SCCA Solo EP Double A-arm Build

[qship510]

What spring rate did you go with?

Thanks, its been a lot of work. The springs are 450 lbs/in which doesn't mean much unless you know the motion ratios and shock angle so please don't try to compare it to the spring rates on 510 or ZX struts.

So what is the wheel rate?

[icehouse]

What spring rate did you go with?

Thanks, its been a lot of work. The springs are 450 lbs/in which doesn't mean much unless you know the motion ratios and shock angle so please don't try to compare it to the spring rates on 510 or ZX struts.

So what is the wheel rate?

x2

[ktberube]

Your killing me Smalls!

OK, without any other info to go on, this really shouldn't mean much to anyone, but the wheel rate is 290 pounds.

[Byron510]

but the wheel rate is 290 pounds.

Perfect - I think!

Our pogo stick front suspension (as Ice likes to put it) seems to work well under duress in the 275-300# range. However, if your homework is good, you may get away with less spring, or you just may be able to use more stick then I ever have!

Nice work; I've loved this thread.

So, no about that rear suspension....

Byron

[ktberube]

The design was to hold the front of the car up at my perferred ride height and install as long a shock and spring that I could get to fit and have the spring hold up the car so that the shock and spring are roughly in the middle of their travel. This resulted in 450 lb springs and working backwards gave it the 290 per inch wheel rate. The rear suspension has to stay a semi-trailing arm per the rules (unless I want to go to a solid axle). The rear swaybar will be mounted internally and mount off of the cage structure. This should give me the ground clearance at 3.5 inches (measured from the front crossmember).

[five&dime]

I just started reading this thread. My mind has been blown!!! Too bad my..... Ahh nevermind

[S15DET]

In your opinion, would there have been any advantage in designing your mounting structure in a way that would make the arms longer? Maybe it's a rules issue, but could you have located the inner mounts closer to the motor and thereby had longer LCA and UCA's?

[ktberube]

Engineering is a game of compromises. True, long arms are better than short, but with the engine where it is, making the arms longer wouldn't have bought me any more and I would get into clearance issues. Remember the upper and lower arms have to work together and both sides have to compliment each other (I have to turn left and right). As stated before, my roll center only moves 0.15" in the vertical and 1.1" in the hortizonatal through the entire range of suspension travel. The only thing I wish I could have gotten more of was neg. camber gain. Even so, I only get 1.3 degrees of positive camber gain through the entire range of motion. Easily accountable with a mild static camber setting.

[icehouse]

Why would longer arms be better? Isn't the difference in length what makes everything work? Wouldn't a sorter upper arm make higher camber gain numbers as the suspension compressed?

[ktberube]

Exactly. And with longer arms, bad characteristics are less pronounced with movement. That's why ground build race cars move the engine out of the way so they can get the longest arms. This gives the most options in deciding what characteristics to build for: roll center, roll axis, camber gain, etc.

[ktberube]

I believe that a steady and well defined roll axis will allow the driver to get everything out of the car. I have seen twitchy cars post some great numbers, but are impossible for a driver to drive consistently. It only takes falling off the cliff once to end up in the wall or to hit a 2 second cone that places you tenth.

[ktberube]

Bump and quick update. Sorry, no pictures yet. After trying many different headers (comp, BRE, etc. (thanks Seth)) to find a set up that gives me the most clearance for the raised tie-rod attachment points on the custom draglink, I have settled on the comp header I have and lowering the inner tie-rod pickup on the draglink just a bit. Not a big deal as I can compensate on the steering arms... After deciding to mount the calipers in front of the spindle centerline to give me freedom on my steering arm design, I can match the height of the inner tie-rod pick up. Funny thing in all this, I found that the steering arm off the box hangs close to the height of the front crossmember. If I hit (3.5 inches of ground clearance) I want to hit the crossmember first, the pan second, and my steering third. Engineering is a series of compromises and I have considered all that I need to finish the project (everything else is done) so I plan on being done by October. I will have pictures before then. The rear bar install came out nice since a full cage allows many attachment points that structurally make sense.

[ktberube]

Well, I got the steering designed and it will actually fit. Ended up sticking with the comp header that I had. Tried three other headers and it did not buy me more clearence where I needed it. With the comp pan, I had to remove the steering box every time I changed headers.

I am shortening the draglink to 20 inches to give me improved ackerman all the way down to a 20 foot radius corner. This leaves me with a little over 1-1/4 turns lock-to-lock. Just takes getting used to. The ackerman looks incredible from 40 feet down to 20. Much better than a stock 510. The draglink design allows me to fine-tune the length. I have finished modifing the pitman and idler arms. I still need to make the actual steering arms. I have them drawn up, just need the material and a weekend day at the mill.

[icehouse]

What dictates your ackerman angle? With increased speeds and tire deflection I never quite understood what would be the best ackerman angle. All I can guess is since you auto-x it must be setup for that type of driving. Lower speeds and sharper corners. I really like the idea of making it adjustable. Making adjustments would maybe connect the math to the car

I can't wait to see a video of it on the track!

[ktberube]

True, at speed, your ackerman angles will change. So you have to start somewhere. Statically for now.

Draw a line through your rear tires, measure out 20, 30, and 40 feet from the car's centerline, draw a line from these points up to the front tires, measure 90 degrees from these lines (and you thought you'd never need trig or geometry after graduation).

You will notice for the same radius turn your inside tire has to turn more. Now measure what your 510 does (and most production cars for that matter). The inside will lag and not turn enough. Your front wheels are driving around two different turns. If you aren't even there statically, you have no chance of getting there dynamically. You can dial in some toe-out, but that's hard on the tires and makes your car twitchy.

Since I had to make my own draglink, I had the opportunity to improve my ackerman, but it comes at a cost by reducing my total lock-to-lock turns at the steering wheel.