Upgrade rear anti-roll bar?

[AndyW]

This pic from my recent track day makes me wonder if I really do need to upgrade to the M3 anti-roll bars. it is about a 65 MPH sweeper turn with a 75-80MPH exit speed. Car felt good and stable, but man...that squat. And this is with clubsports with 750 Swift springs.

[Phloozy]

Going to assume you mean anti roll bar, or sway bar. I run a larger rear and front bar but also have significantly stiffer springs. I'm running 450 front and 1100 rear. I imagine your car is too soft

[AndyW]

Originally Posted by Phloozy

Going to assume you mean anti roll bar, or sway bar. I run a larger rear and front bar but also have significantly stiffer springs. I'm running 450 front and 1100 rear. I imagine your car is too soft

Yes, and I misspoke, I am 450 front, 750 rear. But maybe I should go higher, like you did.

[tsk94]

Squat and/or roll is not nessicarily a bad thing. If the car feels good, what are you hoping to improve by adding the sway bar?

Also, do you have any pictures of the car stationary? Hard to tell how much it's squatting based on the picture. It could be a lot or not much depending on what ride height you run.

[AndyW]

Originally Posted by tsk94

Squat and/or roll is not nessicarily a bad thing. If the car feels good, what are you hoping to improve by adding the sway bar?

Also, do you have any pictures of the car stationary? Hard to tell how much it's squatting based on the picture. It could be a lot or not much depending on what ride height you run.

I hope to improve traction by keeping more tire on the ground. The pic just shocked me a bit. Since I upgraded to the clubsports in 2019 the car has been much more planted and the pics I have all had pretty benign roll...until this one. Maybe it's just the angle.

Not stationary but this is the basic stance, which I think is great.

[tsk94]

Originally Posted by AndyW

I hope to improve traction by keeping more tire on the ground. The pic just shocked me a bit. Since I upgraded to the clubsports in 2019 the car has been much more planted and the pics I have all had pretty benign roll...until this one. Maybe it's just the angle.

Not stationary but this is the basic stance, which I think is great.

Adding a rear sway bar will reduce rear traction. It will actually unload the unweighted tire more and shift the load onto the outside tire. At the front of the car, less load is transferred side to side. Adding a rear sway will change a cars roll couple distribution, that is, the roll stiffness of the front of the car relative to the rear. I don't want to get overly technical, but given your goals a rear bar upgrade is not the route to go.

For your goal of 'improved traction', can you elaborate on that? By rear traction do you mean more generally rear grip, or mostly traction while accelerating? If you mean more generally to just improve rear grip, what section of the corner (entry, mid, exit, all of the above?) There's lot's of approaches you can take to improve these areas, or even just rear traction.

[AndyW]

Originally Posted by tsk94

Adding a rear sway bar will reduce rear traction. It will actually unload the unweighted tire more and shift the load onto the outside tire. At the front of the car, less load is transferred side to side. Adding a rear sway will change a cars roll couple distribution, that is, the roll stiffness of the front of the car relative to the rear. I don't want to get overly technical, but given your goals a rear bar upgrade is not the route to go.

For your goal of 'improved traction', can you elaborate on that? By rear traction do you mean more generally rear grip, or mostly traction while accelerating? If you mean more generally to just improve rear grip, what section of the corner (entry, mid, exit, all of the above?) There's lot's of approaches you can take to improve these areas, or even just rear traction.

Interesting. I am going to have to do some study on this (and I admit I am a novice on suspension tuning). It seems counterintuitive to me that a stiffer sway would always mean less traction. I could see if it was so stiff it locked the train, it would function like a straight axle, but to me it seems less body roll = less weight transfer = more even traction distribution left/right = less potential to overload the loaded tire = more traction.

I don't have any specific "problems" to address...which is why I am hesitant to do anything with suspension because the car feels so balanced to me right now...plus I am well aware that I am the cars biggest limitation right now. But I did upgrade to the clubsports with much stiffer springs in 2019 after analyzing pics with my stock shocks and Spec-R's and saw photos like this, which clearly indicated I was at a skill level that was overloading that setup.



So, when I saw the pic I posted first, it seemed I may be pushing to the point i need to improve something else.

I am very interested in this discussion though, if you are willing to share your expertise...if I had to answer your question I would say "improve overall grip" as I don't feel I have an specific oversteer or understeer problems right now.

[tsk94]

Originally Posted by AndyW

Interesting. I am going to have to do some study on this (and I admit I am a novice on suspension tuning). It seems counterintuitive to me that a stiffer sway would always mean less traction. I could see if it was so stiff it locked the train, it would function like a straight axle, but to me it seems less body roll = less weight transfer = more even traction distribution left/right = less potential to overload the loaded tire = more traction.

I don't have any specific "problems" to address...which is why I am hesitant to do anything with suspension because the car feels so balanced to me right now...plus I am well aware that I am the cars biggest limitation right now. But I did upgrade to the clubsports with much stiffer springs in 2019 after analyzing pics with my stock shocks and Spec-R's and saw photos like this, which clearly indicated I was at a skill level that was overloading that setup.



So, when I saw the pic I posted first, it seemed I may be pushing to the point i need to improve something else.

I am very interested in this discussion though, if you are willing to share your expertise...if I had to answer your question I would say "improve overall grip" as I don't feel I have an specific oversteer or understeer problems right now.

A lot of suspension related tuning is counter intuitive, at least initially.

First thing to understand is body roll doesn't impact weight transfer. At least it does so minimally that it's should not be considered. So reducing body roll, by either going to a stiffer spring or sway bar, has no effect on the amount of weight transfer. Weight transfer is affected by other factors. The classic example is a go-kart. It has no suspension, thus virtually no body roll (other then tire deflection) but there is still weight transfer. And as I mentioned previously, adding the rear sway bar will increase weight transfer at the end you add the sway bar, and decrease it on the other end of the car - again, counter intuitive. So if you wanted to even out the rears left/right weight distribution while cornering, you'd actually want to add a front sway bar and/or increase the front spring rate, or reduce rear spring rate/bar rate. There's some good youtube videos explaining this and other articles online if you're interested to read more on it yourself.

If you're very happy with the current setup that's great! Nothing better then a setup that's fast and inspires confidence to push the car and yourself.

Improving overall grip is more difficult than the initial goal of improving rear traction/grip. Especially since you mentioned you're happy with the current balance, any improvement of rear end grip will result (to some degree) in a compromise of the overall balance, ie. more understeer, to some extent, in certain areas. To improve overall grip you have a few straight forward options. Go to stickier tire, or wider and stickier tire. Next you can increase the front and rear track width. Not sure what tires and what size tire you're currently on, but I suspect going to a wider track and wider tire may be out of the question. A stickier tire however might be an option (unsurprisingly) to improve overall grip. The final option, which from the picture you posted here looks easily achievable, is to lower both the front and rear ride height. Lowering the ride height will reduce weight transfer and help the car develop more grip overall. Again if it's just the rear you're looking to improve you have plenty of options, some the same as above. Increase rear track width, lower rear of car, softer rear spring/sway bar, stiffer front spring/bar. Entry and exit the dampers come into play. So for 'improved traction', that sounds like primarily exit, you could soften the rear compression and/or stiffen front rebound.

[AndyW]

Thx for the detailed reply. I have some studying to do!

[Dru128m]

Don't forget, tire pressures can have some impact when trying to fine tune the feel on track. Sometimes that little extra contact patch is all you need.

[carcar123454321]

tsk94 , great info. If I just want to improve handling overall for a 135i, would a set of sway bars not be the best first move (Cobb is selling their set for a good deal currently)? Many have said to the M3 front suspension arms.

I realize body roll reduction doesn't necessarily = better handing/higher grip (thought it does feel a little more stable in some ways). I went from a solstice GXP to a 135i and the roll is a little annoying, but I realize the GXP has a much lower CG.

I'm not ready to put money into coilovers yet, but that would definitely be something I'd like to do at some point.

[alindquist]

450/750 is way to soft IMO. I run 560/895, thinking of going one step stiffer both front and rear. And as said, don't go for squat, go for feel! How wide tires are you running? E82 are heavy in the front, they need as much camber as possible and a lot of tire.

[bbnks2]

I am not too sure the above info is accurate. You can't say that changing sway bars changes roll couple distribution but then also say it's inconsequential to weight transfer and grip. Roll couple distribution is what determines if the load is distributed more to the front or rear in a steady-state cornering situation. If one end of the car is more heavily loaded than the other then you are leaving grip on the table since the relationship between load and traction is not linear. We want to distribute load as evenly as possible to maximize traction. Keep in mind that tuning for steady state cornering is not the same as corner entry and exit where you may want to give up grip on one end of the car or other to prioritize entry or exit. But, to the point above, you can also manipulate this behavior with rebound/compression tuning.

So yeah, it's true changing body roll does not change weight transfer by much, but changing roll couple distribution F:R absolutely does change where the weight transfer occurs. Thus, either hurting or improving overall grip that is able to be generated by the loaded tires since, again, the relationship between load and traction is not linear. 1700lb front load + 1500lb rear load = 1.3g of grip. 2000lb front load + 1200lb rear load = 1.2g grip (just making up numbers to illustrate the point).

You can read all sorts of stuff that says "stiffer rear bar = more oversteer." What's important to understand is that that is being said within the context of moving from a NEUTRAL setup to a RELATIVLY STIFFER rear bar. Thus, moving the roll couple distribution rearward and inducing oversteer.

If your car is setup with a high front roll couple distribution as is, and you move to stiffer rear springs or bar, you won't necessarily push the car into oversteer but rather return it to a more neutral balance. Everyone likes to make the front end really stiff because we all understand that mcpherson + roll = bad. While a front end see's more dynamic load and a slightly forward roll couple distribution is desirable/faster that's only to a point. You also don't need to leave the rear alone/soft to make the front roll less. You can keep the car balanced while rolling less... thus maintaining a more even roll couple distribution and higher grip.

Another consideration is REAR BUMP STOP engagement. 750lb out back is not much at all. The dynamic load of cornering and accelerating is likely putting you down onto the rear bump stops. Very rough numbers: 750lb = 250lb wheel rate. Just accelerating at .5-.6g you're loading up the rear wheels with ~1000lb /250 = 4" of compression travel at the wheel and ~2.7" at the strut. It will feel great until it doesn't and you hit the bump stop. Then boom you're going from lots of rear grip to oversteering. Go check how much strut travel you have from static ride height to bump stop engagement. I would bet that it's 3" or less. I say all this about rear bump stops because a lot of people complain about a loose rear end so they're afraid to go stiffer in the rear. Well talk about counter-intuitive... it may just be the thing you need to do!

Fyi, I currently run 800/900 true rear which is 800/1800 in a divorced setup.