Duke's'72 Carbed KA project

[510rob]

thank you for taking the time to share [nice work, nice welding, good ideas, etc...] with everyone

(I'm stealing ideas )

[duke]

The next "over the winter" upgrade, and the one that will most likely have the biggest impact on lap times, is the jump to running full slicks. For the past few years I have run quite a few different extreme performance summer tires, and have been happy with them as they give a good compromise between good manners on the street (as long as it's not raining hard) and good grip for auto-x. In the quest to go faster though, I knew that slicks would be in my future. After asking around and reading a bunch, I decided to go with a set of Hankook Ventus Z221. This is their soft compound for autocross and other short sprint type events. They might give up a little time when compared to something like Hoosier A6's (the current fastest tire) but they are less expensive and have better wear characteristics from what I have read.

To mount the tires on I needed another set of wheels. I wanted to stick with the same size and offset that I am currently running (15x7, +0) so the search began. While browsing a toyota forum I found a guy selling a set of 15x7 +0 Volk TE37's. I have always loved the style of these wheels. They are a little modern, but I think they still suit a 510 well. As another plus, they are crazy light (~10 lbs per wheel). This, along with the race tires, added up to a weight savings of 6 lbs per wheel. A good savings considering it is both unsprung and rotational weight.

[Byron510]

Looks great with the new tires Duke.

I like you input on the thicker washers. I had also contemplated this on my last hardware purchase, but I have yet to see a case of deformation. However I like the idea, and it may be incorporated into the future bracket sets just for fun. The cost difference is really negligible between the two.
I like the idea of marking the camber settings right on the bracket, but this would be different for every installation out there. But for a guy who will always be changing the settings due to different track or handling setups, certainly a very good and time saving idea.

Your car looks great as always, thanks for the feedback.

Byron

[duke]

No problem Byron. The camber markings are really arbitrary. More of a reference to see if anything has slipped than an actual gauge of + or - 1* of camber.

This past Sunday was the first autocross event of the season. With the new changes to my car, I was very excited to see how it would perform. The club that I run with is also trying a new format, which will take some getting used to for me. In the past, all runs during the day (typically 8-10) would count toward points for the championships. Now, only the first 3 times runs would count, and the runs after that would just be "fun runs." I got accustomed to taking my first few runs a little slow in the past to get to know the course and then pick up my pace as the day went on. Now the pressure was on to be fast right out of the gate. This is especially difficult to do when you are in the first run group of the day and have to deal with cold and overcast conditions. But that is how it will be for this year, and that is how most bigger clubs run, so I guess I'll have to get used to it.

Lined up for the first run, all I wanted to do was set a solid time and get a feel for the new tires. I went out and promptly spun in a fast left hander...damn, not how I wanted my first run on new tires to go. I dialed it back for the next few runs and put in an OK time to take second in my class. The think that was really annoying me though was that I was slower than guys that I was beating last year when I was running street tires!

For the afternoon session I wanted to experiment with some things that were bothering me in the morning. I had another big spin (went all the way off course and got stuck on a berm...car was OK though) in the same corner that I spun on in my first run. I narrowed this down to a brake bias issue because it was a sweeping left that I was trail braking though. With some adjustment of my driving and my rear brake bias I was able to get the car under control in this section. I was still not super happy with my times though, I didn't feel like I had the grip that I should, and the car was bogging a bit in one of the slower sections of the track. Taking some advise from a fellow racer who runs the same tires on a similar weight car, I dropped tire pressures from 30psi (which I had been running most of the day) to 27psi for my last coulple of runs. Well, that made a difference! I dropped almost a second from my previous runs, and started to experience that kind of "face bending grip" that is associated with r-compound tires.

This newfound grip has brought up some issues with the car though. The major issue being it has started to miss on long sweepers. It sounds to me like it is only firing on two cylinders, and it clears back up once I'm out of the corner. To me, this sounds like a float bowl issue, perhaps fuel sloshing out. Maybe some of you guys out there will have some more insight into this, because I would really like to get this figured out before the next event.

The next issue is that the rear inside tire has started spinning in turns. I'm thinking that the added grip and added weight transfer of the new tires is to much for the differential (WRX STI R180 w/clutch type LSD). Is there anything that I can do to help combat this? Higher front spring rates perhaps? I'm currently running 350# springs with a 1" bar up front so maybe a little more roll stiffness is needed.

Here is a picture from Sunday.

[okayfine]

The next issue is that the rear inside tire has started spinning in turns.

Might read up on this, as S15 seemed to have similar issues:
viewtopic.php?f=29&t=25559&hilit=torsen

[Sleepys-14]

The rear inlift might be caused by a combination of things. Your rear camber might be a little too much, 1.5* static camber should be where it needs to be. I ran 2* last year and it was too much on my car. You are pulling some more g's, so your front might be leaning a hair more, but I would check your rear droop travel. If you cut your springs, they might be a little too short. It should be a min of 1.5 inchs droop. You might actually be taking all of the wieght off of the inside rear... Also if you are constantly touching the bump stops in the rear, thats another ball of wax.

[qship510]

The next issue is that the rear inside tire has started spinning in turns.

Are your rear control arm pivots raised?

What is the breakaway torque of your LSD?

[Sleepys-14]

I have the same diff too, and it has a very light breakaway. Maybe 30ftlbs. It really puts down the power smooth coming out of tight corner. Very rarely it will unload completely, but that is when my car gets airborn anyways!!!

[duke]

Thanks Julian, I followed S15's thread closely as it was full of good handling and chassis setup info.

The rear inlift might be caused by a combination of things. Your rear camber might be a little too much, 1.5* static camber should be where it needs to be. I ran 2* last year and it was too much on my car. You are pulling some more g's, so your front might be leaning a hair more, but I would check your rear droop travel. If you cut your springs, they might be a little too short. It should be a min of 1.5 inchs droop. You might actually be taking all of the wieght off of the inside rear... Also if you are constantly touching the bump stops in the rear, thats another ball of wax.

Droop and bump travel are all good, but you might have hit the nail on the head with camber. I still have some room under the rear fenders to go a little less negative (with the low ride height and 0 offset wheels, there isn't much room under there) so I think I will be doing that before the next event to see what kind of effect that has.

Sorting a car in this manner (making small changes in between events) is interesting. Because the courses change, it doesn't allow you to get a good baseline when it comes to lap times, and because events last only 1 day, it's hard to make changes (beyond shock settings) between runs. Well, I guess that is just how it goes.

[icehouse]

I think it was Clayton over on Ratsun telling me the guys down your way were adding an extra clutch disk in the diff or something like that maybe it was a shim. I had issues with my Subi R160 doing the same thing.

[Sleepys-14]

Do you do any HPDE events? Doing a HPDE event with 15-20 min sessions during the course of one day can pretty much do what a 4-5 auto-x's can do. You can tune soooooo much with those long sessions!!!!

[duke]

The next issue is that the rear inside tire has started spinning in turns.

Are your rear control arm pivots raised?

What is the breakaway torque of your LSD?

The inner pivots are (Byrons camber toe brackets), the outer pivots are stock.

Referring to Sleepy, the breakaway torque is ~30lbs.

[qship510]

The next issue is that the rear inside tire has started spinning in turns.

Are your rear control arm pivots raised?

What is the breakaway torque of your LSD?

The inner pivots are (Byrons camber toe brackets), the outer pivots are stock.

Referring to Sleepy, the breakaway torque is ~30lbs.

Raising the pivots 1" does amazing things for anti-squat, which will limit your camber change and improve traction under acceleration. Sounds like you are having a weight transfer issue, but every little bit helps.

The highly competitive autocrossers I've had the pleasure to pick the brains of consistently run clutch LSD's with 150'ish pounds of breakaway. So tight that it appears that their differentials are welded when making low speed turns, but just enough to keep the axles from snapping. Shims are used to set breakaway, Perrin probably has them.

[Byron510]

Wow, 150 is high. I set mine to just over 100 in the ice racer and it behaved like a welded diff all around. Now I never ran R compounds on pavement on that car, so that may have been enough to make it work. But for gravel on Yokohama gravel rally tires and for heavily studded ice race rubber (which have amazing grip), I found to lbs breakaway 40 to 50 worked pretty good for a balance between understeer on the entry of the corner and balance through the exit of a corner.
On the Bronze car, set up stiffer and lower than the rally car, I also tried 65 lbs breakaway and found that it tight as well - running A-048's. So it set it back at 43-45 lbs and find it to be a pretty good set up

What are you running for a set up that makes a 150 breakaway work well for you? Tires, spring rates? Just curious.

Byron

[bertvorgon]

I too run my LSD at 45-50, did that for all my solo years, with race tires, and nor 048/R888's