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singledownloop
singledownloop Reader
10/11/17 6:45 p.m.

 

Shopping for rotors for my 4 wheel disc brake setup on my pv444 volvo.What are the advantages and disadvantages of both vented and solid rotors?My car weighs 2000 pounds and 60/205/15's are the largest tires that fit.I'm running gm metric calipers.They as most know fit vented discs.Spacers that go in the caliper piston to allow them to fit solid rotors are cheap,thanks.

Keith Tanner
Keith Tanner GRM+ Memberand MegaDork
10/11/17 6:50 p.m.

Vented rotors can handle heat much better than solid - they have twice as much surface area to shed it and built-in air channels inside.

DrBoost
DrBoost MegaDork
10/11/17 7:42 p.m.

Yeah, shedding heat is the reason for vented rotors. If I could do either, I'd always to vented. 

Fr3AkAzOiD
Fr3AkAzOiD Reader
10/12/17 4:55 a.m.

Does one or the other tend to go through pads faster?

Any increased risk of disk cracking at the track with slotted or is that just cheap drilled that do that?

DrBoost
DrBoost MegaDork
10/12/17 7:09 a.m.

I think only cheap rotors would crack under any conditions you could live through. 

alfadriver
alfadriver GRM+ Memberand MegaDork
10/12/17 7:19 a.m.

There is a trade off- weight.  Counting both as unsprung weight and rotational mass.  

FWIW, I've driven my Alfa on tracks for hours and hours, autocrossed it hundreds of times, and never saw the actual need for vented rotors.  Any my Alfa is probably ~2200lb.  

Maybe if I were to endurance race it, but I wasn't even planning on using them for the vintage racer I had.

mad_machine
mad_machine GRM+ Memberand MegaDork
10/12/17 9:53 a.m.

that's just it, Alfa. Unless you are doing serious track time, a lightweight car does not need huge brakes. The Fiat x1/9, one of the best handling cars ever, only came with 9 inch diameter solid rotors.

GameboyRMH
GameboyRMH GRM+ Memberand MegaDork
10/12/17 10:05 a.m.

Vented rotors can take a lot more heat, solid rotors heat up faster (due to lower mass) and stay hot much longer, leading to faster pad wear, all else being equal. The only disadvantage of vented rotors is weight (and likewise, the only advantage of solid rotors is weight). I'd only consider running solid rotors on the rear of a front-heavy lightweight car. Otherwise, all vented all the time.

There's no meaningful increase in the risk of disc cracking with slotted rotors. Drilled, yes, especially the cheap ones. Slotted rotors will eat pads faster though.

Edit: Oh, another role I might consider solid discs for is in a dedicated autocross car. They hardly need to handle any brake heat, so the reduction in unsprung weight is worth it.

iceracer
iceracer UltimaDork
10/12/17 10:21 a.m.

Depends on your calipers.   I see you have already addressed that.

Thing is, the front brakes do most of the work, hence more heat.    That is why, vented in front,solid in the rear.

RossD
RossD MegaDork
10/12/17 10:38 a.m.

I remember seeing a TV show doing a big brake and wheel swap on a GTI (IIRC) and they tossed it on a dyno before and after and there was noticeable difference in the torque/power.

rslifkin
rslifkin SuperDork
10/12/17 10:45 a.m.
RossD said:

I remember seeing a TV show doing a big brake and wheel swap on a GTI (IIRC) and they tossed it on a dyno before and after and there was noticeable difference in the torque/power.

Sounds like a dynojet to me.  More rotating mass will cause slower acceleration, but it does not reduce hp or torque.  A dynojet will claim it does, however, as it can't control to a constant spinup rate and the mass to be spun up is now larger, so if that's not properly accounted for, it'll show less power.  

dculberson
dculberson PowerDork
10/12/17 10:48 a.m.

More rotating mass does use energy, though. I saw a pretty convincing demonstration using two bowling balls where one is run down a ramp bare (so it spins) and the other is on a set of rollerblade wheels so it doesn't spin. The roller blade wheeled one hit the bottom of the ramp way sooner and faster than the other. So if the wheels are using energy to be kept spun up, there will be less energy available to move the car. So while the crank HP might be the same, the WHP will be lower.

Keith Tanner
Keith Tanner GRM+ Memberand MegaDork
10/12/17 10:57 a.m.

We did a test a few years ago that showed inertial losses quite clearly - ran the same car on a dyno with adjustable loading so we could adjust the length of time it took for a run. Made a massive difference in available horsepower. I can dig it out if anyone's interested.

I've got one vehicle with solid front rotors, and I want to change them out for vented. Keep in mind that while doubling the weight of a vehicle only doubles the amount of heat going into the car, doubling the speed gets you four times the heat. Vehicle mass is far less important than vehicle speed when it comes to braking. That's why autocrossers don't need much in the way of brakes.

alfadriver
alfadriver GRM+ Memberand MegaDork
10/12/17 11:31 a.m.
GameboyRMH said:

Vented rotors can take a lot more heat, solid rotors heat up faster (due to lower mass) and stay hot much longer, leading to faster pad wear, all else being equal. The only disadvantage of vented rotors is weight (and likewise, the only advantage of solid rotors is weight). I'd only consider running solid rotors on the rear of a front-heavy lightweight car. Otherwise, all vented all the time.

There's no meaningful increase in the risk of disc cracking with slotted rotors. Drilled, yes, especially the cheap ones. Slotted rotors will eat pads faster though.

Edit: Oh, another role I might consider solid discs for is in a dedicated autocross car. They hardly need to handle any brake heat, so the reduction in unsprung weight is worth it.

I don't mean to be accusational or flippant, but have you ever actually driven a car on a track with just solid rotors?

Some of us have, hours of it.  And I have friends who race their Alfas with solid disks.  Have had no real issues.

Vented rotors are better, only if it's actually needed.  They are not better for the sake of being better.  If you don't have a heat problem, you will never notice that you even had vented rotors.  Well, other the weight and the cost of conversion.

It may be really easy to put a vented set up on a volvo, but it's a royal PITA to do it on an Alfa, so few do it.  Interestingly enough, Alfas still stop fine after over 50 years of racing on them.

Keith Tanner
Keith Tanner GRM+ Memberand MegaDork
10/12/17 11:52 a.m.

Vented rotors are the product of Big Brake trying to drive up the cost of your consumables. Stick it to the man, use solid rotors like your grandpa did! They were good enough for him. wink

Fr3AkAzOiD
Fr3AkAzOiD Reader
10/12/17 12:01 p.m.

May give vented a try on the front of my Malibu if its not a big hassle.

With about 3300 lbs braking on the main strait at VIR I notice I get two laps where I can brake just after the 3 mark but after that I need to back it off 50-75 feet.

Driven5
Driven5 SuperDork
10/12/17 12:03 p.m.

In reply to alfadriver :

So we're all in agreement...Slow cars don't need vented discs.

alfadriver
alfadriver GRM+ Memberand MegaDork
10/12/17 12:04 p.m.
Driven5 said:

In reply to alfadriver :

So we're all in agreement...Slow cars don't need vented discs.

LOL.  It is a fact.

Duke
Duke MegaDork
10/12/17 12:06 p.m.

I guess it never occurred to me that, for a given swept area, a vented rotor would be heavier than a solid one.  That seems kind of like a solid beam being heavier than a truss.

Driven5
Driven5 SuperDork
10/12/17 12:09 p.m.

In reply to Duke :

Except that your truss is made from two half-thickness solid beams, plus the linking members.

GameboyRMH
GameboyRMH GRM+ Memberand MegaDork
10/12/17 12:19 p.m.
alfadriver said:
GameboyRMH said:

Vented rotors can take a lot more heat, solid rotors heat up faster (due to lower mass) and stay hot much longer, leading to faster pad wear, all else being equal. The only disadvantage of vented rotors is weight (and likewise, the only advantage of solid rotors is weight). I'd only consider running solid rotors on the rear of a front-heavy lightweight car. Otherwise, all vented all the time.

There's no meaningful increase in the risk of disc cracking with slotted rotors. Drilled, yes, especially the cheap ones. Slotted rotors will eat pads faster though.

Edit: Oh, another role I might consider solid discs for is in a dedicated autocross car. They hardly need to handle any brake heat, so the reduction in unsprung weight is worth it.

I don't mean to be accusational or flippant, but have you ever actually driven a car on a track with just solid rotors?

Some of us have, hours of it.  And I have friends who race their Alfas with solid disks.  Have had no real issues.

Vented rotors are better, only if it's actually needed.  They are not better for the sake of being better.  If you don't have a heat problem, you will never notice that you even had vented rotors.  Well, other the weight and the cost of conversion.

I have driven a car on track with solid rotors on the front and drums in the rear, until relatively recently that's the setup my Corolla had. Brake fade was definitely a problem, even moreso on the street - 2~3 hard stops from around 140kph would leave the brakes fairly useless.

I agree that vented rotors are better only if needed, but if they're not needed, it's probably on a very light and low-powered car, which is an unusual type of vehicle these days.

alfadriver
alfadriver GRM+ Memberand MegaDork
10/12/17 12:24 p.m.

In reply to GameboyRMH :

The original car in this thread is smaller and lighter than the car I referenced that does not have a fade problem with solid rotors.  So they are not that unusual.  

GameboyRMH
GameboyRMH GRM+ Memberand MegaDork
10/12/17 12:49 p.m.

Fair point, solid discs might be enough for this car. Vented discs on the rear would surely be overkill.

jimbbski
jimbbski Dork
10/12/17 12:56 p.m.

My first car was a V6 Capri. I did open track it a few years later. It came with solid rotors. This car needed vented rotors, heck even the 2.0L version needed vented rotors! Now I later road raced a Ford Fiesta and that car came with solid rotors as well. On this light car (1750 lb) and racing pads it stopped fine with never an issue.

Early Rabbits had solid rotors and they do fine in IT racing. My Scriocco which weights more needs vented which VW give it.

I think the weight/speed question has to be looked at before you decide to do a swap.

wspohn
wspohn Dork
10/12/17 3:26 p.m.

Listen to th guys that tell you that you don't need vented for a 2,000 lb car with decent diameter solid rotors.

I ran a race car with solid disc that weighs a similar amount and could stop repeatedly from 120 -130 mph with zero fade on difficult tracks with significant downhill sections.  I used Ferodo DS11 in the old days and Portefeild R4  more recently.

Only if you have some brake anomaly  - some Mini Coopers with discs about 2" wide, Jaguars (or anything else) with inboard rears that can't cool, - do  you usually need  ventilated.rotors

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