Rear end feels light under braking

[Elombard]

First time on the track with this 911 and 3rd time ever in any 911.

I have done quite a few track days and had an SCCA racing license in the 90s but not a really experienced driver by any means. Previous cars were Datsuns and Mazdas.

This car is an 84 911 stock motor open exhaust. Light weight RS/R gruppe mods FG bumpers, race seats, roll bar, Ac and Radio Delete etc. Weighs a bit over 2400 lbs.

Suspension - bigger T bars (have to look up the size) and fresh bilstiens with the Nurburgring map on them (Sports?)

Stock brakes - fresh discs, street brake pads some brake cooling at the front.

Car has been lowered and aligned by PO but NOT Corner Balanced. Not sure if the alignment is just a street or a bit more aggressive.

Tires - Toyo RA1s in good shape 245/16s rear and 225/16s front. I dont remember the pressures I was using, I played around a bit and I think I was running them harder vs softer. I believe some one there said the RA1s wear longer when they are a bit higher pressure, sacrificing a bit of ultimate grip? or maybe I read that here. I think I was around 34 lbs hot.

Weather was pretty cold low 50s? This was at the Little Taladega track in Alabama in Nov.

I was trying hard to brake in a straight line and as I got more comfortable and more aggressive after a few sessions I noticed that I was losing confidence in the rear. I wanted to adjust the brake bias to have more brake in the rear (like I could on my Datsun 510 race car a hundred yrs ago).

I started researching here to see if there was a PV on the rear of the 911s - looks like there is not.

Then I remembered that the car has not been corner balanced and a friend mentioned set up/alignment.

So before my next track day I want to eliminate as many variables as possible. I plan to Corner balance, check the alignment and install some track brake pads.

Any other things I should look at to minimize this loss of confidence in the rear of the car?

[Cajundaddy]

It sounds like toe out on your alignment. When the car squats in a braking zone, toe out is exaggerated and the rear gets squirrely . There could be other reasons but this would be my first stop in troubleshooting your symptoms.

[Elombard]

Does the rear squat? Seems like it was raising up above stock height? Regardless I need to look up alignment settings for a dual purpose car - sounds like a bit of toe in on the rear is a good idea.

[RichardNew]

I had a similar problem. I solved it by adding a huge amount of rebound into the rear shocks. The rebound pushes the tires into the pavement as the rear becomes light under braking. Find a really good shock guy and talk to them Don't bother talking to a salesman - you need an engineer.

Richard Newton
Facebook

[Matt Monson]

Very common complaint about the 911 and its rear engine design with all that weight out behind the axle. If you are going to track the car frequently the single biggest thing you can do to settle the rear end is to get a proper LSD in there that locks under braking.

[Elombard]

Ahhh - that is where the LSD helps - good I guess, now I can justify the cost of the LSD ;-)

I should do some searches, I am sure this has been a topic before.

[Bill Verburg]

Originally Posted by Elombard

First time on the track with this 911 and 3rd time ever in any 911.

I have done quite a few track days and had an SCCA racing license in the 90s but not a really experienced driver by any means. Previous cars were Datsuns and Mazdas.

This car is an 84 911 stock motor open exhaust. Light weight RS/R gruppe mods FG bumpers, race seats, roll bar, Ac and Radio Delete etc. Weighs a bit over 2400 lbs.

Suspension - bigger T bars (have to look up the size) and fresh bilstiens with the Nurburgring map on them (Sports?)

Stock brakes - fresh discs, street brake pads some brake cooling at the front.

Car has been lowered and aligned by PO but NOT Corner Balanced. Not sure if the alignment is just a street or a bit more aggressive.

Tires - Toyo RA1s in good shape 245/16s rear and 225/16s front. I dont remember the pressures I was using, I played around a bit and I think I was running them harder vs softer. I believe some one there said the RA1s wear longer when they are a bit higher pressure, sacrificing a bit of ultimate grip? or maybe I read that here. I think I was around 34 lbs hot.

Weather was pretty cold low 50s? This was at the Little Taladega track in Alabama in Nov.

I was trying hard to brake in a straight line and as I got more comfortable and more aggressive after a few sessions I noticed that I was losing confidence in the rear. I wanted to adjust the brake bias to have more brake in the rear (like I could on my Datsun 510 race car a hundred yrs ago).

I started researching here to see if there was a PV on the rear of the 911s - looks like there is not.

Then I remembered that the car has not been corner balanced and a friend mentioned set up/alignment.

So before my next track day I want to eliminate as many variables as possible. I plan to Corner balance, check the alignment and install some track brake pads.

Any other things I should look at to minimize this loss of confidence in the rear of the car?

A stock '84 will have a 33bar p/v on the rear circuit
the brake torque figures for that car @70bar line pressure w/o p/v are going to be 1444/1183 for a 1.2 ratio, w/o p/v it's 1444/639 for a 2.26 ratio.

previous 911s used different brakes and had 1444/969 w/ no p/v. If you remove the p/v and use the older rear M calipers and rotors that's what you would have.

Is one end locking before the other? Is that what is causing the tail waggle? On track when braking w/ 3.2 Carreras they seem to lock both ends at about the same point, but that could be w/ a different car set up than what you have.

An effective lsd helps in hard braking, old style was a 40/40 or 80/80, more current think is 40/60, 40/80 ie an asymmetric lsd w/ low preload

corner balance is definitely a good place to start, additionally the more camber an end has the less straight line grip it will have, the tires lose brake and acceleration grip.

[cstreit]

Couple potential causes:

1. Too much toe out in the front (what are you running?)
2. Not enough toe-in rear (what again?)
3. Torsion bars in front too soft (what again again are you running here?)
4. Open differential
5. Rear sway bar has preload
6. Different pad compounds or imbalanced F&R brake setup (are you stock or upgraded?)
7. Corner balance is off.

[Elombard]

No brake lock up issues, it just felt light and I was feathering the throttle at the end of the braking zone on turn in because I was not confident that it would stick. I just did not feel like I could lean on it very hard and was having to tip toe at certain points which was a real confidence killer.

Interesting that there is a p/v in the ircuit I must have read the chart wrong. But I think I will start with alignment and CB and see how that feels.

I almost want to go stiffer also to get less weight transfer but its pretty freaking stiff as is.

[Elombard]

Thanks for the list Chris,

let me get back to the board on some of the specs and I will check the alignment and rear bar pre load.

[Mahler9th]

It seems you have a lot of suggestions here... from folk's own experiences and from theory.

It does not seem that you have a complete handle on "what the car is" in terms of a system and its current settings, like alignment (ride height, camber, caster, toe, rake) and chassis balance.

And it is not clear that you have gone over the car to make sure that everything is working like it should (e.g., wheel bearings, suspension parts not loose, et cetera).

I strongly suggest getting to know this stuff first, before you go changing anything.

Then I suggest benchmarking. How does what you have compare with appropriate benchmarks? Best to find some in your area that drive your tracks.

Even for folks that can do all or most of their own work and are interested and capable in the technical details (like you seem to be), connecting with other similar folks and better yet area shops with longtime pro racing experience can still be helpful.

To me, only once you have done all of that can you start to uncover real shortcomings or preferences for changes.

For example, I run a pretty well-optimized 1975 GT car. I have a handle on the car and some technical knowledge and ability. I have debated the car's builder on his choice of diffs. In my car he used a Torsen, and on others he has built that are otherwise very similar, he uses LSDs. I understand just enough about diffs and their interaction with the car as a system. It is my conclusion that while an LSD can add performance, it would have to be in conjunction with other changes. And in the end, the net benefit might be only part of a second a lap for my $$$ trouble. For example, how many feet or inches of performance would I get in braking? What would the net result be in racing scenarios? Then I have to ask myself (because budget is not unlimited), what other things can I do to gain performance that compete with this prospective investment?

We are faced with a lot of these trade-off decisions for changes that would put more potential performance in our "systems," these 911 race cars. Only once we have a proven baseline does it make sense to consider the parts of seconds left over, in my opinion. Thankfully we have excellent resources like Matt and his company, because without them we'd be screwed.

These cars have been developed in money is no object fairly unrestricted racing circles since around the time they were hatched by PAG. Thankfully, there are plenty of folks around that have accumulated years of area under the curve regarding good baselines. My car's builder i such a person. There are others. I would find one and work with them.

In my past, I once was fortunate enough to speak to Franz Blam in the Atlanta area. I cannot remember the topic, but I remember that he helped me.

In summary, I suggest making sure you know what you have and that the obvious stuff is working, then benchmarking, and then looking for leftovers to tweak. Looks like you are well on your way.

[Mahler9th]

To me, if you take say a 1988 911 and want to make a great DE car from a stock one and you have a range or level of changes you are willing to make (id est, what area you are willing to address and what you are not), then it is likely that the stuff folks figured out in the late 80's and early 90's is still probably the way to go. Sure the DOT and racing tires are better, but for example, they all want to see something like a sixteenth of an inch of rear toe in at some basic ride height. Just because it is 2013 and we have the world wide web doesn't mean these basics have changed.

On such a car, $10,000 shocks may get you some performance, and it may be 0.001 second per lap. Only you can decide if that is worth it.

[JP911]

One additional consideration. Are the RA1s new and unshaven? Fresh Full tread RA1s have an unnerving squirm under braking.

[Mahler9th]

Hello JP!

Back in the day we called that "block wobble." Sadly, it has been 12 years sine I drove a car on track with DOTs.

[Cajundaddy]

Originally Posted by Mahler9th

It seems you have a lot of suggestions here... from folk's own experiences and from theory.

It does not seem that you have a complete handle on "what the car is" in terms of a system and its current settings, like alignment (ride height, camber, caster, toe, rake) and chassis balance.

And it is not clear that you have gone over the car to make sure that everything is working like it should (e.g., wheel bearings, suspension parts not loose, et cetera).

I strongly suggest getting to know this stuff first, before you go changing anything.

Then I suggest benchmarking. How does what you have compare with appropriate benchmarks? Best to find some in your area that drive your tracks.

Even for folks that can do all or most of their own work and are interested and capable in the technical details (like you seem to be), connecting with other similar folks and better yet area shops with longtime pro racing experience can still be helpful.

To me, only once you have done all of that can you start to uncover real shortcomings or preferences for changes.

For example, I run a pretty well-optimized 1975 GT car. I have a handle on the car and some technical knowledge and ability. I have debated the car's builder on his choice of diffs. In my car he used a Torsen, and on others he has built that are otherwise very similar, he uses LSDs. I understand just enough about diffs and their interaction with the car as a system. It is my conclusion that while an LSD can add performance, it would have to be in conjunction with other changes. And in the end, the net benefit might be only part of a second a lap for my $$$ trouble. For example, how many feet or inches of performance would I get in braking? What would the net result be in racing scenarios? Then I have to ask myself (because budget is not unlimited), what other things can I do to gain performance that compete with this prospective investment?

We are faced with a lot of these trade-off decisions for changes that would put more potential performance in our "systems," these 911 race cars. Only once we have a proven baseline does it make sense to consider the parts of seconds left over, in my opinion. Thankfully we have excellent resources like Matt and his company, because without them we'd be screwed.

These cars have been developed in money is no object fairly unrestricted racing circles since around the time they were hatched by PAG. Thankfully, there are plenty of folks around that have accumulated years of area under the curve regarding good baselines. My car's builder i such a person. There are others. I would find one and work with them.

In my past, I once was fortunate enough to speak to Franz Blam in the Atlanta area. I cannot remember the topic, but I remember that he helped me.

In summary, I suggest making sure you know what you have and that the obvious stuff is working, then benchmarking, and then looking for leftovers to tweak. Looks like you are well on your way.

+1
Wise words. Find our where the car really is first and then look at what needs fixin.

[Elombard]

Boy I cant imagine this was tread squirm but maybe - these were fairly new full tread. I forgot to mention earlier the car has adjustable sway bars, I think they are welt meister - not the really fancy ones. maybe stiffening up the rear bar would help reduce the feel. Agreed I need to get more baseline info- make some small changes and see what works. I am not in a position currently to spend loads of money to win the DE grand Prix, I just want to be able to be aggressive on the track. I think this is what people mean when they say a dual purpose car is never a happy compromise - I suspect if i get the car where i like it to feel on the track it will not be much fun on the street.

[Chuck Moreland]

I will say the problem definition is a bit ambiguous. " losing confince in the rear" is a lot harder to diagnose than "left rear lockup prematurely" for example.

that said, your setup description has a screaming red flag: bigger tbars and no corner balance.

a bad corner balance will jack up an otherwise good braking setup. every time. and your corner balance is just by chance, and almost certainly off.


fix this first, then see what you have.

[333pg333]

You said you had no rear lockup issues but under brakes you were really tentative based on a feeling. What was that feeling telling you? Could it have just been nose dive/rear lift even?

[Elombard]

Sorry my description is a bit vague, I guess the lack of confidence is more on turn in? At the end of the braking period the car is front loaded, during the turn and as I transition to throttle I cant 'feel' what the back end is doing, it does not seem to want to take a set and I don't have confidence to lean on the out side tire so I am feathering the throttle. Hard to explain. This particular track has two areas where you can trail brake, as I mentioned before I try to brake in a straight line but even minor adjustments cause a lack of confiedence in the rear.

Need to figure out my existing equipment set up, check out a few things (loose wheel bearings) CB, and go to a slightly more track focused alignment and see where it is. I hope to report back some of the info this weekend and see if any one has more feedback. Probably a month or two from a track day.

[cstreit]

Erik,

One of the things I had to force myself to accept when I first was learnign to race was "more throttle equals more rear grip". It was very counter-intuitive to me a first.

Luckily I was driving my instructors car when he kept pushing me to "Get on the gas!" at corner apex when I "knew" I was at the limit of adhesion.

So I remember saying "Well I'll show him, I'll do what he asks and then he'll see that the car is going to spin". I mashed the throttle and the car hooked up and flew out of the corner. "Wait, what?!"

I guess he was right.