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I've always been unhappy with braking performance of my car. It behaves fine during moderate-hard stops, but try to haul it down hard from speed and the front tires lock up waaaay too easy and feels like the rear brakes never do enough. I don't have video, but I'm pretty sure the rear axle is separating from chassis a lot (back end high). How are you guys controlling that with your cars?
I'm suspecting it's due to my shackle style torque arm. The front mount is over center (a bit in front of mount) and the angle of the shackle imparts a load on chassis that's unweighting the rear tires. Considering going back to a bushing so arm can slide fore-aft. (Obviously the shackle arm works really well in acceleration.)
Hi Qwk, I'm going to start with what you probably already know as an option, the unbalanced engineering decoupled torque arm is supposed to be for exactly this problem. As of now, I can't personally say if it's effective as I'm on their wait list to get one so I'll have to get back to you on whether it works or not sometime down the road. For now I have the same issue you do with 02 front brakes and the stock rear drums that D.D.S. (don't do ****)
I might have never known about this happening, and maybe its not as bad on my car as it sounds like it is on yours, but this photo from last year's UMI autocross event tells me that my back-end is lifting a noticeable amount during hard braking also. When this photo was taken it was right before a heavy brake application spot on the track where the car's needed to do a hairpin turn. The rear end of the car WAS never that high whens stationary but now I added drag race air-bags which makes the rear always sit up higher than before , but I was still playing with the wilwood prop-valve adjustment (and I still am finding the sweet spot), but is this the kind of thing you are talking about? Also, as you mention about your front brakes doing more, I also can attest to that, as I always have MUCH more brake dust on my front wheels, and hardly any brake dust on the rear wheels after an autocross event.
I do have a Wilwood prop valve and it's cranked up near max on rear brakes and I loaded the rear brakes (LT1, same as 1LE PBR's) with much more aggressive pads than the front. I've done everything I can in that respect, short of buying different brakes. They are doing something, there's always lots of dust on the rear wheels (it's the nature of the pads).
I do have a Wilwood prop valve and it's cranked up near max on rear brakes and I loaded the rear brakes (LT1, same as 1LE PBR's) with much more aggressive pads than the front. I've done everything I can in that respect, short of buying different brakes. They are doing something, there's always lots of dust on the rear wheels (it's the nature of the pads).
im wondering if something is wrong. one of the cars i work on is a 4th gen with wilwood prop valve on it. it was setup with full pressure to rears it was getting squirrely on top end even with skinnies/slicks. seems like you would be having issues with locking up the rears?
seems like you would be having issues with locking up the rears?
No, I can't generate enough rear bias to lock up the rear brakes. My front brakes are larger than 4th gen and I have more rear traction than normal because of stiff suspension (less weight transfer). That's partly why my front brakes lock up so easy. But I feel like the rear should stay more planted than it is.
My car feels fine at drag strip but I would not risk a road course right now, things are still too messed up for that. I can generate a lot of speed but the whoa! isn't working as good as I want.
In a general way, if all the forces that are applied to the wheels, get transmitted to the vehicle at its center of gravity (i.e. instant center = center of gravity), the car will be "neutral". In general of course: there are specific ways of altering this, and lots of pesky details that get in the way sometimes, but this is the general principle.
Stock vehicles are RARELY in such a condition. Seems like most of them, the ic of the rear suspension is much lower and farther forward than the c of g; ends up, the rear tries to lift when accelerating. The inevitable result of that is wheel hop. However, in this state, under braking (acceleration applied to the rear), the rear will try to "plant" itself, leading to better "emergency" stability for the untrained driver.
So, to optimize a car for drag racing (maximum possible forward acceleration), this fundamental tradeoff gets altered. The effect is, that when accelerating, the rear tries to "separate" itself from the body. But since it can't go downwards (the pavement is in the way), the rear of the body ends up lifting. Then, under braking (acceleration the opposite direction), the opposite thing happens: the rear tries to LIFT. There's nothing preventing that, except for the body being in the way, via the springs & shocks. Thus, the MORE BRAKING EFFORT you exert on a rear axle in this situation, THE WORSE THE BRAKING GETS. The rear tries to lift, loses traction, and you get wheel hop under braking. Couple that with the natural fact that the car tries to roll over on its nose under braking no matter what, which lifts the rear of the body; and the rear axle, which is already trying to jump up into the tunnel, gets even lighter on the pavement.
Increasing brake bias toward the rear won't cure this.
Keep in mind, "braking" can always be thought of the same as "accelerating rearwards", as far as suspension behavior is concerned. Except that while the rear has more effect on forward acceleration than the front, the behavior of the front would have more effect during braking.
I'd suspect that the solution involves the front suspension more than the rear. You'd need to control its tendency to dive by adjusting where the front suspension's instant center is, much as you adjust the rear's ic with the torque arm and LCAs and whatnot. You could conceivably even alter it so severely that the front would RISE under braking, if the ic was raised to the point that it's above the c of g. Obviously making the front end rise would tend to transfer weight toward the rear, which in turn would aid rear traction. No idea how far you can carry this principle safely; like most things, it would probably be real easy to tip yourself over some edge of counterproductivity.
I don't know the details of how to do that in a practical sense, but surely the SCCA type guys would have some ideas. Even if you don't plan to spend time in the parking lot races, some of the principles that those guys use, would seem to apply here.
Last edited by sofakingdom; 04-24-2022 at 05:39 PM.
I'm suspecting it's due to my shackle style torque arm.
Likely culprit. Also, our cars suffer from too great a difference in the roll couple - front and back.
Our cars suffer from:
- the static roll center height is too low, causing too much roll angle - the dynamic roll center height in dive and roll is too low, causing even more roll angle - front suspension geometry has a jacking effect and a higher front roll angle vs. rear - rear roll center was too high
Add to that the poor stock rear brakes...
What rear shocks, spring rates, SB, and position of PHB do you have?
An easy trial of lowering the PHB just to see what happens should tell you something. Normally, if you lower PHB you want to increase spring rates, but for a trial just lower it and see what happens. Bolt in some brackets to lower just for the trial - you need to weld-in if permanent, cautious speeds for trial and minimize lateral G's - straight line trial. If there's minimal improvement, you might have to unshackle. If the trial is hopeful, weld the brackets, increase spring rate and swap a smaller SB, as well. I don't think you do, but many run around with moisture in their 5-year old brake fluid.
I don't know the details of how to do that in a practical sense, but surely the SCCA type guys would have some ideas. Even if you don't plan to spend time in the parking lot races, some of the principles that those guys use, would seem to apply here.
That's a good idea. I've never posted at FFRAX before, might check if they're still alive.
Yeah, I'm not really understanding the nuances of the suspension. I have a feeling this is something I'll never get a grasp on unless I invest a lot of time learning how things work. I like easy.... this doesn't sound like fun to me.
What rear shocks, spring rates, SB, and position of PHB do you have?
* Rear shocks are Viking Warrior double adjustable. I've added some compression to help traction when I push the Go! Dog! Go! pedal, but I'm on a comfort-performance street setting and not nearly as tight as the shocks would have to be for track.
* Front / Rear springs are 800 / 185
* Front & Rear sway bar are Detroit Speed with the rear adjusted to the lower spring rate of 620 lb/in
* Panhard bar is lowered and can be set to anything I want.
* Rear control arms are about 2° sloping down toward the axle
Panhard bar is irrelevant. Ignore all that CRAP. Some kinda "grad" trying to show off how much he "knows". BULLPLOP.
Problem here is, the same as when you couldn't get the rear to hook up; it would wheel hop, spin, whatever.
Except now, it's the front.
Look at what it takes to eliminate the front dive so that at least SOME weight stays on the rear. Which of course is the obvious root cause of the issue at hand. Too much weight on the front, not enough on the rear. Brake bias won't fix that. Will make it worse, if anything.
Panhard bar is irrelevant. Ignore all that CRAP. Some kinda "grad" trying to show off how much he "knows". BULLPLOP.
Problem here is, the same as when you couldn't get the rear to hook up; it would wheel hop, spin, whatever.
Except now, it's the front.
Look at what it takes to eliminate the front dive so that at least SOME weight stays on the rear. Which of course is the obvious root cause of the issue at hand. Too much weight on the front, not enough on the rear. Brake bias won't fix that. Will make it worse, if anything.
Work on the front dive.
Why the personal insult?
It reflects more negatively on the one slinging it, than it does on the intended target.
Then you assert that "front dive" is the concern as opposed to what I proposed. But I did say the badly mismatched roll couple is the problem, duh "- front suspension geometry has a jacking effect and a higher front roll angle vs. rear."
So who are you arguing against?
Why the insult?
Why do you expose yourself as such?
It is easier to test a hypothesis with the rear roll center. It is done all the time in the pits. Changing the front is more involved and not necessary just to test a hypothesis.
I am arguing against trying to fix this problem by messing around with the rear of the vehicle, most specifically, the PHB position and stab bar. These things affect turning but not braking. (except of course, braking WHILE turning)
PHB has no effect on front-rear behavior of the vehicle, other than the minute extent to which it might allow the rear to shift from side to side unintentionally. It is not material to the problem at hand. Similarly, if the car is in a straight line, the SB is under no tension whatsoever; it is "at rest", therefore its strength is irrelevant.
Note that I DID NOT say "his PHB is fine"; I DID NOT say "he doesn't need a PHB"; I DID NOT say "the PHB is not important"; I DID NOT say anything about "roll couple doesn't matter"; NOT NOT NOT NOT NOT NOT NOT NOT. What I DID say was, that the PHB is not material to the problem at hand, and therefore is not at issue here. It's a classic red herring.
I'm no genius about "roll couple" (or much of anything else for that matter); only a person who employs logical thinking to a problem, and who stays FOCUSED on the matter under discussion. Which in this case is, excessive weight transfer OFF OF the rear suspension (which the only place it could possibly go, is to the front), causing braking problems. Likewise, increasing brake bias toward the rear, which seems tempting because of the complaint that the rear brakes "don't do enough", is likely to merely aggravate the whole situation, by making them lock up and/or cause wheel hop.
You are exactly right that modifying the front suspension on these cars, and on almost any production car for that matter, is not a trivial undertaking. Frankly, I don't know how one would go about it, given that so many of the things that might need to change (front control arm pivot angle for example) are not at all easy to alter. To accomplish the equivalent effect on the front that rear LCARBs and other similar techniques for altering the rear's instant center, I have no clear idea what '"handles" the car has that you could grab onto and work with. All I DO know is, the problem here, and therefore the solution to it, lies at the front of the car, not the rear; and MOST CERTAINLY NOT the PHB. The ONLY adjustment to the rear that might help is, if there is "too much" anti-squat (rear LCAs set to too much of an angle for example), this will create wheel hop under braking, the same as having too little anti-squat creates wheel hop under acceleration, and therefore backing off on that might help. The shackle style torque arm mount clearly could be a contributing factor for this reason. But definitely NOT the PHB or the SB, and probably not the rear springs & shocks, unless grossly misadjusted, which doesn't seem to be the case.
Look at what it takes to eliminate the front dive so that at least SOME weight stays on the rear. Which of course is the obvious root cause of the issue at hand. Too much weight on the front, not enough on the rear. Brake bias won't fix that. Will make it worse, if anything.
Work on the front dive.
I agree with this, what sofakingdom is describing is antidive.....given your Torque arm is correct and everything you have done up until now is adjusted correct,... Because like a Jedi balance is needed......
The front is what you need to look at. For this the LCA's need to be moved, now its not easy to do but it will make a significant difference if done correctly.. I built it into my K-member.
Here are some pics of a hard braking situation going into a stop box, first pic is accelerating going into the box, second pic is hard braking once past checkered flag, and third is full stop. Notice the front of my car doesn't move or "dive" throughout this, also notice no smoke from my tires because they didn't just lock up.....balance
Interesting thread. I dont have anything to contribute but rear bias and pads helped my car….with my c5’s up front and ls1 rears with hydroboost and stock 4th master with stock 89 disc prop valve, i too felt my rears never did anything. On jack stands youd have to really press brake to get rear wheels to stop. When I changed front pads to something aggressive, i got instant lock up. Very sketchy at 140+ mph. I did the grainger spring mod to the prop valve and matching aggressive pads on rear and it fixed all my problems. But im a straight line braker but it got my 4000 lbs down from 180 very smoothly and quickly for what it is. I never felt the rear jacking up or separating, infact it doesnt separate much either on acceleration like i wanted. Stock iroc rear springs, 275 lb front coilovers. Im surprised yours with adjustable prop valve didnt seem to help with the fronts locking up early.
sofakingdom,
I like a good argument, as in debate.
First, let's start with what we are in agreement with. Front dive is the problem. No argument, here.
Second, we differ on how to solve it in this instance. I have not yet offered up a solution. I did suggest to the OP a trial test to see the difference in behavior. If I was imprecise in my language, I apologize. We both agree adjusting the front is complex. That is why I suggest a test on the rear; easy, no cost, see how the behavior changes. You seem emphatic that any solution must involve the front. OK, I hear you, but I think something can be learned from lowering the PHB and reporting back the changes in behavior. It can point us in the right direction so that a solution can be suggested. The OP suggests that his shackle arm on the TA imparts a load on the chassis that's unweighting the tires. The OP is not a newbie. Before we offer an internet solution, let's test...let's test his suggestion that the TA is loading on the chassis and unweighting the tires.
Third, it is not true that the PHB and SB are "at rest" during a straight line. A car in motion is a very dynamic situation. Even the solid axle has many forces acting upon it. How about "under a load on the chassis that is unweighting the tires?" If the chassis and axle are in a dynamic situation, how would one determine those forces at play? Specifically, the interaction between chassis and axle? I say that the chassis and axle are tied together by the PHB; you are free to disagree, even in all uppercase if you so choose. Well, how about a test? Low cost, minimal time, easy to put back to normal afterwards, before offering a solution on the keyboard. A simple often used tuning tool to see what is happening dynamically between chassis and axle - as the OP suggests.
Originally Posted by sofakingdom
I DID NOT; I DID NOT ; I DID NOT ; I DID NOT ; NOT NOT NOT NOT NOT NOT NOT NOT. I DID .
In one sentence you were very emphatic. If I have offended you, I apologize. I am a humble man, and I choose to behave that way. We don't have to agree in order to get along.
I'm running all factory, small disk up front, drums in back. do not have that problem. just had to slam on brakes last Saturday on the freeway. It was a 70mph - 10 mph slow down. everything smooth and controlled. stopping distance not as good as modern car, but still, it was smooth and controlled. or maybe I just had the daylights scared out of me and didn't notice the car all over the place. never tried a 120mph quick slow down, well, not in the Formula anyway
If your Prop valve isn't working well enough to lock up the rears even if you tried, something is probably very wrong.maybe a clogged brake line. corrosion somewhere?Stuck caliper piston?Even a PBR calipers should be able to lock up the rears with a prop valve adjusted aggressively.I run BAER SS4 brakes in the rear of my 92 and my Wilwood prop valve can be easily adjusted to lock up the rears.Even running a 315/30R18 200tw tire.Pic for attention.
Panhard has nothing to do with straight line braking. It could effect trail braking into a corner but I'm not that sophisticated yet.
My rear brakes are healthy, I just can't generate enough rear brake bias. My car is an object lesson in the concept that bigger front brakes can harm your braking performance by over biasing the front. I was duped by the bad advice on the forums. Now I know better but won't spend the money to fix it. I've never raced where the front brakes got real hot, so it might be possible things balance out after front loses some coefficient of friction, but I don't know. Don't get me wrong, my car stops way faster than stock, it's just not behaving the way I want.
I had that front anti-dive in my mind too, I think Heidts k-member has 3° anti-dive built in but I can't remember for sure. I'm actually pretty interested in that but am gun shy after my past experience with other Heidts products, that's what is holding me at bay for now.
Honestly though, there's something not right about my torque arm. I can hear it and feel it driving but it seems totally fine during inspection in my garage. It takes a lot more force to make it do whatever it's doing than I can apply in the garage and I can't figure out what's going on.
10-4. Was curious. Where is your battery? You are very even weight wise but just curious if you'd thought about shifting some weight rearward. Very interesting thread.
I too am at the same ratio and wondering if I need to make changes now.
My battery is in stock location in engine bay. I won't put the battery in back because I refuse to cut up my tail lights for a disconnect switch. They won't let you run at the drag strip without an external switch. I rarely go to the drag strip but I don't want somebody telling me I can't either. Heck, I haven't even run the LS7 down the drag strip and I've had it for 4 years.
i just bought a cheap kit for mine from harbor freight. Take bleeder screw out and thread in the gauge adapter.
just curious what your pressures would be to verify things. again, might not have anything to do with your issues but i would think with the prop valve bias all the way to the rear, you should have strong rear brakes.
Can I lock up the rear brakes if I crush the pedal to the floor and flat spot the front tires? Probably. Do I have enough bias adjustment to ever get the rear to lock first? No.
The brakes work, it's a bias problem. Calculations show it too. I can only develop at best 75% / 25% brake bias with my combo. That might work for a car with stock suspension that pitches a lot of weight on the nose but not mine, I have less weight transfer to front and need more rear bias. Things improved when I ditched the drag radials and went to the autocross tires because I developed more grip at the front (could use more of the front brake). Bottom line is I would have been better off with a smaller brake in the front like a C5 or LS1.
I have a manual trans car and absolutely love the LT1 style parking brake -- it's a serious parking brake, way better than LS1 style, and I don't want to give that up.
I too am at the same ratio and wondering if I need to make changes now.
It never ends, man, just get it on the road and start enjoying it. There's no faster way to sort out your car than to drive it. I guarantee after you drive it for a bit you're going to want to change some things no matter how well you plan. It'll get half torn down every winter for the first few years.
How similar is my car to 7 years ago? Not very much. And I had it all planned out the first time, huh?
It never ends, man, just get it on the road and start enjoying it. There's no faster way to sort out your car than to drive it. I guarantee after you drive it for a bit you're going to want to change some things no matter how well you plan. It'll get half torn down every winter for the first few years.
How similar is my car to 7 years ago? Not very much. And I had it all planned out the first time, huh?
So true. Also, to your point. If I have issued that means I am actually driving it! Ha.
Can I lock up the rear brakes if I crush the pedal to the floor and flat spot the front tires? Probably. Do I have enough bias adjustment to ever get the rear to lock first? No.
The brakes work, it's a bias problem. Calculations show it too. I can only develop at best 75% / 25% brake bias with my combo. That might work for a car with stock suspension that pitches a lot of weight on the nose but not mine, I have less weight transfer to front and need more rear bias. Things improved when I ditched the drag radials and went to the autocross tires because I developed more grip at the front (could use more of the front brake). Bottom line is I would have been better off with a smaller brake in the front like a C5 or LS1.
A little late to the party but I have to ask.
Are saying that less braking performance at the front will help alleviate the excessive dive and allow the rear brakes to have a more active roll?
If that's the case then is your hope is to have a more balanced braking application but not necessarily greater stopping power? Given that the rears don't lock up, this appears to be so.
The brakes work, it's a bias problem. Calculations show it too. I can only develop at best 75% / 25% brake bias with my combo. That might work for a car with stock suspension that pitches a lot of weight on the nose but not mine, I have less weight transfer to front and need more rear bias. Things improved when I ditched the drag radials and went to the autocross tires because I developed more grip at the front (could use more of the front brake). Bottom line is I would have been better off with a smaller brake in the front like a C5 or LS1.
.
Typically, the front force is limited by tire compound. You can over compensate with brake components (heavy weight), but C5/C6 should be fine.
What you are describing sounds more like lbs per square inch on the rear than brake ability, but bias is always a concern. You're springs are still very soft. I found great improvement with stiffer springs and softer bar.
Fastest stopping distance comes when you have optimized the bias (front/rear braking torque ratio) that makes best use of the grip available at front and rear tires. I could put on smaller front brakes and still pitch the car just as hard and still lock up the front. Difference is more % of braking would go to the rear before the fronts locked. Braking is a ratio thing, you'll draw the wrong conclusions if you just look at one end by itself.
Even when balanced, the more total clamping force in your brake package the more grip you need at your tires to use it, otherwise the brake pedal just gets touchy. This is where sizing the brake package becomes important so you can use the range of your pedal travel and modulate. A good handling car with performance street tires can use about 3000 lb-ft braking torque. A car like McLovin's with slicks can handle in the neighborhood of 3500 - 4000 lb-ft braking torque.
Question.
1. what ports on your brake master cly are routed to the front and rear?
On GM master cly, The port furthest away from the firewall is for the rear brakes.
it does matter for OEM master cly.
I've talked with BEAR and Wilwood about it.
So I'm still confused how OP isn't having issues.
when I originally installed my Wilwood Prop valve, I had factory Drum brakes in the rear and 6 piston Willwoods in the front.
I could adjust the rears so they would lock up before the front.(atleast before the drums overheated)
I feel like something else is wrong here.
Tip: Wilwood prop valve ****.
The **** turning clock wise is more rear brake.
Counterclockwise (lengthening the ****) is less rear brake.
The **** arrow says "more brake" that's actually talking about the front brakes.
QwkTrip's issue isn't the brakes its chassis set up......... Qwk, what torque arm do you have? is it shorter than the stock one? Where Is the front mount/pivot of the TA in relation to the OG one? More than Likely you've changed your Instant center location and that's what's contributing to your braking issues
Torque arm is something we made at a local shop. Rear bracket and front poly mount is scavenged from my old UMI short arm. It's a little shorter than stock, roughly near the driveshaft front u-joint. Mounted to a bar that spans between the Heidts (Alston) inner subframes. Bottom bushing is hard mounted to the cross bar. Top bushing is attached to the arm. The two are joined by a swing plate.
The arm is a little bit over center at the front mount. I've shorted the threaded rods as much as possible at the axle end to draw the whole thing rearward but there is no more adjustment left (need to leave a little for pinion angle adjustments).
I know the axle bracket is placed wrong, it's just a picture of the arm during construction.
I've always been unhappy with braking performance of my car. It behaves fine during moderate-hard stops, but try to haul it down hard from speed and the front tires lock up waaaay too easy and feels like the rear brakes never do enough. I don't have video, but I'm pretty sure the rear axle is separating from chassis a lot (back end high). How are you guys controlling that with your cars?
I'm suspecting it's due to my shackle style torque arm. The front mount is over center (a bit in front of mount) and the angle of the shackle imparts a load on chassis that's unweighting the rear tires. Considering going back to a bushing so arm can slide fore-aft. (Obviously the shackle arm works really well in acceleration.)
Thoughts?
In terms of what I do, when I make a Crossmember (Tube welded perpendicular to both Sets of Sub-Frame Connectors) for the Torque-Arm...
I extend the Forward most end of the Torque-Arm for use with a Stock-Type Bushing (usually Poly-Graphite).
Then make a new Clam-Shell for the Bushing and attach it to the Torque-Arm Crossmember.
Depending on the Rear Suspension configuration, I do see a significant amount of "back and fourth" travel of the Torque-Arm.
Does torque arm attach below the crossmember? It looks like it in the pic. Most are above the cross member it seems. I wonder if making that front attachment point at different heights would do anything
Angle of torque arm is essentially inline with stock. In other words the nose would project through stock mounting location if extended. The lower mount is rigid (welded to the cross bar that is welded to inner subframe connectors that is welded to the car.)
All of the following may have been covered but I'll ask anyway.
Do you have lower control arm relocation brackets and if so, what position are the arms attached?
I'm trying to get a handle on how the instant centre might have changed and the result is the rear end jacking under braking that you're experiencing.
My understanding is that the IC is determined by the intersection of the control arm plane and the mount for the torque arm.
Something you've probably read at LS1 Tech...
The angle of the LCA's & the torque arm mounting point create the front to rear IC.Move IC back/upward for traction, moving IC forward/lower improves heavy braking.( less prone to rear brake wheel hop)
Did you say you had a shorter than stock torque arm?
04-22-2022, 12:33 AM
