and FD teams don't necessarily share all of their information with everyone, but I do like to increase the collective knowledge we have as a community if we can when it's not proprietary.
I'll try to dig up some diagrams that explain anti squat and why it is important:
As you can see, if the trailing arm mounting point is higher, or at least higher relative to the base of the tire, you will have more anti-squat and press the tire into the ground harder via geometry alone.
Mike
Last edited by MikeE36; 10-05-2017 at 05:57 PM.
IG: @mikevanshellenbeck
That's excellent! I have the Race Car Vehicle Dynamics "bible" by Milliken and Milliken. I'll look around in there and see if I can find some information that is more precisely on point to our rear suspensions.
I would think working on your tire contact patch and a stiffer rear spring. If you hit the gas and the weight moves to the rear.....if the spring compresses easily then it should be absorbing force you want pushed on the pavement. No expert but just thinking it throgh based on what I fell in the car when I mash the pedal.
ME:"I want to make my car faster and lighter"
THEM:" Get out and let someone else drive"
I don't really have much to add to the convo. You've already said most of what I know. Raise the car and shove the TA mounting point farther up in the car. I think there's kit somewhere to do it.
I remember some drag race video where the rear actually raised when the car set off lol.
Status: Someone put glitter in my oil. Wait. Why's all my oil outside the engine? What's that knocking?
Mike Radowski's E36 barely squatted at all, even though he had a tremendous amount of weight transfer to the rear. It dips a bit at the initial hit, but then the rear comes back up to almost the initial ride height:
awesome, this is what I was hoping this thread would develop in to
https://www.bimmerforums.com/forum/s...-Chassis-Setup
and I was in the same "more squat = more grip" thinking
Well Anti squat is just part of the equation, anti-squat is good to a certain point but what you want to avoid is tire shock. You want to load the rear tires as much as you can with weight transfer up to the point of tire shock. Tire shock is when the load deforms the tires contact patch and reduces grip. That's one of the reasons that serious Drag cars run those big wrinkle wall slicks. The soft sidewalls slow down the load transfer to allow the load to be applied gradually so the tires footprint isn't distorted.
That's why with more anti-squat softer compression dampening and spring rates are run. The load applied remains the same but the speed at which the load is applied is more gradual. Stiffer springs and dampening speeds up weight transfer. The idea is to minimise load loss by utilizing the geometry to reduce excess movement but slow the transfer at the same time to optimize the tires contact patch.
Last edited by Piner; 10-10-2017 at 03:05 AM.
what ride height are you trying out? What were you running before? I just put new out of the box fortunes on my car and still need to decide on a beginning ride height point
Before was approximately 5" or 5.25" from the jack point to the ground, from baremetal at the hole where the pad screws in.
I raised the rear about 1", so it's at 6.25", and I raised the front 1/2" at the coilover. I will measure it again when I am at the shop to confirm. I switched from a 5" 10k to a 7" 8k with no adjusters or anything in the rear.
Mike
IG: @mikevanshellenbeck
Also another thing that helps is to run a double or triple adjustable damper so you can tune rebound and compression curves separately. You want less compression dampening than you would normally have for a road race car but stiffer rebound than what would normally be in that combo. That way it slows weight transfer on squat but it helps keep the tires in pavement when the transfer is complete. Having the ability to dial that in at the track is invaluable. Thats why you see most FD cars running at minimum a double adjustable damper.
Justin is at irwindale, convince him to go measure that distance in kristaps' car haha. Although I think he has wisefab rear knuckles so that probably drops the point where the trailing arm mounts anyways
Well, mid day update at Lonestar Bash. The car feels F**** awesome! Raising it and doing the 8k rears made a huge improvement in grip. Fairly worn scrubs felt like brand new tires, and I was able to run down Aaron Losey's Mustang no problem, which would never happen before. I absolutely love it! Videos tonight.
Mike
IG: @mikevanshellenbeck
So, I set my heights last night at 6.5" rear and 6" front, but now I've been staring at that graphic I think I've confused myself. It would seem that the ride height alone doesn't make a difference, but difference front to back does? If so, would jacking up the front a bit higher than the rear (like you often see 240s) be beneficial to A/S as well as adding a wee bit of caster?
What you are attempting to maximize is the trailing arm angle relative to horizontal, i.e. have the RTAB pocket as high as possible relative to the rear knuckle. When you raise the rear, this happens. While raising the car, you also move the CG higher, which reduces anti-squat because anti-squat is a ratio to the height of CG essentially. 18" anti-squat and 18" CG means 100% anti-squat. Having a little bit of positive rake in the car with the front lower, means the CG will stay a little lower, while the back being high means the trailing arm angle is good. Too much rake and you have less static weight on the rear tires and can reduce grip also. I'd say there's for sure a happy medium. I think mine is still lower than 6" up front but I will measure today.
You can see in this video how much more grip the car has. When I throttle it in 2nd out of drift, the front end lifts up and it just accelerates rather than just blowing the tires off and not moving. Also, self-steer is significantly improved because I can rely on the rear grip now to throttle steer the car. Overall the car just improved significantly on driveability and grip level.
Mike
Last edited by MikeE36; 10-17-2017 at 12:18 PM.
IG: @mikevanshellenbeck
Glad to hear its working out for you. I bet with more fine tuning and optimization it will improve even further.
On the subject of RTABs and RTAB brackets. What are all of you guys running? I have custom brackets to allow more toe in and the garagistic offset bushings. But it seems I may need to go to a bearing style RTAB to allow proper movement of the trailing arm under compression.
And also. What are you guys running for rear toe? I'm at about .25 degrees toe in on each side.
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