I figure some here will have more info on this, but Wilwood is advertising it has a big brake available for 914-4s. Just in case anyone would be interested. I haven’t even looked it up yet, but seen the ad in Excellence.

You guys must see what size wheels will clear the brake. Will it need a 16 inch or will a 15 work?

Best Bob

I did go on their site, really didn’t find much. But here is the ad.

It appears the cost F&R would be around $1800-2000. You have to weigh the benefit vs cost over stock performance for a street car. Maybe on a race set up it would be justified. Stock calipers with Porterfield pads work great.

That is quite interesting!

But I'm with John (Geezer). I historics-race a 914-4 and find that the front brakes are quite adequate with the right pads; in fact, if I had a lot more front brake I'd probably be running into bias problems and possibly front lockup.

For reference, I'm running the BMW calipers, stock rotors, Hawk Blue pads, manual bias valve typically dialed a lot to the rear, and Hoosier Speedster or Continental Extreme tires.

However, one big advantage I see: replaceable rotor rings! It would be wicked cool to be able to do rotor replacments without having to repack/replace bearings.

And, of course, they look pimpy cool (I race Wilwood brakes on the SCCA Produciton car and love 'em).

I sent them an inquiry for more info.

Just call them. I forget his name but they know their product.

Did mine 20years ago when they were $100 a caliber
Click to view attachment

Fact: tire grip is the single largest contributor to stopping distance.

For most people, they would get more out of a better set of tires than they would out of “upgrading” to “big” brakes.

They look smaller than these and I need 16” minimum rims to fit. A big plus is weight. The M calipers that I have for the back weigh a ton, compared to the larger wilwoods for the front.

Click to view attachment

What rotor are you running? 20 mm Vented or 24mm vented.

Just wanted to let people know, I know I don’t need the bigger brakes, but I know plenty of you have bigger hp 6’s, Suby’s, do autox etc…..

20mm

Weigh and horsepower make a difference, but the way you drive it also makes a difference what kind of brakes you might want. Stock worked pretty good for a while. But the braking would become less effective as my drive proceeded. So if you are cruising to cars and coffee in it, the stock brakes with some of those cool pads Eric sells will work great.

with the general premise but . . .

Horsepower makes no difference at all to single stop, stopping distance. Because . . . Physics. Force = mass * acceleration. There is nothing in that equation affected by horsepower. Note: acceleration in the braking case is simply deceleration. Opposite direction of forward acceleration.

Horsepower only comes into play with repeated, hard at limit braking, with short intervals between events that limit cooling between braking events. Track use, canyon carving come to mind. I autocrossed for years on stock brakes with race compound pads and didn’t have fade since average run is only a couple minutes in duration.


Note; not a critique of Shivers. Just get tired of the internet mythology that horsepower has any bearing upon single stop, stopping distance.

You need different pads, brother.

Not speaking to you directly, but it is simply amazing to me how most car guys don't understand how the pad compound you use really makes a substantial difference. Seriously. Many a braking complaint is easily resolved by "the right pad". And many a problem is caused by someone thinking they understand the concepts but getting it wrong ("why are these Hawk Blue pads tearing up my street car's rotors so bad??")

Today's pad compounds are fricken amazing. I would have killed decades ago for this technology*. And today we take it for granted...

I'd be glad to make suggestions, if we could just discuss what you're doing with them and whose pads you prefer.

GA

*Try racing Road America or Road Atlanta with dead-stock pads. That's what we used to have to do in the old Showroom Stock days of the 1980s: stock parts only. I'd go through a set of pads literally every session, and another set 3/4 into the race (seems like the braking got better when it went down to metal-to-metal...but it made a lot of noise).

Lap One Turn One at the Road Atlanta Runoffs smelled like a brake pad factory...

Metal to metal is much higher coefficient of friction than pad to metal. Your perception is correct. And pretty stable (not changing friction) much with increasing temperature.

The rotor wear on the other hand

The main problem though is race pads on the street really suck when pads are cold and first few stops are sketchy.

Ask me how I know

I have done my fair share of racing in my day…most of the time, race parts are just that, for racing, not street driving, even aggressive street driving. Especially suspension and brakes, tempof the brakes is huge! But also race suspension isn’t really designed for street use. Most roads have bumps, potholes, gravel, etc…stiff race suspension won’t keep the tires in contact with the pavement as a bit softer set up.

Either way, I digress, just happy people seen this thread and are at least talking about it.

As for me, keeping it stock in the brake department, but may use a bit better pads for when I feel the need…

OK gang, for those of us street drivers with mostly stock cars, what brand of pads do you recommend for replacement with stock calipers?

Porterfield R4-S pads from PMB performance. Maybe someone familiar with Hawk pads can recommend a part numbers.

This will quickly devolve inot the same kinda discussions as your favorite tires and motor oil...

Think less "brand" and more "application". Pretty much all major brands of pad manufacturers will offer a pad compound to suit your usage needs. Hawk, for instance:

https://www.tirerack.com/images/pdf/product...ound-Charts.pdf

No pad will do everything, but many pads will do a lot these days.

For my SCCA E Production 2600# car, I race on Hawk DTC70, but that's totally unsuitable for the street. If I had that same car on the street ('08 Civic Si) I'd likely drive on the HPS. Track days, maybe the HT10.

Manufacturers I like to support include Hawk, Carbotech, G-Loc because they support racing. Others with good reps are Porterfield, Brembo, StopTech.

But everyone will have their own opinions... - Greg

Surprised to see this offering, but delighted nonetheless. Always good to see more aftermarket support for the 914.

Curious for others' take on Wilwood calipers' lack of dust boots (if I have that right).

Rear kit particularly interesting if it's 1) light, 2) a good solution for keeping an e-brake, and 3) could be paired with a good option for 911 front struts on a 914.

A set of these in black (or plain metal) could be very attractive as an alternative to 986 calipers all around with a 911 e-brake conversion.

Most Wilwood "kits" include the cheaper "non-booted" Wilwood calipers.
These don't have the rubber boots that protect the pistons and the caliper bodies because when used in competition the boots tend to melt.
You are EXPECTED to reseal the calipers once a year at a minimum. These are extremely suspect of electrolytic corrosion. The steel to aluminum transition is not protected at all and brake fluid is highly hygroscopic. End result is a corroded-up caliper by the second year unless you NEVER get it wet...

Wilwood has booted versions of all of their calipers for street/off road applications, but they are commonly $100 more expensive per caliper and the "boots" are not as secure as a stock ATE caliper.

For a street-based car, the big brake kits require far more maintenance that stock stuff.

What I'm after with a BBK on a streetcar is a reduction of un-sprung weight and the "look".

My BBK plans include stock Boxster calipers which are both light weight and have very nice secure boots on all of the pistons.

Bigger contact patch (tires) and appropriate pads for the intended use are the critical things here. Stock stuff should work FINE with upgraded pads.

I chose Porterfield RS-4 because of everyone's recommendations for street use.

My 75 narrow body is using the BMW 320I front calipers, stock rear calipers, 19mm master, stock pressure regulator, and RS-4s all the way round.

My 76 916 clone will be using Boxster calipers and this set up for the pressure side of things. With dual masters and a balance bar I will not need a proportioning valve at all, and I can choose masters that will allow me to use ANY calipers I want.


I've been watching for something like this for a year!

http://www.914world.com/bbs2/index.php?showtopic=368858

For a link to the data.

Far more important to understand what you are buying vs going by brand or word of mouth.

Generally speaking what you want is a nice flat friction (mu) across temperature for street use. - HPS curve.

A mu curve that is rising with temp will help offset that feeling of fade. The tradeoff for the higher initial friction and rising curve of HP-plus will be noise, pad wear, rotor wear, poor disk thickness variation (ie judder) over time, excessive dust, or some other negative attribute.

HPS 5.0 might seem like a good middle ground but that drop off in Mu after 300-400F is going to feel like bad fade. The flip side being most street drivers will never get their brakes much above 400F. Would depend on how you’re actually using it.

The point being, know what you want and what the tradeoff is to get it. You can’t do that without the friction data.

Most pad suppliers only publish vague marketing claims at worst or force you to contact them to get their data.

Click to view attachment

I never understand when 914 owners that drive their cars less than 1,000 miles per year in good weather purchase all-season tires with rock hard rubber that last 50,000 miles.

On the 718 Cayman there are High performance summer tires, that its recommended not to drive under 40 deg and don't even add air if its under 20 deg.

I keep all-season tires on the 914 so I can at least drive it in cold weather. (with no salt on the roads)

I try to have two sets...

An all season on a narrow wheel for everyday driving. I like the Continental DWS versions.

A "go fast" set up on wide wheels with a summer ONLY tire and treadwear at or below 200.

The "summer" tires are so temperature sensitive that I store them OFF the car in my heated shop over the winter. I store them up high, where the warmest air is.

The narrow car has all season DWS on 16 X 6s right now. That tire has 96% of the grip that the summer version has and triple the life.

When you have a set of all season street tires you don't have to choose a longer life tire for the "go fast set".

I can remember when a set of "winter" tires/wheels was common...
Sometimes they were studded!
If you ever have to run a studded tire on clean/dry streets you will remember it for the rest of your life.
NO traction and stud noise drowns out everything else.
Sounds like a tree grinder...

But that (summer only tire) is how Cayman pulls off 1.2-1.3g stops and sub 100’ stopping distance (60-0).

It’s a far better trade (in terms of performance) to run dedicated summer tires and dedicated winter tires. Each of those tires will give you better braking performance than “upgraded” big brakes on a typical all-season tire for example.

Nothing wrong with the choice you’ve made as long as you understand what the trade off is.

I had BMW front calipers and the front brakes were doing 90% of the braking. The rear pads were hardly worn while the front pads were shot. I switched back to stock front calipers with Porterfield R4-S pads and pad wear is even all around. Greg Amy uses BMW calipers up front, but installed a brake proportion valve with heavy bias towards the rear for even braking.

That is the exact opposite to what I would have believed would happen!
With a 3/4" master and the BMW caliper the bias should move rearward anyways.

I HATE the original pressure regulator. In stock form it does not even see the front circuit and is merely an accumulator that starts to fill beyond an adjustable limit.
The stock pressure for actuation is 525 psi from what I remember.
It just "snubs" the pressure at 525...
For a stock car that's fine but I'm betting both you and Greg Amy are using non/stock tires and that threshold needs to be adjusted.

I'll be interested in how the 75 narrow body brakes work when I am finished.

Maybe not. I can't offer that I noticed, but bigger pistons would offer more piston force up front, ergo a lighter pedal pressure required for the same amount of deceleration. With a lighter pedal pressure you're less likely to hit the "knee point" on the rear bias regulator, thus less line pressure in the rear and less pad wear.

Just a WAG, though.

I adjusted my pressure regulator on the stock/street 914 with a washer under the screw to increase the spring pressure to initially bias more pressure toward the rear.

Manual Tilton bias lever on the race car, also typically turned favoring more rear bias.

I think Porsche/VW intentionally biased brakes more toward the front to keep the masses from locking up the rears and spinning the car in slick conditions.

Recall that the BMW calipers with the bigger pistons won't give you any more front pad friction (ignoring pad differences) or tire grip (ignoring weight transfer differences). The bigger pistons give you more piston force (not PSI) for the same line pressure with the compromise of increased pedal travel due to larger diameter/volume swpet area (adding the bigger 19mm MC helps on that last point).

Honestly, I didn't notice a significant difference on the street in braking other than a slightly different pedal feel, mostly because I don't drive on the street like a maniac... Changing pads would likely make a lot more difference. - GA

To give you a sense on what I mean by "knee point", the stock bias regulator works exactly like the Tilton manual brake adjusters, but the stock one is preset at a single (normally) non-adjustable point.

In both cases, the brake pressure between front and rear lines will continue to rise linearly until at some set point the resistance spring will be overcome and the rear brake line pressure "knees" off on a lower-angle pressure rise.

On a Tilton screw- or lever-type, all the driver is doing is changing the initial tension of the internal resistance spring. Porsche/VW set and forgot it on the 914.

That all said, I sent a note to Wilwood and got a reply. I have not yet gotten approval to post his whole reply publicly but I think it's Ok to offer that he said when they bought that ad in Excellence they thought the kit would be ready by Jan 1; they missed the date. They're working on getting this done ASAP. I'll let you know what I learn; I may consider getting it for the historics racer. - GA

Click to view attachment

Maybe horsepower is more the cause than the effect. I should have used something like velocity. When the 930 came out I was struck by the size of the brakes on it. And the 300 mph Bugatti Chiron, holly cow.

Maybe I have the rear pressure accumulator backwards...

A real proportioning valve (that has both front and rear circuits into the unit) hits a knee point that proportionally reduces the rear pressure. As the front pressure goes higher the rear pressure also increases but at a slower rate.

The 914 valve only sees rear pressure and it merely shuts it off at a spring-loaded point.
Once you hit that point the rear pressure doesn't change in any way, it doesn't drop or gain until you release the brakes at which point the pressure drops off and the accumulator section dumps its load back into the master.
This non-knee point is why I don't like them. It can be "tuned" with spring changes, but it is not proportional in any way.

The way I understand it is if you lower the spring rate on the accumulator section you lower the pressure it activates at and instead of the stock 525 you see something less.

OMG are you right on basic OEM brake set ups. They should always put heavy bias to the front.
Morons can deal with front lock up SO MUCH BETTER than rear.

As the CG of the car moves rearward this gets more important.

I just read your second post...
The puppy that Porsche installed looks like a simple accumulator to me.
I've worked with shitloads of these (automatic transmissions!). Normally they accumulate extra volume/pressure and are released to make something happen faster.
I don't think you will ever see any higher pressure in the rear system than what the valve operates at.
Now, I'm going to have to take one apart to investigate further.

A bit of search led me here.
https://tiltonracing.com/wp-content/uploads...ning-Valve_.pdf

That is COOL! the one line only Tilton valve definitely has a knee point. You can adjust when it happens, but you cannot adjust the percentage of proportion.
Still way cool!

The Tilton valve does NOT look like an accumulator, but the Porsche part does...

Thanks @GregAmy just the tidbit about how the Tilton valve works was worth it!
I'm big on balance bar dual masters but that link has shown a deficiency in my thought process. I'll be adding one of those to the 916 clone...

I don't think that's the case; I don't think it's basically a shut-off valve.

I did find this that implies it works the same as the knee valves (with disassembly photos).

https://forums.pelicanparts.com/porsche-914...-revisited.html


My Dodge race cars (L, G, and P bodies) as well as the minivans had a prop valve mounted on the chassis with a lever arm attached to the solid rear axle via a spring; as the car braked the rear end came up, the rear axle would drop relative to the chassis, and the spring would pull on the prop valve, reducing rear brake pressure. We'd cheat in Showroom Stock by modifying/replacing that rear spring to get the bias we wanted... - GA

EDIT P.S. These days, brake bias is managed 100% with electronics via the ABS system. That's been the case for a large percentage of the market since the mid-00s if not earlier. Manufacturers are no longer mechanically limiting rear brake bias, they're managing it via the ABS/TCS systems. We ran into this when we built an '08 Civic Si into a race car for a series that required us to disable the ABS; first time we drove the car (Daytona) we flat-spotted 3-4 rear tires before I drove to Home Depot and bought my add-on aftermarket rear brake bias limiting adjusters to get through the weekend...hey, "brake lines are free" per the regs...

Click to view attachment

Pretty much agreeing with all this

It is most definitely standard practice to bias toward front brakes to avoid rear lockup and inherent instability that comes with it. It would be considered negligent in a court of law to allow rear brake lock before fronts on any modern vehicle.


Good summary GA.

Edit: GA’s bias for race car is distinctly different that what’s suitable for street use.

Thanks Greg!
That thread doesn't change how I feel they work.
He created that graph "I took the only description of the pressure specs I could find and graphed it, shown here"

His blow-up pictures do not show how the adjustment bolt works and that, Gentlemen,
looks like an accumulator to me...

Fiat was one of the early adopters of suspension adjusted prop valves. I'm sure others were before but the spider had one in 1966.
It was sort of a neat rear suspension for a live axle, 4 link with Panhard bar and ride height adjusted prop valve. 4whl disc brakes, 5 speed, DOHC cross flow with Weber side drafts (the first ones).
Had a lot going for it compared to the English stuff that was available.

@technicalninja

This isn’t a personal attack or directed at you.

The chart is in the factory manual and unless I’m misunderstanding your words, they couldn’t be more incorrect.

This incorrect information (pressures, accumulator, regulator, etc.) just keeps getting propagated. I’m not sure where all the misunderstanding comes from when the information has been published for 50 years.

Click to view attachment

The front and rear circuit do “see” each other by virtue of both being the same master cylinder bore diameter. Barring a loss of hydraulic pressure in the front or rear circuit due to failure, the pressure in each circuit is equal.

Which suddenly reminds me of the Chevy Citation debacle. As I recall, the Citation didn't have a rear brake bias adjuster, or maybe it was tuned bad, and there was a rash of crashes where the drivers were locking the rear wheels when the brakes were applied hard, which would cause a skid/spin right into what they were trying to avoid. I think it even killed a few people.

Couple recalls didn't work (don't remember what they tried) and eventually NHTSA sued alleging Chevrolet knew and sold 'em anyway and it ended the car entirely.

The X11 was a good Showroom Stock B car for the times, though (let that sink in...)

I do agree that I'd not suggest anyone dink around with their rear brake bias unless they were very very comfy with what they're doing. The change I made with a thin washer to pinch the spring a tad is not much, but I'd remove it prior to selling the car. - GA

Edit Found an article on the Citation thing:

https://www.washingtonpost.com/archive/poli...6-c4cef1d6d390/

I run a similarWilwood front caliper and a smaller Wilwood rear caliper on my Datsun 510 race car

Wilwood provides excellent engineering service/advice. I would suspect that this same level of engineering and service will be at play with this product

Interestingly after evaluating the braking load that my race car sees, they recommended their high performance street pad compound for my application. It performs very well with consistent performance from first cold application to the last corner after 30 minute sessions of hard braking. These new compounds are indeed incredible.

In addition, the weight of these calipers is amazingly light. The reduction of unstrung weight on the 914 can not be a bad thing.

I do run a stock system on my six. Perfectly adequate for spirited street driving

Chris Foley,(Tangerine Racing) experimented with the rear brake pressure valve. He shimed the spring 1/4" using washers. Another test he did was to turn the set screw 2 turns in. Both improved the pedal feel and gave more balance front and rear without locking up the rear brakes. I also read that
3 turns was on the edge of locking up the rears. I adjusted mine 2 turns in.

Taken under advisement Superhawk996.

There is a TON of misinformation regarding the rear proportioning valve on this site as well as others. Easy to pick up erroneous information.

There is a full theory of operation in the factory manual if you’re truly curious.

IMO, the main reason for the complexity of the 914 proportioning valve vs a Tilton is:
1) German design

2) The incorporation of 4 wheel disc brakes (not very common in 1969) and the increased awareness that rear brake lock needed to be avoided at all cost. Especially so in a mid engine, low moment of inertia, relatively short wheelbase like a 914.

3) Evolution of a component design over time (decades in this case) moves toward smaller, more elegant design with fewer seals, pistons, springs, and moving parts. Cheaper to manufacture and generally more reliable as design evolves.

Edit: this topic brings back fond memories of Brent Brock for me. Brent used to love provoking me by referring to the rear proportioning valve as an early ABS system which it most certainly is not. Brent knew this to not be the case but it became an inside joke just to me. Man, I miss Brent!

Without a wall of text I’ve been very happy with my wilwood brakes. No complaints.

The only thing that I've read that makes sense to me is on the PMB website.

This is a direct quote:
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Repeat after me, "Pressure Regulator" -- "Pressure Regulator"

That's right, this little beast is a pressure regulator, "NOT" a proportioning valve. It proportions "NADA". It is the first anti-lock brake device ever to be installed on a Porsche. Here's the story:

The 914 is a mid-engine automobile. As such, it has amazing handling characteristics... to a point. Once that point is reached, it can bite you like a rabid dog. We're all familiar with the child's play toy the "top". It's a physics principle called "Polar Moment Inertia".

Porsche knew the issues with rear and mid-engine automobiles so, they installed this pressure regulator in the system to prevent the rear wheels from ever locking up. You see, if you lock your rear wheels in a rear or mid-engine car and your front wheels are anything but straight, you can be that "top" we were talking about and, that's not a fun toy at that time. This pressure regulator senses a panic stop (which it is set and locked at 525psi). Once it sees that pressure on the system, the valve body slides over and activates a microswitch which shuts off pressure to the rear wheels. This is a panic stop mind you, this device flows fluid like a "T" fitting until you need it. That's why we insist that it proportions "Nothing". Once the pressure equalizes in the chamber, it brings the rear calipers back into the circuit. This is "split second" stuff gang, not a leisurely event.
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

That is the source of my "misinformation".
I thought it was a good source.

They are calling that a pressure regulator...
They're pretty insistent.
I thought they knew what they are doing.

I'm guessing that was typed by Eric Shea.

I believe him more than anyone else regarding 914 hydraulics.
Maybe I'm reading that wrong.
If it has a proportioning feature, I would have thought he'd mention it.

I believe a reduction in rear pressure vs front pressure makes it a proportioning valve.
The Tilton is a p valve IMO.

A regulation of rear pressure at 525 psi regardless of front pressure makes it a regulator in my book.

It might have proportional response past the operation point of 525psi.
I don't think it will.
I'm in agreement with Eric, I think it's a pressure regulator.

I'll let you tell him he is wrong SuperHawk...




I'm going to purchase a pair of these:

https://www.summitracing.com/parts/sum-8001...BSABEgIJ8_D_BwE

And test it myself.


Even Eric calls it "It is the first anti-lock brake device ever to be installed on a Porsche" which is being nice IMO.

I call it "never lock".

Anti-lock to me means something is modulating the individual brake line pressures due to a difference in the decelerations of the wheels.

It is true that factory manual refers to it as a pressure regulator (German nomenclature) by name.

We can argue about the semantics and translation of pressure regulator vs proportioning valve.

However, the factory manual is clear both in written description that it functioning as a proportioning valve (English nomenclature) as well as the published chart which in previously posted which clearly shows a proportioning behavior with a changeover or “knee” point. That knee point differs between the 914/6 and the 914/4. The slope of the proportioned line pressure is determined by the difference in annular surface area on the piston that separates chambers (6) and chamber (7).

I’ll respectfully disagree with what PMB has chosen to publish and will defer to the factory manual which was reviewed by the engineers that designed the component.

Please take the time to read the factory manual rather than referring to second hand internet information (including my own).


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Click to view attachment

I’ll post one last bit of information to further support that there are various nomenclatures used to describe regulators vs proportioning valves.

The graphic is from the Bosch Automotive Handbook.

Braking force limiter (a) - this is what we would call a regulator (English nomenclature) - it allows output pressure up to a set limit and then flatlines, holding (limiting) that fixed output pressure despite increasing input pressure.

Braking force reducer (b) - this is what we would call a proportioning valve (English nomenclature) - it allows output pressure to build up to a particular point (knee point) and then the output pressure growth is reduced, but continues to grow, though at a lesser proportion than the input pressure. This is what the 914 rear valve is doing.

Red lines added by me to call attention to the shape of the line pressure in otherwise very busy charts.

Click to view attachment

@Superhawk996

Couple of questions:

Have you ever tested this yourself?

How many times has "factory service information" lead you down the primrose path?

If the answer to the second is never, you're a virgin!


I'm not 100% satisfied with anything I've read but I do know that I can easily test this for myself.

It will require a fresh valve from PMB, an old valve, and putting the 75 brakes back together.

I did read everything you posted!

I'm still not convinced of actual in-service operation which I'm going to test.

I do now understand better why so many folks have trouble getting brakes bled though the valve now.

If your valves older than 20 years, then it should be rebuilt.
If you have trash in the hydraulics this critter is a trash catcher!
And, I don't believe you can remove all of the air pockets from this valve without operation past the activation pressure.

New add for me regarding bleeding brakes, "At some point in the bleeding operation the pedal SHOULD be hammered hard at least 3 times..."

NON (OLD) PORSCHE CONTENT: Ninja tip for getting air out of an ABS unit...

You've screwed up and allowed an air bubble to enter the ABS solenoid unit.
The "factory manual" advised you to use the factory scan tool to clear it.
As you don't work at the dealership that scan tool is not available.
Your aftermarket scan tool doesn't have the ability or it has it, just not in the right sequence.


Ninja way to get past all this shit...
Operate the ABS unit.
Simple as that.
I've never had it fail and it doesn't matter where it's built.

Where to operate- you need a slick surface which can be a PIA to find here in Texas during the summer when it hasn't rained in two months.

What works fine is a parking lot that has the tiny brown/red pebbles in it.

Do this when the parking lot is EMPTY and the owner is not around!

You don't have to go fast. 10-15 mph works fine.

Want to be a nice guy?
Take a broom to get rid of the ruts you will leave.

I've got a Mexican restaurant near the shop that has the perfect parking lot out back.

https://www.donmelquias.com/

Pretty decent food there as well!

Post the data when you have it.