As the modifying process continues with my E30 (that was bought to NOT modify) my next task is control arms. I am installing my new suspension and wanted to do an entire front end refresh while I am at it. The car only has 130k miles (which is low for E30's), but its still 17 years old. I looked into my options and I have two with the actual control arms, and several with bushings. I can go steel or aluminum with the actual pieces. I can then go with stock bushings, upgraded polyurethane bushings, or go all out with Delrin bushings.
My view - my new suspension is nearly a full out race suspension, albeit progressive. Similar to J-stocks, actually just about the same but for a different class. I am replacing the shortened/valved Bilsteins with my Koni components that will make my front double adjustable and my rear rebound adjustable. I like the aluminum control arms, had them on my E30 M3, but still roughly $450-$550 new they are a bit steep. On top of that you add the bushings and its all of a sudden a $600 project. There are the cheap steel counter parts, $85 brand new each. Of course from my E30 side, cheaper is better. From my Porsche side, cheaper = spend more in the end. So I know now that I won't be happy with steel since there is better available. The problem with better is that it comes at a significantly higher cost, close to 200% more for about a 35% benefit.
Another view is the mechanical side of me. Coming from a heavily modified and lowered Porsche, I quickly became familiar with what happens on lowered cars the original geometry...stuff starts to break. Well not necessarily break, but wear out significantly faster. Control arm bushings to name one of the easier parts to name. With the help of a fellow 968'er we measured the suspension geometry...I am talking every square inch of it to get wheel rates, motion ratios, etc... I was in the process of building a bespoke coil over setup (Koni 2812's) and the other member was trying to figure out why his car didn't handle as well since he lowered it and why his stock control arm bushings were becoming toast. Furthermore there were reports of ball joints binding. When I was measuring the motion ratios of the front suspension, I realized that I doubt we will have our ball joints bind, they are at an incredibly odd angle. This promotes wear as well as taking the car out of its stock geometry.
There is a point where you can actually lower your car too much. The point where the central mass of the vehicle is below the effective suspension. This essentially means that your shocks and springs are higher up than the central mass of the chassis. This is not necessarily a bad thing, but the suspension was not designed from BMW to accommodate this. With my shocks and springs I will be approaching this point. Anyone on J-Stocks, IE Stg 3./HR Race without pads, or coil overs at full droop will be in the same category. At this point your suspension is being worked beyond its measured limits - control arms, sway bars/links, bushings, as well as the struts/shocks getting more wear from excessive work.
So what is the fix for us on super low E30’s? Well the simplest answer is raise the car up. Get the car back to its stock geometry. The problem with this solution is the performance and aesthetic of me likes the performance suspension. There are two parts attached to the strut, the tie rod and the control arm. The tie rod has no structural integrity, but is used to control toe and steering movement. Thus the control arm is responsible for the suspension geometry. The option is to have geometry correcting control arms created. This is not my idea, in fact it is taken directly from the Porsche 944 aftermarket division. They suffer from the same issues as we do with the E30.
How are geometry correcting control arms made? This is the question I am struggling with. Currently I am working with an engineering friend on some CAD drawings to contemplate. The first option is to use square or round tubing welded together via the CAD drawings. Since CAD has the ability to determine pressure points and weak points, these would be taken into account. This is the cheapest option. Welds would all be professionally done (you don’t want to see my weldingJ) using the strongest TIG medium possible. The filler has a tensile strength of 84,000PSI which is more than that of chromoly, aluminum, or steel. Most likely these would be done with thick walled stainless tubing, which could then be polished or powder coated.
The other option is a CNC milled piece of billet aluminum. This would be structurally stronger than welded tubing, look better, and be lighter. The downside is the cost of CNC tooling and machining. Literally it is about double the cost.
Cost - This depends on what method we choose to take with manufacturing. I am willing to front the costs for machining or welding based on deposits or contracts, but as a whole it depends on a lot. It will cost me nearly the same amount to make 10 pieces as it will to make 1 piece. I am not looking for a profit…I honestly just want a set myself. Thus it would be my cost divided by the number of orders. I haven’t received the CAD drawings yet, but my CNC machinist guessed roughly $4500 for 10 sets. Welding would be less, but I would need to tac a prototype together for them to weld. There would be a lot more R&D associated with the welding option. I am also need to see what the cost will be to sonic test all the welds to insure the quality.
My view - my new suspension is nearly a full out race suspension, albeit progressive. Similar to J-stocks, actually just about the same but for a different class. I am replacing the shortened/valved Bilsteins with my Koni components that will make my front double adjustable and my rear rebound adjustable. I like the aluminum control arms, had them on my E30 M3, but still roughly $450-$550 new they are a bit steep. On top of that you add the bushings and its all of a sudden a $600 project. There are the cheap steel counter parts, $85 brand new each. Of course from my E30 side, cheaper is better. From my Porsche side, cheaper = spend more in the end. So I know now that I won't be happy with steel since there is better available. The problem with better is that it comes at a significantly higher cost, close to 200% more for about a 35% benefit.
Another view is the mechanical side of me. Coming from a heavily modified and lowered Porsche, I quickly became familiar with what happens on lowered cars the original geometry...stuff starts to break. Well not necessarily break, but wear out significantly faster. Control arm bushings to name one of the easier parts to name. With the help of a fellow 968'er we measured the suspension geometry...I am talking every square inch of it to get wheel rates, motion ratios, etc... I was in the process of building a bespoke coil over setup (Koni 2812's) and the other member was trying to figure out why his car didn't handle as well since he lowered it and why his stock control arm bushings were becoming toast. Furthermore there were reports of ball joints binding. When I was measuring the motion ratios of the front suspension, I realized that I doubt we will have our ball joints bind, they are at an incredibly odd angle. This promotes wear as well as taking the car out of its stock geometry.
There is a point where you can actually lower your car too much. The point where the central mass of the vehicle is below the effective suspension. This essentially means that your shocks and springs are higher up than the central mass of the chassis. This is not necessarily a bad thing, but the suspension was not designed from BMW to accommodate this. With my shocks and springs I will be approaching this point. Anyone on J-Stocks, IE Stg 3./HR Race without pads, or coil overs at full droop will be in the same category. At this point your suspension is being worked beyond its measured limits - control arms, sway bars/links, bushings, as well as the struts/shocks getting more wear from excessive work.
So what is the fix for us on super low E30’s? Well the simplest answer is raise the car up. Get the car back to its stock geometry. The problem with this solution is the performance and aesthetic of me likes the performance suspension. There are two parts attached to the strut, the tie rod and the control arm. The tie rod has no structural integrity, but is used to control toe and steering movement. Thus the control arm is responsible for the suspension geometry. The option is to have geometry correcting control arms created. This is not my idea, in fact it is taken directly from the Porsche 944 aftermarket division. They suffer from the same issues as we do with the E30.
How are geometry correcting control arms made? This is the question I am struggling with. Currently I am working with an engineering friend on some CAD drawings to contemplate. The first option is to use square or round tubing welded together via the CAD drawings. Since CAD has the ability to determine pressure points and weak points, these would be taken into account. This is the cheapest option. Welds would all be professionally done (you don’t want to see my weldingJ) using the strongest TIG medium possible. The filler has a tensile strength of 84,000PSI which is more than that of chromoly, aluminum, or steel. Most likely these would be done with thick walled stainless tubing, which could then be polished or powder coated.
The other option is a CNC milled piece of billet aluminum. This would be structurally stronger than welded tubing, look better, and be lighter. The downside is the cost of CNC tooling and machining. Literally it is about double the cost.
Cost - This depends on what method we choose to take with manufacturing. I am willing to front the costs for machining or welding based on deposits or contracts, but as a whole it depends on a lot. It will cost me nearly the same amount to make 10 pieces as it will to make 1 piece. I am not looking for a profit…I honestly just want a set myself. Thus it would be my cost divided by the number of orders. I haven’t received the CAD drawings yet, but my CNC machinist guessed roughly $4500 for 10 sets. Welding would be less, but I would need to tac a prototype together for them to weld. There would be a lot more R&D associated with the welding option. I am also need to see what the cost will be to sonic test all the welds to insure the quality.
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