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Hard Braking causing the car to hunt

 
Old 03-24-2019, 06:24 AM
  #1  
67greengt
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Default Hard Braking causing the car to hunt

I've been slowly getting some more time with my car (not on track yet) but have been playing with braking and steering inputs etc. The car is completely stock, stock sway bar setup front and rear, Sport cup 2's normal pressures etc. I've noticed under heavy braking the car wants to dance around a bit (has the PCCB) is this normal? Would the DSC Controller help with this? Looking for some input from the track guys out there as I don't want to take it to my local track with this issue - hopefully its an easy fix or maybe even a change in driving style for me?

Thanks everyone
Tom
Jet Black GT4, PCCB, LWBS, NAV, PDLS
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Old 03-24-2019, 08:30 AM
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2slow2speed
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What are the current alignment specs for the GT4?
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Old 03-24-2019, 08:31 AM
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67greengt
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I'm not sure the exact specs, but I know that its factory spec when I bought the car CPO, I want to say around -1.xx camber ? not sure on toe/caster
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Old 03-24-2019, 10:09 AM
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Bill Lehman
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This issue is common and much info is here. Adding rear toe fixes the problem. TPC rear toe link also helps. I've done both.
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Old 03-24-2019, 11:12 AM
  #5  
paradocs98
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As Bill says, it's a known issue, so you're not crazy. Rear toe-in, TPC rear toe links, and DSC sport controller are all fixes. It's especially disconcerting on track the first time you experience it, particularly if you're coming from a 911 where the rear weight bias is extremely stable under hard braking.
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Old 03-24-2019, 11:35 AM
  #6  
Pyrat2
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Toe in is the key spec that you can adjust without mods. Slightly more toe in will help. Might want to check where it is set and have the shop tweak it slightly towards the maximum spec setting or even slightly out of spec. Too much though and you'll be scrubbing the tires and will tradeoff turn-in.
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Old 03-24-2019, 12:00 PM
  #7  
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If you are planning to track the car or do a lot of high speed canyon driving with aggressive braking I'd reach out to Tom at TPC racing. Along with the availability of adjustable toe links, Tom has a vast knowledge of the GT4 & offers various settings for different applications (track, street, etc). He along some other bright engineers at TPC designed the DSC controller. Working with Randy Probst a seasoned racing veteran, they sorted out the shortcomings of the GT4 early on & "fixed" these issues.
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Old 03-24-2019, 12:48 PM
  #8  
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As mentioned above.....normal for the stock car. Correcting the rear toe helps but the improvements with TPC rear toe links is noticeable and worth the price. I later added the DSC module with a TPC custom mapping for their toe links and braking at high speed is just perfect.
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Old 03-24-2019, 01:50 PM
  #9  
Alan C.
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Had the problem on my GT4. Added TPC links and DSC along with Tom's suggested alignment. Took care of my problem.

Anyone wanting a DSC V2 mine is for sale. Getting a new one for my 991.2 GT3
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Old 03-25-2019, 12:28 PM
  #10  
Tom-TPC Racing
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Originally Posted by Alan C. View Post
Had the problem on my GT4. Added TPC links and DSC along with Tom's suggested alignment. Took care of my problem.

Anyone wanting a DSC V2 mine is for sale. Getting a new one for my 991.2 GT3
Alan's DSC is in good working condition for anyone looking for a V2!
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Old 03-25-2019, 01:47 PM
  #11  
Tom-TPC Racing
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Thanks for all the inputs guys!

To the OP, here's what's going on and how each of the mods will help-

What's going on is during hard braking is the car's front dives and the rear lifts. As the rear suspension lifts it is by factory design that the rear toe(alignment setting) goes outward, causing what's referred to as Toe-out. Rear Toe-out in this case makes the rear of the car unstable. This is not very confident inspiring, for example on a race track at 130+mph.

DSC makes the car more stable by managing the peak damping force and the timing of the damping force. Exactly what DSC does is during a hard braking event DSC commands the front suspension dampers to be firmer by increasing front compression damping force and simultaneously commands the rear suspension dampers to by firmer by increasing the rear rebound damping force. The amount of damping force increase is relative to brake line pressure and g-force in real time. This DSC function slows down the rate(and in some case the total distance) of suspension travel movement(front down/rear up) producing the same type of positive effect as having adjustable racing dampers and stiffer front springs. Substantially, DSC slows down the rate of rear Toe-out enough to get the through the braking zone. Other benefits of this DSC function is by managing the rate of suspension travel movement, which improves dynamic weight transfer to help prevent overloading of the front tire contact patch(going to ABS) at the same time keeping the rear tire contact patch loaded for longer duration of time for better braking bias and get more braking done in this duration of time. DSC has other great functions as well, this is one function that's relevant to hard braking event. Bottom line is damper control is about managing timing.

TPC Offset Rear Toe Links makes the car more stable by reducing the amount of rear Toe-out per the amount of suspension travel distance. This is achieved by the suspension geometry change.

In summary, DSC improves the timing of the suspension travel, while TPC Offset Rear Toe Links improves the geometry of the suspension travel distance. Having one of the two is good. Having both are even better! Especially since DSC has a specific calibration file(aka map) for the TPC Offset Rear Toe Links that commands the rear compression to be softer to get better rear corner exit grip taking advantage of the improved geometry.
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Old 03-25-2019, 01:57 PM
  #12  
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Can anyone here say about how much rear toe-in we should dial in to mitigate this problem? What have you guys used?
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Old 03-25-2019, 02:27 PM
  #13  
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Originally Posted by RocketGuy3 View Post
Can anyone here say about how much rear toe-in we should dial in to mitigate this problem? What have you guys used?
Factory spec rear toe is +10' per side. Converted to millimeter is +1.4mm to +1.6mm per side. You'll have to run double the factory spec amount(or more) to keep rear toe from going below zero and into a negative number during hard braking if the shocks, springs, suspension controller, and rear toe geometry is stock.

Edit: There's a lot for variables on how much rear instability as a car's front dives/rear lifts- how sticky the tires are, braking torque from the brake pads, driver's brake application, hard braking during downhill or uphill...

Last edited by Tom-TPC Racing; 03-25-2019 at 02:38 PM. Reason: See edit
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Old 03-25-2019, 02:28 PM
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Dr.Bill
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I was happy with zero toe in the front and 2mm toe in for each rear wheel (i.e 4mm total toe in).
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Old 03-25-2019, 03:24 PM
  #15  
Bill Lehman
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I ran 3 mm per side without the TPC links and now run 2 mm per side with TPC links. I am a late braker.
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