Could it be front end lift at 100+ mph?

[tony-muscle]

So yesterday I was at a HPDE event at one of the tracks near by. In a few occasions when braking hard at the end of the front straight away I got a unpleasant shake coming from the front end in addition to some tire skidding. I didn't get this effect anywhere else in the track when braking hard but when going at 100-115 mph. My theory is that the front end may be getting lighter at high speed which is leading to locking the front tires. It was not a continuous lock, but it was more like on and off which made the front end shake. I didn't lock the tires anywhere else in the track except at this spot at high speed. I talked to others and no one had problems in that section of the track. I fixed the problem, without thinking of the front end lift theory, by getting on the brakes lightly at the beginning before the hard braking. This may have helped push the front end down and reduce lift, in addition to reducing the speed. Any thoughts? Next time, which will be next year, I will try to shift some brake bias to the rear but maybe I should look into making some type of lower front spoiler to use for track days only. Right now I have the stock spoiler and I don't want anything lower for the street.

 

[Sheriff41]

Hmm. Couple things came to mind that you've already thought of: 1) control weight transfer via brake modulation/pressure and 2) need to adjust the brake bias.

@71Rustang may have some insights from researching his build.

 

[Mister 4x4]

If your engine temps are comfy, maybe tape-off some of the grille toward the top to reduce the amount of air getting shoved in and cavitating under the hood.

Get a 'grille-to-radiator' plate to reduce the amount of 'stray air flow' under the hood (the air going into the grille would be forced through the radiator and fan shroud, rather than deflect up and over the front cross-member).

Do you have Ram Air, block off plates, or are your NACA ducts open? If the ducts are open without Ram Air, close 'em off (they're not doing any good like that anyway).

I like the idea of making a bigger spoiler for the front... maybe something that either replaces or attaches to the bottom of your stock spoiler to increase its area (like a thick plexiglass 'splitter').

Might be counter productive, but maybe change the angle of the trunk spoiler to increase lift on the rear end (just slightly) which could help transfer a little bit of weight to the nose (long shot, that could upset the car).

Just some thoughts. Hope it helps.

 

[Don C]

This is thinking outside of the box, somewhat, but you might want to introduce some turbulence at the front of the hood. An airfoil (as in airplane wings and propellers) functions by increasing the rate of air flow across the top of the airfoil, which creates a low-pressure area and lift. I haven't read anything about it, but it seems like a spoiler that is mostly aesthetic may not be creating low pressure under the car, and the shape of the hood and roof may be lightening the load on the front. It would be interesting to see some wind tunnel studies on some of these older cars.

 

[Kilgon]

I would think shifting some brake bias to the rear should help. As you mentioned it sounds like partial locking / skidding which can cause wheel hop/shaking. I really don't know if our front spoilers really provide any measurable amount of benefit due to their height off the ground. With the undersides of our cars being anything other than aerodynamic there is going to be a considerable amount of lift at higher speeds due to all the air traps and turbulence under our cars. To get any really noticeable benefit the spoiler needs to be closer to the ground to deflect air around from the underside of our cars and front tires. Front spoilers usually won't provide any noticeable effect under 90 mph or so to begin with, so you are just reaching the threshold of the car actually benefitting from it at the end of the straight away. As Don mentioned, it be nice to have a wind tunnel test done on our cars to see just how efficient, or more likely inefficient, the body design and spoilers are.

 

[Stanglover]

Tony, you're NUTS!!!
Be safe out there eh!

 

[MikeGriese]

If the front end is lifting at speed, you should feel it in the steering wheel. Does your steering feel lighter the faster you go? The front end may tend to wander a bit as well if there is significant lift. If you suspect there is lift, there are devices you can put on the car to measure changes to ride height.

What is the condition of your suspension bushings, ball joints, and bearings? Are they all torqued to spec? Have they been replaced lately, and if so, what bushing material are you using?

What are you feeling in the brake pedal and steering wheel when you experience this phenomena? Is there any pull to either side, vibrations, brake pedal pulsing, steering wheel feedback? Did you examine your tires after the session to look for obvious scrubbed areas or flat spots?

 

[tony-muscle]

If your engine temps are comfy, maybe tape-off some of the grille toward the top to reduce the amount of air getting shoved in and cavitating under the hood.

Get a 'grille-to-radiator' plate to reduce the amount of 'stray air flow' under the hood (the air going into the grille would be forced through the radiator and fan shroud, rather than deflect up and over the front cross-member).

Do you have Ram Air, block off plates, or are your NACA ducts open? If the ducts are open without Ram Air, close 'em off (they're not doing any good like that anyway).

I like the idea of making a bigger spoiler for the front... maybe something that either replaces or attaches to the bottom of your stock spoiler to increase its area (like a thick plexiglass 'splitter').

Might be counter productive, but maybe change the angle of the trunk spoiler to increase lift on the rear end (just slightly) which could help transfer a little bit of weight to the nose (long shot, that could upset the car).

Just some thoughts. Hope it helps.

Great ideas in regards to the grille. My engine runs at good temperature so I can add some type of shield and/or a grille-radiator cover. My RAM air is functional so that won't help.
I have to start thinking about what can I do in regards to a lower spoiler. I am going to start looking at earlier Mustangs or Camaros that are raced more.
I thought about the rear spoiler thought. Right now I am running the spoiler flat which is about 15 degrees to the air flow coming through the sportsroof to create a little of downforce However, I wonder how much is it creating.

 

[tony-muscle]

This is thinking outside of the box, somewhat, but you might want to introduce some turbulence at the front of the hood. An airfoil (as in airplane wings and propellers) functions by increasing the rate of air flow across the top of the airfoil, which creates a low-pressure area and lift. I haven't read anything about it, but it seems like a spoiler that is mostly aesthetic may not be creating low pressure under the car, and the shape of the hood and roof may be lightening the load on the front. It would be interesting to see some wind tunnel studies on some of these older cars.

It would be nice to see some wind tunnel results. The front spoiler's job is to create a zone of high pressure in front of the car to help create some downforce. Also by limiting the amount of air under the car, the lift is limited. That's why a lower spoiler or air dam may help.

 

[tony-muscle]

If the front end is lifting at speed, you should feel it in the steering wheel. Does your steering feel lighter the faster you go? The front end may tend to wander a bit as well if there is significant lift. If you suspect there is lift, there are devices you can put on the car to measure changes to ride height.

What is the condition of your suspension bushings, ball joints, and bearings? Are they all torqued to spec? Have they been replaced lately, and if so, what bushing material are you using?

What are you feeling in the brake pedal and steering wheel when you experience this phenomena? Is there any pull to either side, vibrations, brake pedal pulsing, steering wheel feedback? Did you examine your tires after the session to look for obvious scrubbed areas or flat spots?

I think if the front end is lifting we are talking about very little. Just enough to take some weight of the front and allow from the tires to lock. At that moment I was going straight and more worried about not overshooting the turn. You don't want to be turning when heavy braking. The car definitely was going straight and not pulling in any direction so whatever it was equally on both sides.
In regards to the suspension. I have a Mike Maier coil over suspension that is relatively new. This suspension has no bushings so no issues there and all is very tight. I check my suspension before every race to make sure all is good.
Tires had not flat spots. This was not a complete lock up. It was more of a border line lock up that made the car shake the front end. Luckily there was no pull to any side. I didn't feel any unusual at the brake pedal or steering wheel. However, remember that I was going at 100+ MPH and all this happened so fast that there was not enough time to think about anything else besides keep the car straight and try no to overshoot the track.

 

[71ProjectJunk]

This is not my area of expertise but I will give you some thing I learned while working on a Factory Five GTM I used to have. When I bought the car the front of the car was slightly higher than the rear, I can't recall how much exactly, but it was not much, if I recall correctly maybe 1/2" higher. At 90-100MPH plus the car would get a little squirrely and you could feel the steering wheel get lighter. Since the car had coil overs I just lowered the front end to be 1/2" lower than the rear of the car, measured from the chassis to the ground. After that the car was rock soil at speed. Check your ride heights and make sure that the car is lower on the front than on the rear. If the car is even or even a little higher in the front, air will get under the car and lift it. On an older Muscle car whose aerodynamics are not really that good, I would probably try to get the front at least an inch lower than the rear. Maybe as you are coming into the corner, there may be a slight dip in the corner, as the front suspension hits the dip and goes down a bit and then up again air gets under the car. Air under your car at 115 mph exerts a huge amount of force, and will unsettle the car.

I would try to lower the car a bit in the front from where you are at currently, maybe 1/2" and see how it feels. And get a front spoiler that ends up closer to the ground. You could get another repro spoiler and add plastic or aluminum riveted to it to it to extend it closer to the ground.

 

[swice]

I changed out front springs to one inch lower, added strengthened the front/rear stabilizer bars, strengthened the front lower spoiler, Ram Air 351c 4V,
the stang actually does pretty well 100 mph, I have not tried hard braking at that speed, but steering feels ok, I need to change out to a better radiator as the temp will slowly rise if I do not slow it down. The above discussion is interesting, these are old cars and I am always a bit cautious to over do it. What I like is the fun of having a fast driver within reasonable speeds with good handling! Stay safe out there!

 

[Cleveland Crush]

Agree with lowering the front end. As to brake bias, if not already done, rig up an IN cabin adjustable proportioning valve, (mine comes up thru the the center floor tunnel on the driver's side of the shifter). Next, check your lower control arms and strut rod bushings for deflection and/or looseness. I believe you may be experiencing or already have an excessive "toe-in" condition during hard braking at speed. During an excessive toe-in condition at high speed the front brakes will bite harder and erratically. Tire air pressures will effect your brake bias at speed also. Just my thoughts and experiences. Good luck with it. Your Mustang looks great on the track!

 

[tony-muscle]

This is not my area of expertise but I will give you some thing I learned while working on a Factory Five GTM I used to have. When I bought the car the front of the car was slightly higher than the rear, I can't recall how much exactly, but it was not much, if I recall correctly maybe 1/2" higher. At 90-100MPH plus the car would get a little squirrely and you could feel the steering wheel get lighter. Since the car had coil overs I just lowered the front end to be 1/2" lower than the rear of the car, measured from the chassis to the ground. After that the car was rock soil at speed. Check your ride heights and make sure that the car is lower on the front than on the rear. If the car is even or even a little higher in the front, air will get under the car and lift it. On an older Muscle car whose aerodynamics are not really that good, I would probably try to get the front at least an inch lower than the rear. Maybe as you are coming into the corner, there may be a slight dip in the corner, as the front suspension hits the dip and goes down a bit and then up again air gets under the car. Air under your car at 115 mph exerts a huge amount of force, and will unsettle the car.

I would try to lower the car a bit in the front from where you are at currently, maybe 1/2" and see how it feels. And get a front spoiler that ends up closer to the ground. You could get another repro spoiler and add plastic or aluminum riveted to it to it to extend it closer to the ground.

Very good points. I can lower the front a little more but it won't be practical for street use since the headers are already pretty low. I could do it for the track but then I lose the alignment. That said, I am definitely considering lowering the front a bit more, maybe 1/4". I have coilovers so it is easy but I know it will affect the alignment.
The front spoiler is definitely lower than the rear. I am definitely looking for solutions for the spoiler. Ideally I want to come up with something that I used for track only and then I can remove for street use.

 

[tony-muscle]

Agree with lowering the front end. As to brake bias, if not already done, rig up an IN cabin adjustable proportioning valve, (mine comes up thru the the center floor tunnel on the driver's side of the shifter). Next, check your lower control arms and strut rod bushings for deflection and/or looseness. I believe you may be experiencing or already have an excessive "toe-in" condition during hard braking at speed. During an excessive toe-in condition at high speed the front brakes will bite harder and erratically. Tire air pressures will effect your brake bias at speed also. Just my thoughts and experiences. Good luck with it. Your Mustang looks great on the track!

My brake bias valve is next to the MC so I can play with that at the end of each session. I have solid/delrin mounted strut rods and solid mounted LCA so I don't think that it should be an issue. However, next spring I will be redoing the alignment so I will know how the toe-in is while standing.

 

[detritusmaximus]

Consider this...the front valance panel. It has an nice large opening between it and the bumper. As the air flows thru and down under the car. This might be a low pressure area. It is also probably level or even angled up and potentially acting as a wing. With the spoiler under it, it could be a higher pressure area and the source of some lift. I don't know of any data to show if that front spoiler has a positive or negative result...but they look cool!

I would block off that lower opening and/or make sure all the air going thru the grille only goes to the radiator

 

[MikeGriese]

Lowering the front would be my first choice. It's easy, doesn't involve construction, and can easily be put back.

 

[tony-muscle]

Consider this...the front valance panel. It has an nice large opening between it and the bumper. As the air flows thru and down under the car. This might be a low pressure area. It is also probably level or even angled up and potentially acting as a wing. With the spoiler under it, it could be a higher pressure area and the source of some lift. I don't know of any data to show if that front spoiler has a positive or negative result...but they look cool!

I would block off that lower opening and/or make sure all the air going thru the grille only goes to the radiator

I have been thinking a lot about this. It is not that difficult to block air going through the grill or above the valance. That opening is huge. I now get enough cooling so probably can do a lot without affecting engine temp and if that's an issue is a matter of just drilling a few holes in whatever I use to block. I think that the air going through the grill or above the valance creates an area of high pressure and part of the air that doesn't go through the radiator will tend to push up against the hood or a block plate and the other part will push down against the flat under the radiator. That huge opening in front of the radiator doesn't help unless like you say making sure that the air goes through the radiator. At that high of a speed there is a lot of air going though there so blocking some of it should work because I see no way of making all that air go through the radiator. Maybe blocking the air through the grille and leaving the opening above the valance open. Definitely something to try but to tell you the truth it is not the most comfortable affair to be testing wheel lock up at 110 mph. I will try a combination of brake bias, blocking the grill and lowering the front. In addition, I will work on modulating the brake pressure at 100+ mph. This track has another straight where I was reaching upper 90s, with a high of 103. I did not have any issues hard braking there, so it seems that my issue is 110ish or above. Oh well, any solution will have to wait until the next season.

 

[rvrtrash]

It sounds like you're focused on blocking excess air into the engine compartment, which isn't bad, but long ago when I raced on road tracks, I lowered the rear end 1/2" and the front end 1", and didn't have any problems at 110-120. Yes, you have to be careful of speed bumps, etc. on the street with headers, but that's what worked for me. As mentioned earlier, make sure your strut rods, lower control arms, tie rods, etc. don't have play in them and your wheel bearings are torqued correctly. Little things add up at high speed.

 

[bkdunha]

I am no aero expert or road racing expert, but I am not thinking front end lift would lead to intermittent braking problems. What brake fluid are you running? At the end of a long straight at high speed I would think you would be on the brakes pretty hard. You may be boiling the brake fluid at high temps and if you have not changed the fluid you may have some small water deposits in it vaporizing and causing air bubbles in the lines. You may want to explore changing your brake fluid out to a more race based high temp fluid and flushing the whole system. Just my opinion without much race experience under my belt is my disclaimer.