I'm getting very close to aligning and corner weighting the car. What sort of percentages should i be looking for in cross and front to rear? I would guess 50% cross with me in it and closer to 45% front/55% rear.

Yes that's about right. These are mine with me in the car.


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Woody you need to take those numbers and use the corner balance formula to calculate ideal balance

What do you mean ideal balance?

there is a formula to calculate Ideal balance using your actual corner weights...


it will tell you how far off you are... what each corner should ideally weigh given the weight distribution of the actual car.

your pretty close woody... within 3 lbs on each corner
good enough really
I usually quit trying in the 1 to 1.5lb range.

brant

Having a cross weight of 50.04% (second pic) is darn good.

Thanks. It wasn't too hard to get it there.

Hi everybody. We are getting ready to start building a 914 track car. Is the ideal weight distribution on a 914 45%F/55%R and not 50/50?

Thanks in advance.

Van

50/50 would be optimal on any car. 45/55 is where I see most mid engine cars end up. Anywhere from 40/60 to 43/57 on a rear engine car.

Okay, thank you. 914s are new for us so I didn't know if the chassis preferred a slightly rear weight bias.

Van

Oh yes, the magic calculator. Probably does more for handling than anything, IMHO.

Just out of curiosity, why not 50/50? Is everybody dead set against adding ballast?

Van

After busting my chops REDUCING weight, it pained me to add ballast to make class minimum.
Adding weight for any reason beyond class requirements is an anathema.
Shifting weight to get ( let's say ) side to side balance is SOP.

Personally, I like a rearward weight bias percentage in the mid to high 50s.
spending a lot of time fooling around with corner weights becomes moot under most non-static activities...makes ya feel good, tho. Note tires contact.
Care to guess the percentage of car weigh on that left rear?

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I would only add ballast to meet a class minimum weight. Moving stuff around in the car such as the battery is how you would optimize your F-R and L-R weight. Like JP, I also prefer a slight rear weight bias.

We had to add about 80 Lbs of ballast to our RX7 to get to 25/25/25/25. I wasn't wild about adding the weight, but without it the car felt as though it was trying to kill you in every corner (the transition from understeer to oversteer was very abrupt) and 90 minutes of that was very fatiguing. We had already done everything we could to eliminate/transfer weight.

I haven't driven a 914 in about 25 years and I can't remember what the handling was like. Is the preference for a slightly rear weight bias because of the open diff?

What wheels are on the back of that car?

Thanks!

Just my.02 based on what you've written.
Suspension set up is the place to cure an unwanted transision. Consistent handling be it slight understeer or oversteer with no transitions is ideal.I've not heard of any design flaw in the RX7 that would cause this (such as is the case in the 911).
Class rules may preclude taking the steps necessary,( limits on allowable suspension mods, tires, whatever.) however. I have heard of guys building in a transision using progessive springs tho.

Anything to avoid adding weight.

The only dishonest feature of the 914's handlingis the snap oversteer that comes when messing with the absolute limit.....there is no warning, it just goes and there is no catching it unless one is readyfor it, It takes a lot of butt time in the car to know when you're on that edge.....even then it will "gotcha" occasionally.

The rear wheels were made by Monocoque....now defunct. 10 X 16 and 11 lbs each.
22,5 X 10 Hoosier R35 slicks.
I was fooln' with rear tire pressures & got em' a bit low. They were OK at 20 psi...but not at 18. Damn near ran the tire off the rim .....didn't go there no mo.

The rear bias helps with traction. TB diff in place. Helps the car rotate controlably in the tight suff at AX.

I don't know if you would call it a design flaw, but the suspension on a 1980 RX7 is nothing to get excited about. Mac struts in the front with no adjustment for caster or camber and a live axle with a Watts linkage in the back. And we are very limited in the amount of suspension modifications that can be made.

We didn't weigh the car until after our first race and the corner numbers were:

Total weight: 2100# (with driver)
LF = 37%
RF = 30%
LR = 18%
RR = 15%

Thus, the demonic handling. There simply wasn't enough weight that could be removed or moved to get anywhere close to a better balance, thus the ballast. Not surprisingly, the handling was vastly improved with equal weight at all corners (& we simply added about 30% more HP to overcome the additional 5% weight).


Sorry to go OT here and pardon my ignorance, but what is a "TB Differential"? I Googled it and ended up reading about tuberculosis. Is TB a manufacturer? Pretty sure that neither of the transaxles we have are factory LSD but the 914 is still in the trailer and we haven't looked.

Thanks for all the great info.

Van

Sorry, TB is Torque Biasing. na LSD with an extea rweak.
It acts like an open diff till you put the power to it.

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Right, and TB diffs make a huge difference as well.

I used to have a very unpredictable snap oversteer problem with my car when I was running a square setup. When I went to the 205/225 Hankooks the problem lessened. With my current setup of 225/275 A6s the problem is non existant. I have the car set up just a tad loose in the rear because thats how I like it. If you go to hot into a corner the car will let you know and the back end will start to come around but it is easily caught. Same with corner exit, the backend will start to come around and is easily caught with a quick jab of the wheel. Mid corner is neutral. I am not running a LSD yet either. I need to because it will spin up the inside tire on corner exit.

After I did all the suspension upgrades, corner balancing and figured out the tire issues I literally couldn't get in front of my car. It was super hard to find the limit.

Well that was because it was finally "balanced" and didn't have any of the old major issues anymore. Fun actually as you have to learn how to drive faster around corners.

Last year at the Spring Enduro in Medford I was a bit aggressive on my braking point and overcooked the first turn beginning the second lap. I thought I was toast, but with driver adjustments the car just came right back and I carried on. That first lap ended up being the fastest of the day.

BTW TBD are wonderful. Hardly know they are there, they just work.

Its next on the list after the new engine is completed.

I can still feel my TB Diff...
If I go into a high speed sweeper and the car doesn't feel like its going to make it... I can mash the throttle and feel the line change and make the corner.

If I white knuckle and don't mash the throttle I usually drive off track
but the TB diff will slightly turn the car if you trust it and floor it right at the moment you don't think you will make the corner...

my old locked diff did the same thing but 4times more.
the only way around the corner with a locked diff was to
- kick the rear end out and then rotate the car..
- mash the throttle when the front end was pointing the direction you wanted to go in the rotation sequence.
- and go straight in the direction it was pointing

the big advantage to the locked diff was:
1) over cooking the braking zones in every single corner and making it due to the fact you were sliding sideways through the braking zone and cutting speed due to the slide.
2) being slightly faster due to the constant slide and practicing this style of loosing the car-catching the car... loosing the car-catching the car, in every single corner. The style taught you better reaction and better skilll at catching it. but it also made it a lot easier to mess up since you had to catch it 10-15 times per a lap.

The big advantage to the TB diff is
- the abilility to drive off line in the marbles or on wet or imperfect surfaces.
- the TB diff is a lot smoother. Just as good at full throttle-out and only gives up anything under braking. But MUCH better tire life and MUCH easier to drive due
to the technique differences.
- more forgiving of driver error, easier on the equiptment, and easier to maintain concentration...
- easier to be in traffic, since loosing controll side by side at 80mph is such a fine line when your 2 foot away from the other guys door.

if there was a driver error with the locked diff it was less forgiving
you would loose 5 car lengths to the next guy if he shut the door or you misjudged anything. I think it was faster on open laps, but you could never let up and never loose your momentum.

Nice post Brant. Very informative. I was never intending to run a clutch type LSD since its an AX car. It is nice to see the comparison though. I have heard of successful cars running a clutch type for AX but its not the route I want to take. I enjoy my turn in.

Dang, this is pure gold. Thank you for sharing your experiences.

[quote] [quote name='brant' date='Mar 1 2014, 04:20 PM' post='2004815']
I can still feel my TB Diff...
If I go into a high speed sweeper and the car doesn't feel like its going to make it... I can mash the throttle and feel the line change and make the corner.

If I white knuckle and don't mash the throttle I usually drive off track
but the TB diff will slightly turn the car if you trust it and floor it right at the moment you don't think you will make the corner...[/quote]



Interesting comments: I've never had my car on the track, just autox and hillclimbs. A hillclimb isn't where you want to experiment, so big sweepers there are very measured, from a drivers point of view.

Although a great (tight) sweeper is the first turn at Medford (Fall Enduro Course). http://www.ssccmedford.org/track.html It's right after the straight, so very fast and scary. Should have tried your thinking there, but didn't know and now it's too late.
High speed sweepers in autoxing are unfortunately rare. But when they are there I go through them flat out, so guessing the TB diff is allowing that to happen.

Finally finished my (race) alignment and corner balance at Iron Canyon Motorsports. We were there for about 8 hours! It took forever to align the car as the passenger side rear was being difficult and needed some "engineering" to get enough toe-in out of it. Once we got past that and the initial set up things went pretty well.

The corner balance was damn near comical. I spent a ton of time getting the ride height and rake "just so". When it came time to do the corner balance all it took was about 3/4 of a turn on the front passenger side torsion bar adjuster and it was there! I guess the chassis set up god's punished us enough with the alignment.

I can't recommend Mark at Iron Canyon enough. He has an amazing shop, endless patience, and is more thorough than I would be with my own car. A rare personality to find these days!