How do you size a swaybar?

Here is my current situation:

The car is currently used for both autocross and rallycross. I will be setting it up with more focus on rallycross over the next few months. I am learning how car setup works and have gotten some conflicting information from the internets.

Question 1: Is this amount of roll bad? If it is not bad, how do I make it happen faster?
Question 2: How do I adjust sway bars on a car that already understeers quite badly?

My general understanding is that I need a combination of stiffer springs and bigger sway bars to remove the body roll. If a larger front swaybar increases understeer but decreases body roll, is that a good thing? Do I just upgrade the back?

To solve the understeer issue, you first have to find out why it's understeering. It could be because the roll stiffness at each end of the car is biased to cause understeer, or it could be excessive body roll causing camber loss up front due to the nature of the suspension.

If it's the first case, a bigger front bar would cut body roll but make the understeer worse. If it's the second case, a bigger front bar would actually reduce understeer, as reducing the amount of roll in the front suspension would reduce the undesirable effects that can occur during roll in some suspensions.

I would imagine a car like tha already has some well thought out solutions. Calling a BMW performance specialist will likely answer that question very quickly. (I suspect there are a few wandering around here)

Technically, whether an amount of body roll is bad (at least for steady-state cornering) depends on the car's camber curve. But eyeballing the angles of your wheels in the pic...yeah it's bad.

To find out what effect the existing sway bar setup is having on the handling balance, you could try disconnecting the front sway bar. If that makes the handling balance better (more oversteer), you need more anti-roll stiffness in the rear. If that makes it worse, you need more anti-roll stiffness in the front.

MacP strut - try to let it move as little as possible.

Body roll isn't the end of the world. It just isn't as responsive to input and strut based suspension has junky camber curves.

Softer suspension actually tends to be better for maximum lateral grip and is more forgiving for driver mistakes. At least at higher speeds and/or rougher courses.

For autocross, responsiveness is often more important than overall maximum overall grip.

For rallycross tends to be a bit more rough and running softer suspension could be beneficial.

So, I would do a couple of things:

Get a ton more negative camber in the front end. You'll need to experiment with settings, depending on tires used and spring rates chosen, but you definitely need more.

Increase the spring rate by maybe about 30-50% which should still be comfortable enough to daily driving/rallycross and improve responsiveness for the autocross.

You can go ahead and buy/borrow some larger bars for the car, but I'd do the above first and test a bit before adding them.

Two things I found with struts. Limit body roll and start with 3 degrees negative camber and more caster.

In my case, FWD, a bigger rear bar, slightly stiffer and lower springs worked.

Lol. Ross2004 not true.

Yeah, I don't think I'd say let the McStruts move as little as possible so much as don't let them move more than they nee to for the suspension to remain compliant and grippy. And definitely add some more negative camber, and for Autox, more caster will help too. The extra caster will give you more camber gain when turning, although it will un-weight the inside rear tire more in turns (rotates the car better, but can make it harder to put down power on corner exit with an open diff).

Start with stiffer springs. Give Ground Control a call. Sway bars are good for fine-tuning but I wouldn't use them as a replacement for the correct springs

What class are you running? What tires are you running? The answers to those will heavily influence what you want to do as far as autocross suspension is concerned.

The car is running in the Prepared RWD class in rallycross and currently in STX in autocross. At some point, the rallycross mods might bump the car up to street prepared. For autocross, I am running on Bridgestone RE-11a. For rallycross, I run on all season or snow tires with the potential upgrade to rally tires in the future.

Maybe this isn't a realistic goal but I would like the body roll to happen faster. The car feels predictable when it is neutral or leaning over. I don't like how it feels in between the two states. It feels like it takes a long time to change right now.

It sounds like changing sway bars is getting ahead of myself. I was already looking at springs because I am hitting the skid plate more often than I would like after a bump. The tough bit on e28s is getting aftermarket springs that don't lower the car. Camber plates seem like a good idea too.

In reply to ojannen: The easy fix is to have ground control, eibach, or another good company your car's specs so they can custom make you springs that are stiffer without lowering your car. They may only be able to suggest up to 50% stiffer springs since you use it for rallycross too, and you will need the shocks to be tuned to the springs.

Junkyard "upgrade" time.

Measure the existing springs for length and diameter as well as the thickness of the material.

Hit the local yards and look for springs that meet the specs you think you'd like. You may find that larger BMW's like 5 and 7 series cars might provide the springs you need.

where are you running rallyx? what exactly are the courses and conditions like? how often do you drive the car and are you willing to do some work between events to swap parts of a setup for autox vs. rallyx

I'd start by making sure all suspension parts are in good shape. Check shocks, bushings and all mounting points. Then once you are sure those are good reassess the situation.

A week later, the car is still rolling

It turns out you can't get stock height custom springs for this car very easily. Trying to figure out what to do next. Sport springs and thick spring pads is an option. If I can figure out if any of the available coilover kits let the car run at stock height, I will give it a try. Considering I will need new or revalved shocks and camber plates, the price difference is low.

Have you explored options using e34 parts? I know there's some interchange.

My advice would be to get a pair of these in whatever size works for you to jam into your front coils: spring spacers.

I've used those and similar on various RallyX cars to get a bit of lift and an increase in spring rate for cheap. You can also cut them into 3/4, 1/2 turn, etc. sections for fine tuning. If you have more budget then maybe build your own coilovers with some threaded sleeves.

^Good idea as a quick fix. Threaded coilovers are a good long-term solution, but they're not a cheap solution.

In reply to ¯_(ツ)_/¯:

You can buy those at Oreilly for $6. You can also cut a bit out for smaller springs. Common circle track item, you can get them in different durometers if you want to get spendy.

Also, I've rallycrossed an e28 on snow tires. The technique I used to get it to turn was basically "stand it on its' nose for turn in, then drive with the throttle". I'm not sure what that car's setup was but it sure didn't feel like it had much roll stiffness and it worked well that way. If you're trying to use a gentler, road racing type gradual turn in technique that car will probably ALWAYS push on a loose surface.

My mind was just blown. I thought those spacers went on the other side of the spring perch. I was dreading taking apart the suspension to put in temporary rubber. I will give these a try this weekend.

In reply to ojannen:

Some of them do! Depends what you have, but if it's got a "U" shape on both the top and bottom then you're in business.

Sway bars came in before springs. I put the rear on before I ran out of time. Maybe not the best decision.