[AXR] Autocross setups

View previous topic View next topic Go down

[AXR] Autocross setups

Post by jtmineha on Mon Jun 29, 2015 6:16 pm

I figured we'd start a thread where we could post info on our chassis setups to compare notes. I know there's just a few of us with the AXRs but some of the info may cross over to the other models.

Some of the things Scott and I have been working through this year with the autocross setup is reducing understeer. The big 14" rear tires and other chassis settings had the car quite pushy the beginning of the year.
We have made a lot of adjustments and are starting to see some progress I the right direction.

We've softened the front springs to 150# and stiffened the rear to 500#.
We've adjusted the dynamic rear toe to stay close to zero change- before it was toeing in under compression. We have also played with ride height/rake and currently run the back about 1.5" higher than the front.

The rake changes seem to really make a noticeable difference.
Also key things have been watching the tire temps- particularly front tires. They tend to heat up a little more then the rears. So watching the temps and cooling between runs as needed makes sure you aren't overheating the front tires.
Another key thing is alignment. Getting the camber in the right place (-1.0 front and 0 rear) and the toe zeroed out and square to make sure the alignment isn't causing ther issue. Lots of ride height changes and such can move those things around so always good to double check that stuff and make sure it's where you want it to be.

avatar
jtmineha

Posts : 35
Join date : 2015-05-02
Location : Austin, TX

View user profile

Back to top Go down

Re: [AXR] Autocross setups

Post by dgoetz on Fri Jul 03, 2015 7:47 pm

The rake changes seem to really make a noticeable difference.
I am running close to the same set up as you. You said you changed the rake, from less or from more rake? I am running 3/4 in rake now, what can I expect in the change of handling when I adjust to more rake?
avatar
dgoetz

Posts : 9
Join date : 2015-06-30

View user profile

Back to top Go down

Re: [AXR] Autocross setups

Post by jtmineha on Sat Jul 04, 2015 5:18 pm

I'm running about 1 1/4" rake. Measuring points being from the frame under the control arm being 4" and measuring the rear frame just in front of the rear tires is 5 1/4".
The increase in rake - higher ride height in the back really helped resolve some of the understeer issues. Turn in was much better as well. This was after about a 3/4" adjustment at the last practice day. I went from 4.5" I the back to 5.25".
avatar
jtmineha

Posts : 35
Join date : 2015-05-02
Location : Austin, TX

View user profile

Back to top Go down

Re: [AXR] Autocross setups

Post by jtmineha on Mon Aug 17, 2015 12:20 pm

Made some more changes to the car for this past weekend and had some pretty decent results.
Went from 500# rear springs to 650# (on the middle rocker arm setting), and reduced the rake a little (back to 1" overall). I also got a pair of the 20x10 Avon A11's from John Berget to try out on the front to replace the 20x9.5 Goodyears.
The combination of the new front tires and stiffer rear springs seems to be working much better to improve turn-in. The event was on a lower-grip asphalt lot that I usually struggle with understeer really bad, and the car was pretty balanced this weekend.
not very scientific, but I never was able to get the car to work well on the Goodyear front tires, so I think I'm going to stay away from them for a little while. I still have the Goodyears on the back, and will probably just continue to run them until they are gone.

My tire plan for Nationals will be a fresh set of Hoosiers. I'll probably test the Hooisers and Avons on the front on the test course before Nationals to see if there is any difference.

In playing with the Rear rocker arm settings the past few weeks is interesting. It makes for a great tool that can be changed relatively quickly. If you don't change the length of the pushrod, the changes in rake when you move the rocker arm setting sort of work in the same direction with the wheel rate. So if you are at the middle setting, and experencing understeer, an adjustment to the lower (stiffer) setting, will increase rake about 1/2" and increase the wheel rate- both good things to help improve steering response on turn-in.
Like wise, if you are at the middle setting and the car is too loose, the change to the higher (softer) setting, will soften spring rate and reduce rake. Both of these should make the car more stable.
So given the quick turn-around between runs at a typical autocross, I think the rocker arm setting is a good tuning tool to make some fairly noticable changes in the handling very quickly. Similar to what I have done with a swaybar adjustment on previous cars. If you feel the rake adjustment to be too much, it's also pretty easy to adjust the spring perch to fine tune the change a little, also.
avatar
jtmineha

Posts : 35
Join date : 2015-05-02
Location : Austin, TX

View user profile

Back to top Go down

Re: [AXR] Autocross setups

Post by dgoetz on Mon Aug 17, 2015 8:54 pm

It is very interesting how we came very close to the same settings but i think we both got there differently.  
Last winter i had my machinist friend come over with his dial indicators and adjustable magnetic basis and we calculated the motion ratios both front and rear, the rear in two holes (std & stiffest). We did this by aligning the dial indicators on the spindle and the top of the shock directly in line with the travel of each to be as accurate as possible.  Then measured every .1 in or one rotation of the dial with the car jacked at ride height. I then calculated the springs weights and rocker position to get into the ideal autocross wheel rate to corner weight of between .8 to .9.
I think you may have done more by seat of the pants feel of the car which for me in the middle of a +300 in of snow winter is impossible to do.
Below are the charts i came up with. The red numbers are my specs and the green is yours for the rear. I think our fronts were already very close.
What air pressures are you running?
why drop the rear ride height 1/4 in ?


avatar
dgoetz

Posts : 9
Join date : 2015-06-30

View user profile

Back to top Go down

Re: [AXR] Autocross setups

Post by jtmineha on Tue Aug 18, 2015 11:24 am

Scott had shared your spreadsheets with me, and that was what I used to base my new spring choice for the rears. I had been running the 400#, then the 500#, then tried 500# at the stiff setting- which I liked. So I went up to the 650# at the middle setting to basically match the 500# at the stiff setting. That would give me room to either go softer or stiffer to adjust handling one way or the other. Since the 650 spring was a little stiffer wheel rate than the 500 at the stiff setting, I figured a reduced rake would get the handling about the same.
I've been talking with local autocross guru, Erik Strelnieks about getting the handling right, and so I've been following a lot of his advice at test and tune days to test things out. He's the one that got me to play with the rear rake and adding ride height to the rear to get the car to be more responsive. When I asked him about extreme amounts of rake, he said that it typically means that you don't have enough rear spring rate. So go stiffer on the rear spring and bring the ride height back down should get the responsiveness you want in the front, but keep the rear end more stable and predictable. So that's really the direction I am working with at this point. Haven't really proven it out, but the car did work pretty well at the past event so I think it's going in the right direction. Still need a couple more events to really know what's working.
Also, I was having some interference issues with the outer CV boot and the lower control arm/pushrod mount when I was running the high ride height. So the reduced ride height was sort of a move to protect my CV boots and get me through the weekend, until I could send my control arms back to Scott to have him modify the mounts.

I am still running 150# front springs, on the stiff setting (the soft setting was causing interference between the two springs).

As for tire pressures. I have been running the fronts around 26# and the rears at 19#. The temperatures look pretty good across the tire, but might look at trying to add some air in the rear tires and see what that does.

I'm really looking forward to Lincoln and getting time in on the test and tune course with fresh tires. Then maybe I'll have a better idea of how well the setup is working.
avatar
jtmineha

Posts : 35
Join date : 2015-05-02
Location : Austin, TX

View user profile

Back to top Go down

Re: [AXR] Autocross setups

Post by dgoetz on Tue Sep 01, 2015 9:46 pm

Jason
I think you are using Penske double adjustable 2&1/2' dia shocks. You went from a rear wheelrate of 288lbs to 468 lbs, a 62% increase, what adjustments did you make to the shocks?
The reason i am asking is Mine was the first AXR built and mine came with the std 1&7/8ths Gaz shocks.  I couldn't fit the 2&1/2 shocks without totaling modifying the frame shock mounts and rocker mounts. Instead i went with a company called Advance Racing Suspensions out of Indiana. they custom built me an all aluminum  1"&7/8ths" by 13 & 1/2" long shocks at at at savings of over 2lbs per shock & spring. They built them according to the specs i sent them along with the charts you have. I have increased my rear wheel rate by 25% and wonder if i need to send my rear shocks back and get them revalved for the increase?
If i don't hear from you before you leave for the Nationals....Good Luck, i hope to join you there next year.
avatar
dgoetz

Posts : 9
Join date : 2015-06-30

View user profile

Back to top Go down

Re: [AXR] Autocross setups

Post by jtmineha on Wed Sep 02, 2015 3:21 pm

I have the same Penske shocks as Scott and my Dad have on their car.
So far, haven't played with the shocks since the spring change. They may be getting to the point where they may need to be revalved to work better with the stiffer springs, but I don't know for sure.
At some point I was planning to get the adjustment knobs installed (the standard "sweep" adjuster is a major pain in the butt), so if I need a re-valve I'll work that in at that point.
I really haven't put enough time in the car with the new springs to know what is right yet. Hopefully I'll get it sorted out in Lincoln on the test course.
avatar
jtmineha

Posts : 35
Join date : 2015-05-02
Location : Austin, TX

View user profile

Back to top Go down

View previous topic View next topic Back to top

- Similar topics

 
Permissions in this forum:
You cannot reply to topics in this forum