Topic: rear brakes for dragbike, drum vs disc

[gschuld]

I'm curious about brakes.

My current understanding is that the main advantage of a rear disc over a rear drum brake is potential weight savings and resistance to heat soak induced brake fade.  But otherwise the braking power between the two is comparable.  Is this correct?

Drag racing wouldn't present an opportunity for heat soak I'd think, so brake fade shouldn't be an issue with drums in this application, correct?  And yes, I am talking about a fast clutch bike, not an auto, and with a generous disc brake front wheel.  So is there any functional disadvantage to using a rear drum, front disk brakes system other than the modest extra unsprung weight of the rear wheel?

George

[TurboD]

George Either setup is more than adequate to stop a dragbike. my bikes that still have a drum brake will lock the rear tire with less effort than my disc bikes. Heat buildup or heat soak is of zero concern with both setups drag racing.

A little something to think about.. A great many of the Grudge racers around here do not have rear brakes at all, none zero. We're talking speeds almost twice of what these SOHC Honda's run.

When given a choice, I like using a disc setup (GSXR rear wheel), because it is lighter than a stock CB750 setup.

[RP]

I think the disadvantage of the drum rear for drag racing, compared to a lighter weight disc set up, is a heavier wheel assembly hurts the bikes acceleration because it is taking more horsepower to turn the heavier wheel.

[gschuld]

Oh certainly.  I get that.  Anyone know the difference in weight between a factory rear disc brake wheel/brake system vs a factory rear drum wheel?  For those who are going for heads up drag racing, this is a solid issue.  But for something like bracket racing with 45yr old equipment, the modest extra weight really wouldn't mean much I'd think.  In regards to the vintage appeal, a drum rear wheel to me seems more the classic or vintage style of the two.
 
My interest is regarding an early-mid 70s period correct (appearing at least) style sohc4 dragbike/dragster.  Drum brakes were common on the earlier dragbikes/dragsters(running in the 9s), and plenty had them in the later 70s as well.

Going without a rear brake on a fast dragbike, no thank you.

So in the catagory of safety and stopping power(highest priority) for a (FAST) vintage dragbike/dragster, drum rear brakes are equal or better(?) than disc rear brakes(factory cb750 drum vs factory rear disc)?   

George

[TurboD]

I don't know exact numbers, but I will say the drum setup is lighter than the stock disc setup (75-76F's).

Haha Racers around here run speeds approaching 200 without rear brakes, its routine.

To answer your last question (if it was a question). As I said earlier, my drum brakes have more stopping power with less effort. But again both setups are many times more than up to stopping a SOHC.

[Bill/BentON Racing]

Dem rotors are heavy, especially stock Honda ones.

[TurboD]

Dem rotors are heavy, especially stock Honda ones.

Yes and so are the Rims, center hub, and brake calipers. Lol

[gschuld]

I don't know exact numbers, but I will say the drum setup is lighter than the stock disc setup (75-76F's).

As I said earlier, my drum brakes have more stopping power with less effort. But again both setups are many times more than up to stopping a SOHC.

I was kind of thinking that a factory drum setup might be lighter than a factory disc.  I only held and weighted a factory KZ disc wheel with SS spokes and did aluminum 3.5x18 rim.  It was not light...

A factory drum rear wheel, with SS spokes, and a 3.5x18 DID aluminum rim with cush drive, brake plate, and 54 tooth 630 solid alloy rear sprocket and vintage road race slick with tube weighs 43lbs all up.

George

[RP]

Period correct early 70's and mid 70's would be two different periods for rear brakes on dragbikes using 18 inch rears.

Around 1975 most serious races started using aftermarket lightweight hubs and rotors.....I don't remember anybody using new O.E.M. heavy discs and hubs back then.

The Goodyear 18 inch wrinkle wall from that period has not been produced for awhile now......and might not be the safest choice even if you find a good looking one,  and soft compound probably is not soft by now.

I think M&H still makes a 6x15 motorcycle slick which is what went on most Pro class dragbikes in the early and mid 70's.

[NalleyRacing]

Just some food for thought. I've run my bike up to 102mph and am able to safely make the first turn around at every track ive been too. I have NO rear brake of any kind, just a SINGLE drilled front rotor and aftermarket master cylinder. But it all is very heavy.

Ive been on the lookout for a good aftermarket wheel conversion.. buddy of mine did a single sided swingarm and dual disk front end off an early 2000's honda of somekind. Looked sharp and made the bike 3 inches longer and a few inches lower.

[gschuld]

Trey,

It's not uncommon apparently for grudge bikes today to run without rear brakes on 200hp bikes.  No thank you to that...

Disc brakes on two piece bolt together Kosman style wheels were becoming commonplace by 1973 on the highly competitive drag bikes, and the only real option for multiple engine bikes that needed off the shelf car slick tires for traction.

For example: RC Engineering's first, custom built for a customer(for Adam's Racing, Alberta Canada), double engine dragbike, in late 1972, ran a custom made 5" wide "Super Trick"  style bolt together two piece rim with an aftermarket lightweight rotor.  It was good for 9.32s at 150mph in 1973 with RC himself onboard(a national record at the time in Top Gas)

The popular mid 70s(early 1974 through late 1975) D&K/RC single engine dragster chassis came with a seat, fender, chain guard combination in welded aluminum.  The spacing on the fender and chain guard allowed for no more than a 3.5" to 4" rim width(4" slick max)  So for example, a 5.5 slick would not fit as designed.  With the replacement/modification of the seat/fender/chain guard, a 5.5" slick could fit easily on a 5.5" rim/wheel width, assuming you dealt with the necessary offset sprockets.

The RC catalog states that the basic rear wheel package is the factory rear drum wheel for that single engine dragster.  D&K offered a 3.5x16 Kosman style bolt together disc brake wheel as an option for that frame.  Without doubt, the top contenders were using the aftermarket rear disc wheels to save a few precious pounds.  But the aftermarket lightweight single puck disc brake systems were very unlikely to be any more powerful or safer than the big cb750 drum hub wheel.  By 1976, guys like Eraldo Ferrari and others were down in the mid 9s at 150mph in pro comp(dragster chassis, N/A big bore engines) with these lightweight, small caliper disc rear wheels.

I'm just wondering if ...say ...that last tenth of a second and 2mph in the 1/4 is not really that important, but safety IS, is running a factory drum rear wheel with a wider rim and good slick a reasonable thing to do?  And yes this is regarding a big bore dragster capable of fast times.

George

[seanbarney41]

is a powerful rear brake ever really useful or safe on any bike ever?  I have always found that locking the rear wheel can often be the quick, easy way to catch a nasty high side

[gschuld]

On a road racer, absolutely.  On a drag bike with a skinny front tire and a fat rear tire, rear brakes get used more than usual. 

That's a good point though.  How much do guys rely on the rear brake once your into the 10 second or faster 1/4 mile times category?

Also, how much does the width of the front tire,wheel affect this?  Super skinny front tires have little contact area for the brakes to act on, but a 90 series tire on a 1.85 front rim might be more useful.

George

[BPellerine]

some guys used to use ferodo?green linings on their shoes in drum brake roadracers but you had to warm them up or they were very grabby.billp

[RP]

George,

Most tracks will have plenty of shutdown area for a 10 second or quicker bike to slow down to make the turn off.     You should not even need to get on the brakes all that hard to worry about traction with a skinny front tire or stopping power of the rear brake.  It is pretty safe going in a straight line when you have the whole lane to yourself.

[gschuld]

Yes, I figured as much.  But safety is number one, and I wanted to be sure that drum rear brakes were not inferior in any measurable way that I wasn't taking into consideration.  The extra bit of weight isn't my main concern.

George