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Rear Suspension Bottoming Out

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Hey Guys,

After recently "trackifying" my Boss, when driving on the street the rear end hits the bump stops and bottoms out quite a bit on dips and relatively minor pavement irregularities.

Running Steed Boss springs, so not only .8" drop in back. Also running the shorter FRPP bump stops, but there's barely an inch between the bump stop and the chassis. Wondering if the lack of bind from the spherical bearing B302S LCAs and Steeda UCA bearing have something to do with it.

So is this a common issue with this type of setup?

BTW...Happy Memorial Weekend!
 
1,022
99
Exp. Type
HPDE
Exp. Level
10-20 Years
Texas
Do you have lower control arm relocation brackets? The suspension geometry will be way off if you don't and the car will bottom out easily. If you do have them and they are adjustable put the arm in a lower hole. Also, make sure that your upper control arm wasn't installed upside down on the axle side. It will contact the diff housing when the suspension articulates and you will hear a metal on metal sound when they contact. Good luck

Adam
 
Flyboygsxr said:
Do you have lower control arm relocation brackets? The suspension geometry will be way off if you don't and the car will bottom out easily. If you do have them and they are adjustable put the arm in a lower hole. Also, make sure that your upper control arm wasn't installed upside down on the axle side. It will contact the diff housing when the suspension articulates and you will hear a metal on metal sound when they contact. Good luck

Adam

Hi Adam,

Thanks!

Yes, FRPP relo brackets installed with LCAs bolted to the upper hole. I did notice that the bump stop looks to be slightly rearward by maybe 1/2" from the flat part of the bump stop cutout, further reducing the amount of travel. So bringing the axle forward slightly by bolting into the lower hole makes sense. How much would this change the DS angle?

Good point about the UCA. Since only the UCA bearing on the diff housing was changed, don't think that's the case but here's a photo...
IMG_7723_zpsaa2fu5xr.jpg [/URL][/img]
 
1,022
99
Exp. Type
HPDE
Exp. Level
10-20 Years
Texas
Depending on your suspension setup I would set your pinion angle at -1 to -2 degrees down. If I remember correctly the FRPP relocation brackets only have two holes. Check your upper control arm to make sure that's not the problem and if it's not then move the LCA into the lower hole and take it out for a test to see how it feels. It's not just about how much room you have between the frame and bump stops, it's about correcting the rear suspension geometry. By correcting the geometry it will actually make the car feel more stiff in the rear which in turn will make it not bottom out as much. If the rear control arms are inverted or nearly inverted (it's called thrust angle) then the car will bottom out more easily. I hope my terrible explanation makes sense. There is a much more scientific explanation to the issue but that should get you going in the right direction.

Adam
 
Flyboygsxr said:
Depending on your suspension setup I would set your pinion angle at -1 to -2 degrees down. If I remember correctly the FRPP relocation brackets only have two holes. Check your upper control arm to make sure that's not the problem and if it's not then move the LCA into the lower hole and take it out for a test to see how it feels. It's not just about how much room you have between the frame and bump stops, it's about correcting the rear suspension geometry. By correcting the geometry it will actually make the car feel more stiff in the rear which in turn will make it not bottom out as much. If the rear control arms are inverted or nearly inverted (it's called thrust angle) then the car will bottom out more easily. I hope my terrible explanation makes sense. There is a much more scientific explanation to the issue but that should get you going in the right direction.

Adam

Makes sense. I'll have to move the LCAs to the lower hole and see what difference that makes (yes, there are two adjustment holes in the FRPP relo brackets). Thanks!
 
675
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Are you sure you aren't hearing some contact between your Bob's/Shelby KR axle overflow reservoir and the axle? I had a similar setup and it would clunk and clank over very minor bumps, no matter how I configured the hoses and fittings. I went to a JLT one and never had any more issues and my car sits lower than yours.

EDIT: I meant to say contact between the reservoir or it's fittings hitting the axle, differential, and/or overaxle pipes.
 

302 Hi Pro

Boss 302 - Racing Legend to Modern Muscle Car
2,009
441
Southeast
As Adam said, changing mounting locations will change the geometry of the rear suspension, and moving the rear mounting point of the RLCA will bring your rear axle forward. This changes a few things including your pinion angle. I do not believe it would help with a rear "bottom out" situation.

The rear coil springs sit on top of the rear axle tubes, so changing RLCA mounting point below the axle tubes won't help.

Look at your pinion angle, it should be 2.5 degrees downward. It's hard to check the actual degrees as it should be measured against the line between the trans bell housing and engine block. It's not done by just sticking on magnetic dial.

If you have a 1 piece DS, Make sure the rear driveshaft angle is similar but opposite of the angle of the DS to transmission. This will get you close, enough so that it won't clunk.

Unless you have adjustable UCA and RLCA's the fine tuning your pinion angle is impossible. The rear diff housing bushing come in a steel sleeve. It is important that this is installed correctly. The steel sleeve has one flaired end and this should have a 4-5mm (IIRC), clearance to the housing. Make sure it is centered in the top eyelet of the diff housing. That will cause a clunk sound.

Check exhaust pipe OTA clearance.

Remember, when you trounce the throttle, the noise or DS flange of the pinion shaft is driven upward. It has quite a force working on it. The DS must not reach an angle that is above level. It must stay slightly below the level plane. This is why the PA should be 2.5 degrees downward. This may decrease to -1.0 degree to -0.5 degrees on WOT with traction. More traction equals more thrust.

Look at everything above the rear spring bottom mount, and make sure your spring seats were reinstalled. The rear diff housing moves in an arch on either the UCA or the LCA's depending on which mounting point you are adjusting.

Good luck and keep us up dated.

302 Hi Pro
 

Grant 302

basic and well known psychic
dabossinne said:
Hey Guys,

After recently "trackifying" my Boss, when driving on the street the rear end hits the bump stops and bottoms out quite a bit on dips and relatively minor pavement irregularities.

Running Steed Boss springs, so not only .8" drop in back. Also running the shorter FRPP bump stops, but there's barely an inch between the bump stop and the chassis. Wondering if the lack of bind from the spherical bearing B302S LCAs and Steeda UCA bearing have something to do with it.

So is this a common issue with this type of setup?

BTW...Happy Memorial Weekend!

Perhaps you are just hearing the spherical bearings? Hitting the bumpstops is more of a feel than a noise IMO. Are you hearing more of a clank or a thud?
 
It's a sharp metallic "clank", like the noise a wrench would make if you tap it sharply on a metal pipe. I think Champale has it nailed.

After getting under the car to look at things, I am guessing the end of the UCA diff bolt slightly impacts the diff overflow reservoir at close to full jounce, which forces the tank into end of a mounting bolt for the Watson roll bar directly above it. And that is what I'm hearing in the car.

So, looks like I'll need to replace the Shelby expansion tank with something else in a different location.

Anyway, thanks Champale, and everyone else, for your ideas and input. It's what makes this forum so great!
 
dabossinne said:
It's a sharp metallic "clank", like the noise a wrench would make if you tap it sharply on a metal pipe. I think Champale has it nailed.

After getting under the car to look at things, I am guessing the end of the UCA diff bolt slightly impacts the diff overflow reservoir at close to full jounce, which forces the tank into end of a mounting bolt for the Watson roll bar directly above it. And that is what I'm hearing in the car.

So, looks like I'll need to replace the Shelby expansion tank with something else in a different location.

Anyway, thanks Champale, and everyone else, for your ideas and input. It's what makes this forum so great!
Check out this thread for different diff breather options.

https://trackmustangsonline.com/index.php?topic=7851.0
 
6,363
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FWIW we use the stock locations for our IMSA and PWC cars, in any case, the cooler lines and PHB (or whatever) links get pretty close to the diff cover. I don't see the suspension bottoming out with those springs unless it's on a bolt or a fitting .
 

Fabman

Dances with Racecars
6,519
8,155
Exp. Type
W2W Racing
Exp. Level
20+ Years
Pleasanton: 1/2 way between Sonoma and Laguna Seca
Flyboygsxr said:
Depending on your suspension setup I would set your pinion angle at -1 to -2 degrees down. If I remember correctly the FRPP relocation brackets only have two holes. Check your upper control arm to make sure that's not the problem and if it's not then move the LCA into the lower hole and take it out for a test to see how it feels. It's not just about how much room you have between the frame and bump stops, it's about correcting the rear suspension geometry. By correcting the geometry it will actually make the car feel more stiff in the rear which in turn will make it not bottom out as much. If the rear control arms are inverted or nearly inverted (it's called thrust angle) then the car will bottom out more easily. I hope my terrible explanation makes sense. There is a much more scientific explanation to the issue but that should get you going in the right direction.

Adam
Control arm angles/anti squat:
anitsquat1.png
anti-squat.png
LCA%20relocation%20bracket.png
 
1,022
99
Exp. Type
HPDE
Exp. Level
10-20 Years
Texas
You explained that perfectly with the diagram and so many fewer words. That's what I was trying to explain when I said moving the control arm to the lowest hole will help with anti squat. I have also experienced this first hand. The car was bottoming out too much when I was on the Nurburgring but when I moved it to the bottom hole I no longer bottomed out so I can say from experience that it makes a difference. Check everything else though that was recommended too. Also remember that because the car is lowered the rear axle will be twisted slightly meaning that the bump stops won't be lined up on the original chassis bump stop locations. Due to this misalignment the housing around the bump stop can contact the chassis making a metal on metal noise. If you look where the bump stops are mounted on the axle and the chassis there may be dents or marking from the axle contacting the chassis. Hope that makes sense.

Adam
 

TMSBOSS

Spending my pension on car parts and track fees.
7,530
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Illinois
dabossinne said:
It's a sharp metallic "clank", like the noise a wrench would make if you tap it sharply on a metal pipe. I think Champale has it nailed.

After getting under the car to look at things, I am guessing the end of the UCA diff bolt slightly impacts the diff overflow reservoir at close to full jounce, which forces the tank into end of a mounting bolt for the Watson roll bar directly above it. And that is what I'm hearing in the car.

So, looks like I'll need to replace the Shelby expansion tank with something else in a different location.

Anyway, thanks Champale, and everyone else, for your ideas and input. It's what makes this forum so great!

Can you simply turn the bolt connecting the UCA around? This will have the long end facing away from the can.
 

Mad Hatter

Gotta go Faster
5,237
4,227
Santiago, Chile
I liked this diagram that Grant 302 put up some time ago. It describes exactly what you feel when you lower the car without LCA relo brackets

QYtkfQWatSUGgWWOqrHsuapaYrZlRJvW0OCIIHpt1vRQUNtIWQ.png
 
675
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Hey you are very welcome! I hope it is really that simple.

I highly recommend the JLT setup. You can 'clock' the reservoir at any angle you need to clear the diff because it only mounts with 1 bolt.

http://jlttruecoldair.com/ZenCart/index.php?main_page=product_info&cPath=524_525_533_357_399&products_id=642

Good luck!

John
 
6,363
8,187
I think you'll find (and this was not my end of the business) that having the rear suspension bottom out on the bump stops is not a horrible situation a slong as it just "lays" down on them and does not "hit " them which would upset the car's balance.

7W3nZJ2l.jpg
 

Fabman

Dances with Racecars
6,519
8,155
Exp. Type
W2W Racing
Exp. Level
20+ Years
Pleasanton: 1/2 way between Sonoma and Laguna Seca
I would expect that any possible benifit from laying on the bump stops would depend on whether or not it was a hard stop or a progressive stop, (soft durometer or cone shaped for example) which would serve as an additional spring with a rapidly rising rate rather than a hard stop which would induce snap oversteer. Be tough to plan that out just right so the rates were complimentary....but maybe some of the after market stops already address this. Personally, I worry about this very thing. On oval tracks it was easy to check travel indicators as the coilovers were out in the open and tracks were relatively smooth and corners were always in one direction. Now with the Mustang on road courses it's more difficult to check and re set travel indicators as well as whether excessive travel was for one or more bumps or re entering the pits or if it's laying on the stops excessively. I'd love to mount a gopro under the car and watch the video to see what's really going on under there....I can't think of a better way. Ideas anyone?
 

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