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MG MGB Technical - Cross Drilled Brake Rotors

Living, as we do, in the rain capitol of the nation, I need to do something to eliminate or minimize the problem of very little braking on the front wheels after driving any distance on rain covered highways in our 66 MGB. Victoria British and Brit Tek both have cross drilled rotors, while Moss carries rotors that are cross drilled and slotted. My question, are the cross drilled and slotted rotors sufficiently better when wet to justify the higher price and what other things can I do to help wet braking on our MGB. Thank you - Dave
David DuBois

Thats not rain David. It is liquid sunsine. Since slotted and cross drilled rotors are designed to prevent heat buildup, I don't think they would have any effect on wet weather braking.

Jim
Jim Lema

David-
There's a lot of myths about cross-drilled rotors. They were developed primarily for racing use. Some people think that they're intended to facilitate cooling, but rust quickly builds up in the holes and acts as insulation. Other people think that the holes are for water to be displaced into by the brake pads. #4 Son put a set of cross-drilled rotors on his 1974 MGB and found no improvement in the New York rain. The primary purpose of the holes is to reduce unsprung weight in the suspension system. Few of the over-the-counter aftermarket cross-drilled rotors have enough holes to make a worthwhile difference. The cross-drilled rotors used by racers have so many holes in them that few would buy them for street use because they'd be too expensive. Slotted rotors do a much better job of getting rid of water. The brake pads force the water into the grooves which act as ducts and centrifugal force slings the water away. I have Tarox slotted rotors on my car and can assure you that they're much better than a smooth-faced rotor when you hit the brakes after going through a deep puddle.
Steve S.

Jim - In that case, I had about an inch and a half of sunshine on my basement garage floor before I could get the drain cleared at the bottom of the driveway apron last night.

Steve - I was hoping that you would weigh in on this. Where do you find the Tarox slotted rotors that you are using? Do you have any experience with the slotted rotors that Moss is offering?

Thank you Jim and Steve for the input - Dave
David DuBois

David-
I got the Tarox Rotors from the UK. The MGOC used to sell them, but they've been replaced in their lineup by UK-made Red Dot rotors. I presume that the strong UK Pound is responsible for this as the Tarox rotors are Italian-made. I have no knowledge of the quality of the slotted rotors currently offered by Moss Motors. Perhaps Craig Cody can give some insight into just who makes them? (Hopefully not by Tienamin Square survivors undergoing "political rehabilitation".)
Steve S.

Steve - Thank you for the information, I'll check with the MGOC regarding the rotors. Regarding driving through deep puddles, driving from Bremerton to Seattle in the winter is like driving through a 70 mile long puddle and with the OEM rotors, it gets real exciting whenever brake lights come on in front of you. Cheers - Dave
David DuBois

Interesting thread.
One thing I have read is the wheel makes a significant difference to the amount of water ending up on the discs and therefore to the performance in the wet.

Another point as well is drilled rotors are sometimes not allowed by scrutineers at racing circuits due to the tendncy for them to crack.
This may not be of any significance here however.

Cheers to all, Pete.
Peter Thomas

David-
If it's that bad, you might want to consider a pair of slotted drums as well. What kind of brake pad material are you using?

Pete-
Yep, wire spoke wheels let lots of air through to cool the brakes, but that means lots of water gets through, too. Disc wheels are the best in the rain, but the rotors can get real hot in the dry! Engineering is always a matter of compromises.

Since drilling creates stress lines in the vicinity of the holes, you're right about the cracking problem. The more holes drilled into the rotor, the weaker it becomes. To make the reduction in unsprung weight worthwhile, well over a hundred holes have to be drilled ($$), which results in lots of stress lines. All of these holes should be champhered slightly aftersurface grinding, then the surface reground (more $$), then the rotor has to be stress-relieved in a furnance (more $$). That's why racing rotors are so expensive. The majority of aftermarket rotors that have far fewer holes drilled in them remain sufficiently strong after drilling so that the stress-relieving process isn't necessary and can be eliminated (thus saving $$$), and are surface-ground to reduce the need for champhering (thus saving more $), but the reduction of unsprung weight is insignificant. The brutal truth is that the fewer holes in such rotors are in effect nothing but a cosmetic sales gimmick aimed at the "monkey see, monkey do" market niche.
Steve S.

Per Brembo
The main advantages of drilled and slotted discs are the same: increased brake "bite", and a continuous refreshing of the brake pad surface. Drilled discs have the additional advantage of being lighter and running cooler.

For road use I would agree with Steve and opt for slotted.

Paul
Paul

Steve - At the present time I am using the standard OEM (?) pads. I figure on going to simi metalic, or even the kevlar pads with the next pad change. Dave
David DuBois

Dave-
Today's brake pads and shoes are available in a wide variety of materials. Materials intended for racing applications are unsuitable for street use as they perform well only when hot. At the temperatures incurred outside of a race track their performance is actually inferior to that of materials intended for street use. Rather than use racing brake material, install a set of MGB GT V8 brake pads in the calipers. They will fit without modification and, due to their larger surface area dissipating heat more easily, are more fade-resistant. Avoid the use of pads made of the Original Equipment organic compounds as they are the least heat resistant, have the poorest coefficient of friction of .32mu, and produce more brake dust. There are essentially three options for high performance brake friction material. The first and perhaps the most commonly available material marketed for a high performance street application are the Carbon Metallic compounds such as those marketed by Hawk. These seem to come in two categories: those suitable only for racing and those suitable for only street use. Those suitable for street use have a coefficient of friction of .36mu, which is too small an increase in performance over that of stock materials to make them worth the additional expense. The second choice is the Semi-Organic/Semi-Metallic type. Being more heat resistant than organic compounds, they do not compress up to a temperature of F 700, thus offering consistent performance while producing little wear on the rotors. They also have a superior coefficient of friction of .48mu, a fifty percent improvement over that of stock materials. These are available from Carbotech Engineering. They have a website at http://www.carbotecheng.com . While these may be popular, there is another material which has an equivalent coefficient of friction but yet an even greater resistance to heat: the Carbon Kevlar type (F 1,050). These are available from TSI Automotive (Pads- Part# CKPMGA/B, Shoes- Part# CKSMGA/B). They have a website at http://www.tsimportedautomotive.com . Be advised that whatever material that you choose for the front brakes should also be used on the rear brakes as well so that the coefficients of friction will be equal, otherwise one pair will prematurely lock up under heavy braking.
It is possible that under the heavy braking loads generated by stronger brakes the rear brakes may lock up prematurely, creating tail drift. This can be tuned out of the braking system by installing a proportioning valve or by changing the slave cylinders to ones with a smaller size piston. The latter change may require modifying the rear brake backplate in order to fit the different size slave cylinders. Another solution is the fitting of tires with more grip, although this can be said to be treating the symptom rather than the cause.
Steve S.

Steve - Thank you for all the information and the links. I think that I have enough information to make an informed choice as to which way to go now. Dave
David DuBois

Hi folks

MGOC are currently advertising Kevlar green-stuff road sport brake pads from a company called EBC (English Brake Company?) that seem, perhaps, to be the greatest thing since sliced bread!? They also do many versions of both slotted and drilled brake disks.

Does anyone have experience of these products?

Regards

Peter
Peter Hills

Peter,

Try this article for shoud be an unbiased view on brake pads.

http://www.minimania.com/MM/BRAKES___Pad_types___choices_936.htm

I use green stuff and IMO they live up to hype.

Interestingly the discs have wider slots and are dimpled for the reasons discussed above.

Paul
Paul

I am using EBC Roadsport green on my B GT V8 which has substantially enhanced performance, and weight (on rallies).

They appear to be excellent.

NJSS
Nigel Steward

This thread was discussed between 03/01/2003 and 06/01/2003

MG MGB Technical index

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