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MG MGB Technical - Disk Warp Theory.

Hi all.

I have a theory about the cause of disk warp, and would appreciate any thoughts.

If you have been braking hard the disks will be quite hot. If you stop at traffic lights or for some other reason the disks are able to lose heat through convection and radiation over MOST of their surface, but a significant part of the disk is insulated by the brake pads, and will cool more slowly (those parts will cool to some extent by conduction to cooler parts of the disk of course).

We all know that metals contract / expand with heat, could the difference in cooling cause warping ?.

I noticed that my disks appeared to have slightly warped after a 'hard' stop as described above, was this just coincidence ?.

It is now my practice to move the car forward a foot or so during such stops, to reduce this effect.

Comments please.. Don
Don

I like your theory Don. I wonder how much effect that has. I know some cars have multi-pot calipers and cover a lot of the disk which might even cause more problems. People don't often think about what a little heat will do. You can permanently bend/form steel plate by heating it just on one side...
CDD Dewey

I think the biggest problem is when the brakes are used very heavily after a period of none use. The sudden very heavy use creates severe heat on the surface while the inside, brake calibers and center are still cold. This has been known to break disks on some cars with 'light' discs. I am assuming it will also lead to distortion just before they break.

The prevention is to warm up the brakes. Not much help if a sudden emergency comes up.

werner
werner haussmann

Volkswagen had bad warping problems and advised their clients not to apply the brake at the traffic light after exiting the highway. So I think your theory is right!
Willem van der Veer

Don
If you can get OE discs they give less trouble with warping than repro ones, but I think your theory also makes sense.
Ron
R. Algie

I believe that warpage is caused by higher than normal temperatures caused by hard stops, decline stops, or any circumstance that requires greater than normal brake pressure. I believe that if you heated a rotor to the "hard stop" temperature, even off the car, it would warp. After working as a service tech in a city that has many roads over 20 degrees incline, I've repaired brakes on most makes of cars, for most types of drivers. The worst cases are those who admittedly ride the brakes - and create more heat.

I could see where holding the brakes after a hard stop could cause the pads to leech heat from a rotor, but only if there is a greater than normal amount of heat present. That would probably vary with pad compounds. Or is cooling faster in the exposed area of the rotor? All of this would vary car to car, taking in to consideration the condition of the pads, rotors, calipers, bearings, backing plates if present, etc...

Now if airflow cools the rotor and warps it, how do drums warp?
Jeff.

Jeff Schlemmer

Check out the Stoptech Website - lots of interesting information there.
Phil

http://www.stoptech.com/tech_info/wp_warped_brakedisk.shtml
Phil O

Nice link, Phil. Thanks!
Willem van der Veer

Hi all.

Thanks for the comments.

Phil's link is well wort a look, though I couldn't find the 'Figs'.

Don
Don

Don, your theory also sounds ok to me. Darn, when I looked at the thread title I thought we were going to have a discussion on theoretical physics! ;-)
Bill Young

Actually the theory is correct. Don't pads weld themselves to the discs if you stand on them too long in the pits while racing? Used to happen quite a bit in F1.

Luigi
Luigi

Hi Bill.

Warp speed in an MGB is an interesting idea, but not really practical as you would need at least a 61 digit mileometer, which wouldn't fit in the speedo head.

Don
Don

Interesting article - red hot discs twice last winter (don't ask why)- clouds of smoke - no warping.
Roger
RMW

For some reason which I can't figure out some discs seem more prone to warpage than others. Here in the US the Ford Tarus cars were notorious for warping discs while other Ford vehicles didn't seem to suffer from this problem. The design was similar for most, an internally vented rotor attached to the hub. Perhaps some random combination of material, thickness, and mass.
Bill Young

Ford research on brake judder warranty which went through all possible reasons that there might be variability in the shape of the disc after running.

Disc profiles of vehicles under warranty that had problems were measured. They had 'flats' on them coincident with the location of the wheel studs.

Using a turned down wheel disc runout was measured with wheel studs torqued to extremities of tolerance. When you have two adjacent studs at bottom limit and three at top or vice versa there is varing conformity to the wheel mount face and run out changes. It doesn't by much, but enough that if you do a lot of motorway miles the constant brushing of the disc on the pitched out parts of the disc gets worn. Under braking torque the spot on the discs that is low cannot generate the uniform braking torque to remain shudder free, and if there is high ground wheel offset there with be steering wheel pitching.

If the rotational frequency of the reduced braking corresponds with the natural frequency of the bushings in the suspension it will feel worse at that speed and even worse still if it is the same frequency as the steering column.

What makes it even worse is when people replace the steering wheel with a light one. BMW steering wheels have steel rims rather than magnesium which is commonly used - the reason being that it proveds inertia that damps out pitch of imbalanced wheels and shuddery brakes.


Paul
Paul

Super site, Phil - Thanks for the link - clicking on the words "Figure 1", etc. pulled up the pictures nicely, but the new windows are not self closing. They included an important caveat, and that was concerning proper tightening of wheel bolts/lugs as a proper first step to avoiding warpage.

Several years ago I had one of those Ford Taurus station wagons as a company car, hauling myself and about 800 pounds of parts and test equipment regularly, and I did experience severe brake rotor warpage, usually within 10,000 miles. Although I usually put on new ones (as that was almost as cheap, and the company condoned it) I once had them turned when new replacements weren't available, and the warpage looked to be about .010". Finally, after the Goodyear store that I frequented broke a wheel lug, I watched the young man replacing the wheels using an impact tool. Thank Goodness I never had to replace a wheel on the road! I don't think my factory lug wrench would have been up to the task!

Anyway, I demanded hand tightening only from that day on, and the problem was virtually eliminated. This may not help for everyone, but I might even demand use of a torque wrench today. Some car companies do. Cheers - - Alec
Alec Darnall

Alec:
I too have seen some shop apes use an impact wrench in an attempt to pull the wheel studs out of the wheel hubs. I now carry a cheap torque wrench in the car and insist that it be used.
Harbor Freight (www.harborfreight.com) have them on sale very reasonable on a regular basis. I think I paid $10.00 for mine.
Phil
Phil O

This thread was discussed between 14/09/2005 and 16/09/2005

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