Rake

carolinachill

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Interested in knowing how much rake do you guys run? 2014 gt working on setup for CAM autocross. I have coilovers, so I can adjust ride height and rake. Also I will be running my 1st event on 315 BFG rivals, what tire pressure do you run to help get me in the ball park?
 

lakeside

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While rake or stance certainly improves looks, the more important thing to go for is corner weights. See if you can find someone or a race shop that will "scale" your car. You'll need to read up on how to do this, but if you have coilovers, you might as well take advantage of them. Or you can learn the hard way like most of us have had to, and adjust it til it looks good and hope you retain some good handling.
 

oldVOR

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Starting tire pressures can vary based on ambient temps and track surface temps.
Since this is your first event, I'd recommend 36F and 31R. From there, chalk the shoulder of the tire where the tread blocks roll into the side wall, an inch will do, 1/2" on the tread and 1/2" over the shoulder. If you're rolling the tire too much, add pressure. If the tire isn't rolling enough, remove pressure. I'd recommend chalking the tires for each event and making adjustments from there. If you want to record ambient and surface temps for each track, you'll have a good starting point for the next event at that track.

Can you adjust camber? If so, try to get -3 degrees or more worth of camber.

Personally, I'd set the chassis up using scales to balance out the weight across the suspension.
 

kerrynzl

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I just measured my for car for you to compare.
It is a Grand-Am FR500C for reference.

From the ground to the top of the wheel arches
Front = 26" [26.00"]
Rear = 27-1/8" [27.125"]

Hope this helps


Kerry
 

SoundGuyDave

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Kerry, I just want to check this... What size tires? You still have the FR500C extended-length ball joints on it, right?

Only reason that I want to bring this up is to make the point that "everything depends on everything else." (Not aimed at Kerry, as I know he knows this)

Ride-height does determine CG, and generally speaking, lower is better. HOWEVER, when you lower the car, you also change the roll centers, instant center, and put the front suspension in a VERY unfavorable portion of the camber curve. You can correct for all of that, however that takes hardware as well as a rule set that allows it.

Please remember that the suspension is engineered as a package, and changing one aspect of it usually has unintended consequences elsewhere unless you really know your shit in terms of suspension design and engineering.

Using the FR500C as a template is not the worst thing you can do. Those ride heights (factory service manual has specific heights to measure that are not ground-referenced) are also engineered as a package, and one that works well. As long as you have the damper travel, spring rates, front R/C correction (ball joints and tie rod ends), and IIRC, the rear %AS has been adjusted as well.

Unless you put that type of package together, you MAY be better off with a slightly higher ride-height, even at the expense of a slightly higher CG. Absent an engineered package, you're looking at trial and error. I would trade a higher CG for a roll center above ground, and a favorable camber curve any day.
 

kerrynzl

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Dave, The tyres are Hoosier 315/30 18's on all 4 corners.

It shouldn't make any difference if they were 185/70 14's [if they could fit]
The distance from the axle centrelines to the top of the arches would still be the same so the rake would be the same [just the ride height changes]

I didn't know about the extended ball joints so I just crawled under to have a look, Yes it has them.

These ball joints alter the instant centre and roll centre not the ride height. That is determined by the distance between the upper platform and the stub axle.

Back to my car, I'm learning new things about it all the time. The level of detail is amazing and would be cost prohibitive to replicate.
My particular car was campaigned by "Blackforest Motorsport" in Florida.
 

Norm Peterson

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For autocross purposes, I think you should be focusing on individual front and rear ride heights and letting the amount of rake fall wherever it may once you've found the ride heights that work best for you. It's things like the front vs rear geometric roll center heights and the amount of rear axle roll steer that will affect transient handling, not the side view appearance.

As a datapoint that might be useful, lateral accelerations that peak above 1.3g lateral are entirely possible for an S197 on 285/35-18 MPSS tires (25.9" tall) with the car still on its OE springs. So OE ride heights, OE rake, about 0.6" lower than OE CG height and roll center heights. Just stiffer sta-bars, Koni yellows, and about -1.8° camber.


Norm
 

SoundGuyDave

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Dave, The tyres are Hoosier 315/30 18's on all 4 corners.

It shouldn't make any difference if they were 185/70 14's [if they could fit]
The distance from the axle centrelines to the top of the arches would still be the same so the rake would be the same [just the ride height changes]

Absolutely agree, HOWEVER, the spec you gave was from ground to fender arch, and tire OD will affect that number. The FR500C manual lists two places to take measurements from to determine "proper" ride height, and they are tire-independent. I'm not certain where it was up front, but I believe it was a height differential between the bottom of the ball joint and the center of the FCA forward pivot bolt. I do remember that in back it was the distance between the top of the axle tube and the frame arch. I do remember the manual recommending the fabrication of "go-no/go" sticks for each for rapid setup.

I didn't know about the extended ball joints so I just crawled under to have a look, Yes it has them.

These ball joints alter the instant centre and roll centre not the ride height. That is determined by the distance between the upper platform and the stub axle.

Back to my car, I'm learning new things about it all the time. The level of detail is amazing and would be cost prohibitive to replicate.
My particular car was campaigned by "Blackforest Motorsport" in Florida.
Yessir, the FR500 program came out with three extremely well-sorted cars, developed by Multimatic. FR500C came first for the Grand-AM series, the FR500GT was built to euro-spec, then the FR500-S was developed for the proprietary Mustang Challenge. Well built, well-sorted race cars, all of them. In theory, you COULD build a "privateer" version without buying one, but as you mentioned, the labor involved would be insane to try to duplicate. If you WERE building a car, you could certainly take some design cues from the program, though, and even use some of the specific parts to help speed you along. I have a FR500-S chassis harness in mine, as it was more "cost effective" to buy the de-contented harness than it would have been to do the work myself. WAY too many hours! The front roll center correction is a perfect example of how the cars are designed and built. The development team had their crap together, and came up with a well-engineered package The lowered ride-height, taken in isolation, would have wrecked the camber curve, the front roll center (and thus roll couple with the rear) etc. The Sachs dampers are shortened, and they relocated the front roll center by extending the studs on the ball joints, which in turn necessitated the extended studs on the tie rod ends to correct the resulting bump-steer... Take any one of the pieces in isolation and you have drama. All three? Functioning race car.



For autocross purposes, I think you should be focusing on individual front and rear ride heights and letting the amount of rake fall wherever it may once you've found the ride heights that work best for you. It's things like the front vs rear geometric roll center heights and the amount of rear axle roll steer that will affect transient handling, not the side view appearance.

Absolutely agree.

As a datapoint that might be useful, lateral accelerations that peak above 1.3g lateral are entirely possible for an S197 on 285/35-18 MPSS tires (25.9" tall) with the car still on its OE springs. So OE ride heights, OE rake, about 0.6" lower than OE CG height and roll center heights. Just stiffer sta-bars, Koni yellows, and about -1.8° camber.


Norm
This, right here, is a telling point. If this is your first outing on a new tire, plan on using the outing to figure them out. Pressures, temps, etc. If the car has been doing "fine" on the current chassis setup, I would avoid any changes simply to keep the variables down. Otherwise, you'll be hard pressed to tell if the oversteer you're experiencing is due to rear pressures, rear roll-steer, or damper/bar tuning. Change one thing at a time. If this is your first event, period, then just focus on getting used to the environment and the style off driving you're going to need to get through the course.
 

kerrynzl

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For autocross purposes, I think you should be focusing on individual front and rear ride heights and letting the amount of rake fall wherever it may once you've found the ride heights that work best for you. It's things like the front vs rear geometric roll center heights and the amount of rear axle roll steer that will affect transient handling, not the side view appearance.


Norm

I also agree with you here. I just answered the OP's question [about Rake] by spending a few minutes with a tape.
Having adjustable ride heights is a quick way to tune the roll couple at the track.
We also use adjustable height watts links to alter the RCH [these can be set up to tune the handling from the drivers seat]

Autocross Racing [also called Gymkhana or Motorkhana] is almost non-existent in NZ. I haven't been at one in 30 years.
Most car clubs can lease [for 1 day] some public roads from local govt's and we have a race called a "Bent Sprint" [timed from point A to B]
This is where Rally cars can race against Circuit cars etc.

The other popular form of feeder type racing is the "Dual Sprint" on our road race circuits.
You have 10-12 cars at the start line, and the start in pairs in 5 second intervals.[racing 1 other car and the stop watch]
This eliminates a lot of attrition you get from over-enthusiastic novices together.
We need to adopt the "Sin-Bin" system mentioned on other threads.
 

SoundGuyDave

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Kerry: driver-adjustable Watts RCH??? I'm assuming it's set up with some sort of jack-screw, and has the pivot joint in a captive cage? Any chance you would have a pic or two? I can't get my head around the amount of leverage needed for the cockpit control.

I COMPLETELY hear what you're saying about minimizing "stupid human trick" based attrition, but I honestly have to say that there is nothing like a proper race start, particularly if it's a standing start.

When the green drops, you're on cold tires, with cold brakes, and you go into T1 HOT, usually 2-wide, and occasionally 3-wide, with all the cars slipping and sliding at the absolute traction limit. It's absolutely heart-pounding action, but occasionally somebody does "run out of talent" and bad things can happen. Still exciting, though! In particular with Vintage classes, I could see why minimizing the carnage would be a good thing.

If you want to have some SERIOUS fun, you should try (at least what we call) an Australian Pursuit race. EVERY car in the race is given a handicap time, and sometimes penalty laps, based on their best race/qual lap that day. They are released FROM PIT LANE in sequence, separated by that handicap time. If all the drivers PERFECTLY nail their potential lap time, they all cross the finish line AT THE SAME TIME. Instead of the big stack-up at the beginning of the race, it happens at the end, when the Vipers are catching up to the Miatas... with a Mustang sandwich in the middle, usually! EPIC fun.
 

kerrynzl

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Adjustable Watts Link

Dave, The adjustable watts is mounted opposite to normal. The links are fixed to the rear end and the bellcrank to the frame [with a screw adjustment]
They are very popular in Aussie as a spin-off from V8 supercars. And available as a kit [photos attached]

My brother has one in his 69 boss race car. he said it is great for fine tuning for different track conditions [he doesn't use in-car though]
Photo attached of my brothers race-car [a genuine "G" code 69 boss]

Aussie pursuit racing is known as "Sandbag Racing" over here. They are popular for getting large grids of mismatched cars.
We now have a break-out rule with a "drive-by penalty" being used to prevent sandbagging.
You are allowed in-car lap timers in NZ to assist not breaking out [as long as you Don't break out]

Me and a friend with a Falcon V8 touring car devised a clever strategy to win in these races [using innovative methods that others called cheating]

After a very high win percentage we moved onto other types of racing
 

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SoundGuyDave

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I figured it had to be something from the supercars; beautiful engineering on that whole kit, particularly in the pillow-block retention, which is the key to everything. I still don't see how an in-car adjuster (electric motor, maybe? with a gear-reduction for the needed torque?) could possibly turn that jack-screw when it was under load, though. Maybe I'm missing something in the physics, I don't know...

"Sandbag Racing." Gotta love it! I figured that it would be called something else down your ways, but I never figured it would be that colorful (colourful?)!

NASA, the sanctioning body I run with primarily, has only done Pursuits a couple of times, and while I loved it, I think it was just a bit too "weird" for most of the racers, who preferred running a more traditional 3-race weekend with 35-40 minute sprints.

We also ran "Matrix" races for a while, which (again) I thought was a blast. This was a pure dial-in race, where you could either take your sprint-race qual time OR declare a lap time. All of the entrants were broken into groups of 6-8 cars, all with similar pace, and released as "waves" each taking it's own green. If you broke out, you were DQ'd! No contingency, no trophies, just for fun. I liked it because you got the opportunity to run with guys you wouldn't normally be racing against. You might find yourself as the only car from your class, in with the back-markers from a faster class and the front-runners from a slower class, suddenly having to RACE against each other, rather than just manage passes. Getting mixed up like that gave the back-markers and mid-packers a chance to see the "fast guys" racecraft, and we all learned a thing or three along the way. Also a victim of the 3-race weekend...

Still fun stuff!
 

kerrynzl

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Dave,

That pillow block needs to be lubed up, and you use a "Sprintcar Weight Jacker"
They are good enough to dial wedge into a car while racing.

What I love about where this thread is leading is ............

A type of racing you refer to as "Australian" we refer to as "Cheating" [Sandbagging]
I assume sandbagging is a term you use over there as well.

I cant wait to tell my Aussie friends :gr_grin:
 

SoundGuyDave

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LOL!! Yes, we use the term "sandbagging" as well, for holding back pace to gain an advantage when there are handicaps involved... FWIW, during the Matrix and Pursuit races, while there was no outright "ban" on lap timers, use was discouraged... Made it more fun that way. The race formats didn't stick around long enough for anybody to start gaming the system, so maybe that's why I have such fond memories of it. Just balls-out racing, no gamesmanship (outside of normal racecraft, of course) and nobody getting stupid about things. It was all for fun.
 

kerrynzl

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Another form of racing we have at a club level is "Timed Groups"

Basically everybody qualifies and put into race groups of similar lap times.
A novice in a good car could be up against an experienced driver in a slower car [the stopwatch levels the playing field]

Again we have a breakout rule to stop sandbagging [usually time added on short sprint races]

This format is popular for racing in the off season
 

SoundGuyDave

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Sounds like our Matrix races. It's fun stuff, actually. Hell, in any event, RACING is fun!
 

kerrynzl

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Adjustable Watts Link

Dave, back to the subject of Adjustable Watts Links.

one of the pitfalls of Watts Links, is the vertical Bellcrank leads to a high RC.

I saw on an old RX8 TransAm car years ago, which had a Watts Links with the Bellcrank that was horizontal and pivoting on the underside of the rear-end.

On the RX8 the centre of the bellcrank had a spherical bearing. and the rear end pivot was threaded . The Bellcrank was located with 2 locking nuts which could raise and lower the RCH.

I think there is too much emphasis on Watts Links in this forum.[ I was reading " Watts Link for improved street manners " in this forum ]

Most of the unstability the driver feels in the rear is "Roll Steer" caused by the outer trailing arms moving on an arc, instead of the rear end moving sideways.
Those bolt-in kits for relocating [lowering] the trailing arm pivot at the rear-end actually cause Roll-Oversteer which is unstable.
 

Norm Peterson

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I saw on an old RX8 TransAm car years ago, which had a Watts Links with the Bellcrank that was horizontal and pivoting on the underside of the rear-end.

On the RX8 the centre of the bellcrank had a spherical bearing. and the rear end pivot was threaded . The Bellcrank was located with 2 locking nuts which could raise and lower the RCH.
I've seen where that approach was used before, might have been on a Camaro or Firebird track car. Probably requires a fairly hefty rear sta-bar because you're limited there in how high you can locate the geometric RC.


Those bolt-in kits for relocating [lowering] the trailing arm pivot at the rear-end actually cause Roll-Oversteer which is unstable.
They're mostly popular because they help with launch traction for the folks who think corners on a track are the ones you're stuck having to make so you can line up again for another quarter mile drive.

Drilled in the proper locations, you can use them to correct roll steer for lowering - lowering the car without revising the LCA pivot point locations increases the roll-understeer, which might not be optimal either.


Norm
 

Norm Peterson

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For the car in my sig as it now sits (about 1/2" lowered from the sig pic) measurement from the ground to the top of the rear wheel openings is a quarter of an inch more than the ground to top of front wheel openings. That's with a full-size spare aboard, nothing else in the trunk, and maybe 3 gallons of gas in the tank, so it's representative of where the car will sit about midway between refuelings at the track.


Norm
 

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