VW- Acceleration and traction VS stopping power

My Challenge car is a 1st Gen GTI with a 16V 2.0.

Last year, it had excellent power, but lacked suspension and traction.

I have stiffened the chassis and swapped to better springs, coilovers, and struts.

My question is one of tires and brakes.

Time and budget dictate that I have only 2 choices:

1)- I can put back the tires, wheels, and brakes it wore last year. 15"x 7" Rotas with Azenis RT615's (worn, but usable). This would allow me to keep the 10" vented disks it wore last year, but offer no traction improvement.

2)- I've got a nice set of Diamond Aeros with Hoosier A3SO4's (worn, but usable). They are 13"x10", which gives a lot more contact patch, and would also improve the final drive ratio significantly (they are 3" shorter in height), as well as lowering the CofG without screwing up the geometry. They will also widen the track. The problem is they won't fit over the 10" brakes. I have a set of 9" brakes that would fit.

Acceleration vs braking... What would GRM do?

Wouldn't you have been tire-limited on braking with the 15x7s anyhow?

Sounds like you might lose a little braking (though I wouldn't be surprised if you actually didn't; as long as the 9" disks are capable of locking up the 15x7s, you weren't actually gaining power with the 10s), but you'll gain on acceleration and cornering.

Secretly, I'd bet $5 you'll gain braking as well. If the 13s are 10% shorter than the 15s, you wind up with the same lever arm (rotor diameter:tire diameter), and heat shouldn't be an issue for an autocross...

Buy a set of V710s in 225/50 15". OR, if that's not in the cards, use the wider 13's. The 1" reduction in rotor diameter isn't going to make a difference in braking on a light weight car like yours. They WILL help cornering.

In reply to ransom:

Don't really know if the 9" can lock up the 15x7s. It has always worn 10" till now.

In reply to hrdlydangerous:

V710s are not in my budget.

Why are you worried about the brakes anyway? You've got a momentum car. Using them is just slowing you down.

You have a bit of a point.. I never use them!

Just figured better drivers would like to dive deeper.

In reply to SVreX:

Are you running the 3rd gen (a3) steering knuckles with the removable caliper carrier or the 1-2 gen (A1-A2) with the forged-in carrier? I assume the 3rd gen version since you cant fit 10" rotors on the 1-2 gen.

Remember you need to shorten the caliper carrier to go back to 9" rotors, or switch back to 1-2 gen steering knuckles and calipers. Assuming you changed the master cylinder out when you went with the 3rd gen calipers to maintain balance, you should switch back to a 1-2 gen master cylinder as well.

When I went to the 3rd gen from the 2nd, it was due to sourcing issues with 2nd gen steering knuckles. I run Hawk HT10 pads in both configurations and havent really noticed a lot of difference in braking. The larger rotors probably take longer to overheat but with the kind of racing I do that isnt an issue.

Option 2 is the best. With decent pads, 9" brakes should be more than enough

What you really need to be asking for is throw away kumhos/hoosiers for your 15" wheels.

Yeah, but even that wouldn't change the width and contact patch area.

But, OK, anyone have any 15" throw aways?

Unless he's going to run a 275 on the 15s, the 13s will be faster. It should be easy to get some pads that will overpower the AS04s

Plus, the brakes are just more weight. There is a reason nationally prepped CSP Miatas run the brakes from the 1.6L cars (or custom lightweight tiny brakes)

OK, I'm convinced.

It just seems weird to shrink the brake size. Kind of counter-intuitive.

But faster is better, right?

I have an '83 GTI that I've run with both tire types - 205/50/15 Azenis 615s on 15x7s and V700s(years ago) on 13x9s. At Grissom and comparable courses comparing my times to other good, consistent drivers, the 13s were 2 - 2.5 seconds faster on a 60 second course. Also, R compounds will benefit from Formula V while Azenis will effectively not. A3S04s would be ancient, though, and even the miracle juice that is Formula V might not revive them very well. The stock vented 9.4" rotors have had no problem dealing with either configuration. In fact, years ago when A1 VWs were competitive in FSP, the fastest examples used 9.4" solid rotors because they were lighter. I don't remember hearing any of those guys complaining of brake fade during an autocross.

Everybody I know locally that seriously autocrosses A1 Chassis VW's use 13" wheels. From 110hp JH's to the 250+hp Turbo cars. All of them run small brakes with stock 81-84 calipers. Most of them even run solid rotors since they weigh a bit less than the vented.

Every one of the can lock up 10" wide kumho slicks without issue. These are dedicated race cars, stripped to 1500lbs. Most cars run hillclimbs as well with no fade issues.

As competitive as these guys are, if there was a benefit to larger brakes they would use them.

Honestly, ive done brake upgrades on at least half a dozen of my cars over the years, and it's always really been for 'feel '. Deceleration vs effort applied. It makes you FEEL better to stop harder with less effort.

But i dont think ive ever really improved max effort braking with larger brakes (thats what tires are for..). When it comes down to it, even the tiniest stock brakes can usually lock up most tires you can bolt to a car. You just have to cram on them a lot harder, and if you're doing it from speed you will have major heat issues on some brakes.

On an autoX course, i dont think it matters. As long as you are well braced by your seat, you should be able to cram on the 9" brakes hard enough to make them do the same as the 10" brakes. They just wont 'feel' as good doing it.

i'd go along with the more grip less stoppers...

the bigger stoppers are all about being able to remove heat from themselves... unless you are somehow overheating them in the auto-x there is no need for larger.

Work is almost finished. Now that I've got them on the car, it's pretty obvious. There is going t be a pretty significant gain in the speed department.

I can always cut out the floorboards and do my stopping Flintstone style!

BTW, even if I could afford the Kumhos, I would also loose in the weight department. The Kumhos weigh 24# each compared to the Hoosiers @ 16#.

8 lbs per wheel = 32 lbs rotational weight. That's a LOT in a momentum car.

SVreX wrote: OK, I'm convinced. It just seems weird to shrink the brake size. Kind of counter-intuitive. But faster is better, right?

Unless you're cooking them in the autocross, why not?

Arrgh... The time and money WASTED on upgrading my 86 GTi to A3 knuckles/calipers/brakes... When I should have just spent the money on a good set of brake pads. And to find out going from 14" wheels to 15" wheels is bad too? I guess that is how we learn. Auto-x is unique, I am not out there flogging the car for half an hour at a time...

BTW, this makes me glad I did not toss the old knuckle assemblies into the scrap heap for quick cash... Sometimes procrastination does have it's virtues... Thank you all once again for pointing me in the right direction.

I do have one issue I didn't really think about...

I intend to keep driving it as a street driver. I guess it is pretty unlikely I'll be cooking the brakes in that scenario either. I had far more brake than necessary- I could always overpower the tires (of course, I had a habit of driving it on crappy tires).

I'm gonna try the smaller ones for a while. If I ever change my mind, I can switch them back.

Thanks for keeping me straight, guys!

I'll just add my support for more grip less brakes. You can get away with quite crappy brakes for autocross and drag, in fact you're more likely to have problems with them on the street if you drive fast.

noddaz wrote: And to find out going from 14" wheels to 15" wheels is bad too?

Different scenario. What 14" wheels with what 14" tires to what 15" wheels with what 15" tires?

GameboyRMH wrote: I'll just add my support for more grip less brakes. You can get away with quite crappy brakes for autocross and drag, in fact you're more likely to have problems with them on the street if you drive fast.

You must go to some pretty lame autocrosses.

ProDarwin wrote:

GameboyRMH wrote: I'll just add my support for more grip less brakes. You can get away with quite crappy brakes for autocross and drag, in fact you're more likely to have problems with them on the street if you drive fast.

You must go to some pretty lame autocrosses.

I've never seen a course IRL or from any videos that could be significantly helped by anything better than decent stock brakes. They might help you save time, sure, but you're not going to have heat problems in an autocross and if the decision's between brakes and handling then the handling will help you much more.

I've beat plenty of cars with modern vented discs and ABS with my old non-vented discs and drums - in fact I'm well known for my late braking and quick stops. The brakes get hot but not enough to fade.

You must go to some pretty lame autocrosses.

How so? The vast majority of cars at any autocross and at all autocrosses back to the beginning of friggin autocross have been using stock brakes. Even the poopiest of stock brakes on any car thats likely to get Autox'd will lock up tires, especially while not going perfectly straight.

In fact, id argue that not only will suspension work be better for your times vs brake upgrades, but suspension work could even be better for your braking than brake upgrades, because if you can already lock up tires with enough braking effort, you will gain more deceleration by limiting weight transfer than by lessening the pedal effort required to lock up a tire.