I'm going to venture to say this is from the race rubber.

Lotus has a similar issue with the Elise/Exige as an example. If slicks are fitted and proper mods are not made to the car the suspension cannot handle the increased cornering/braking loads.

I would think that people on street tires will hardly ever see this.

 

It looks like that should be easy to have some welds added to those points to strengthen the connection.

 

Jason thanks for the pics.

I now see the problem area but still don't understand why braking is the culpret. Looks to me like you would need hard left/right cornering to put the upper part of the A-Arm in tension. In the case of the passager side as shown a left corner would in essence pull the upper A-Arm away from the body as the tire is loaded. Race rubber very well could be the problem then.......

As for the trailing edge being under tension when braking.......I still don't see it.

As the brakes are applied the tire is the part that grips the pavement. The momentum of the car continues to move forward. The A-Arm is the connection between the tire and the body. This would cause the A-Arm to want to twist in a clockwise direction if we're talking about the right side of the car. By this I meet clockwise in "plan view" or in a plane parralel with the ground.......This would compress the trailing leg forcing it back into the body putting the leading edge in tension.

OK now I'm really confused!!!

 

Well, I can confirm that this has happened to at least one more S2000... Good thing we found a good welding shop at Ft. Myers!

Jason, I'm not sure I'd agree with you on the braking thing, but who knows... until an engineer plots out the forces, it's all speculation. I think it's the severe load under cornering with R-compound tires, combined with some of the bumpy surfaces we run on.

For you track junkies, I wouldn't worry about it as much unless you like to pound the curbs. The ashpalt surface of most tracks doesn't create the type of cornering force as some of the concrete that we run on, and when you combine those forces with bumps it's like using an impact tool to hammer away at those brackets....

When I visualize it, under cornering the bottom edge of the tire is taking the most load... that's where the friction of the tire changes the direction of the mass of the car in a turn. If the bottom edge is being pushed in like that, It's pulling out on the upper arms. That's why I'm guessing they made the upper mounts so flimsy, with only a couple tack welds. So with all that force pulling out on the upper arms (mostly the rear one, if you can imagine the forces in a corner).

Anyhow, I think anyone who autocrosses or runs their S2K on the track should keep an eye on these brackets... I have pictures of ours, once we get them uploaded I'll post.

 

Mack, when you brake, you're using the caliper to grab the rotor and slow the wheel down. This produces a torque. The upper and lower control arms anchor the caliper to the body of the car, keeping the caliper from just spinning round and round the wheel. The torque on the caliper from slowing the wheel is transferred to the chassis as a rearward force on the end of the lower control arm and a forward force on the end of the upper control arm.

In addition to that torque, there is a net rearward force that is shared between the upper and lower control arms. That's probably what you're thinking of.

I don't know which Jason is correct about the real cause of the failure, but both are good hypotheses that fit the known facts. Maybe it's a combination of the two.

 

John-

Thanks for the clarification. I wasn't thinking about the torque/twist that the braking would cause......... I think I got it now.

 

No problem...You know, I had been unhappy about the Miata's sway bar brackets being prone to fatigue failure. Now I realize it's not so bad: at least that can't cause the wheel to fold under the car when it breaks! Keep an eye on those buggers.

 

I think its time to resurrect this thread.

I've just made an appointment for monday to get my A-Arm brackets welded. While doing some routine maintenance I noticed what appear to be the beginnings of the above pictured cracks. I haven't AutoX'd the car in about 6 months but I track it at least a couple times a month and lately I've been using race slicks at the track. I imagine that the use of slicks has resulted in this but just incase I urge everyone who pushes the car hard to go and check their brackets and to make it a habit to check them whenever they have the front wheels off the car.

Jason & Jason, In talking to the welder we decided to weld the inside vertical seem and the outside vertical seem. Is that what you had done with your fix? Or, did you actually add reinforcement to the bracket as well? Maybe a bottom cap that bridges the open end of the bracket below the bolt and is welded to the frame as well?

 

I would like to know also. I will be checking mine shortly.

Dave, are you going to Buttonwillow? Are you all ready for the OTC?

 

I'll be going to BW on the 21st with SV and WS on the 12th with OpenTrackRacing.com I need to get in that last little bit of DCB practice before OTC.