The Stovebolt Site Forums The Parking Lot Big Bolts 47' 6400 disk brakes

I need some opinions on converting my truck to disk brakes. I have already found a rotor that will work with a spacer on the hub, I had the spacer made, then i took the rotor and hub to a machine shop to see if they could re drill the holes in the hub to match the rotor, the hub has 6 holes and the rotor has 12, they said they wouldn't do it because they thought the hub or bolts could brake if i put too much of a load on them with the rotors that big (14 1/8"). the other choice of rotors is 11.1" and i think that's too small. What do you think?

here's some pictures of the parts
http://community.webshots.com/album/566266796krtyLu

Brakes are too important to do a poorly conceived conversion.

Looking at your pictures I'm wondering if you'll have enough clearance around the spindle and tie tod. I'd suggest you confirm that if you have not already done so.

A 6400 is a 16,000 GVWR truck, so you need serious brakes up front. I think I'd have considered welding up the holes on that hub and redrilling, the approach your local machine shop rejected. They probably know more about it than I do.

Have you considered machining new hubs?

I don't know if the spindles are the same or not, but you might consider using the entire spindle/hub assembly from a later model Kodiac truck. They also have a dual disk brake system on the rear that could be adaptedd to your existing axle.

the rotors are for 90's Chevy 5500/2 ton tilt cab trucks. The machine shop said they cant weld the holes because it is cast iron.
Their biggest issue was only having 6 bolts holding the rotor on even though that's all the hub originally had, that with the disks they might shear off because the rotors are designed for 12. they're second fear was that the hub could break off where the rotor mounts.

could a machined reinforcing plate be machined and added to the back. may push the wheels out a tick, but would be a way to possibly make it stronger. Just a thought.

Really cool idea though. I'd be interested in options, as I would love to convert the 52 that I am picking up next weekend to discs.

I would listen to the guys at the machine shop, you don't want things to fall apart.

About the spindles from a Kodiak / TopKick they are much larger. I used the whole front axle from a TopKick on another truck (61 F350). I did not think any part of it would be suitable for an AD bigbolt, and for sure not the whole thing.

The best option I have found for disc brakes on an AD bigbolt is the P30 or 3500HD axle swap.

John and I are working on a pretty good tech tip covering these upgrades / swaps. I expect it will be done very soon.

Grigg

i do think listening to the guys at the machine shop is a good idea, but they are not an automotive shop they only do industrial type things. the only reason they would do it is because my dad knows the owner. i got confused when they said the increased stopping power was going to be the problem. i wouldn't think it would be different than a good set of drum brakes, is it?

The whole idea of the disc brakes with a booster is you have increased stopping power, is it not?
If disc are not going to be any better than good original drum brakes then stick with the drums and save the trouble and risk of a possibly cobbled together disc brake setup.

Grigg

If you are not going to gain better brakes why are you doing this? Do you know anyone with a degree in mechanical engineering? They could probaly explain how the stresses on that hub are changing with the disk versus the stock drum.

Can you modify the hubs from the donor truck or make adapter sleeves to mount on the 6400's spindles? That might be a better way to approach this problem.

I absolutely agree with Grigg that your stock drums are better than a cobbled together disk setup.

i was thinking about being able to lock the drum brakes while the truck is moving. wouldn't that be the same as locking the disk brakes?

just to show you what I'm trying to do hears a picture form oldGMCtrucks.com.

http://oldgmctrucks.infopop.cc/eve/forums/a/ga/ul/3141001801/inlineimg/Y/rotor.JPG

With a full load do you think you can lock the front drum brakes while driving down the road?
If so then no, the disc can't apply any more sopping power than that.
However, I think you will have a difficult time getting the front drum brakes to lock if you have lots of traction.

The disc will allow you to more easily and safely come to a stop with the same load, and will likely lock if you ask them to even with a full load.
I still think disc brakes have the potential for more stress on the system.

Grigg

i will usually just drive this truck for fun without a load, but i am also going to want to pull my dads 25 foot flatbed that he pulls with his 3/4 ton and a 18 foot car hauler, both with electric brakes it usually wont weigh more than 12,000LBS about the same as i haul with a '79 i ton GMC with no problems.

Just because you don't think it will be used hard is not an excuse to leave a weak link in the design of the brake system.
The only weak links in a good brake system are the tires on the road.

On a vehicles brake system you must not compromise on qulity, durability, or safety.

Grigg

i don't really think there is going to be a weak link in the hub and rotor assembly, I'm not sure about the caliper parts but the more i think about it the machine shop probably just didn't want to have anything to do it. the guy i had make the adapters said there shouldn't be any problems, he owns a drive shaft/fabrication shop and has built a few trucks. do you think the hub would break with the power of the disks?

BIG RED X!

The setup in the photo is an AD 1-ton with a Dodge rotor. If I remember correctly no new holes needed to be drilled.

Are you going to drive this truck faster than a stocker? Are you going to load it heavier than a stocker (ever, not just usual use)?

If you answer is no, you may be going to a lot of trouble for nothing. But I'll bet your answer is yes. So my next question is are these brakes you are swapping in place designed for something that will match or exceed both your weight and speed plans.

The guy who did the truck you pictured above (assuming I really do recognize that picture) spent a fair amount of time looking for rotors that would adapt and chose one that easily adapted. You may want to look at some more rotors hopefully finding one that will fit your hub's pattern.

Please work carefully to do this right. I don't want any new laws in Washington limiting how I modify my trucks!

yes it will go faster than stock but not carry more weight than stock. most of the reason for doing this is because the drums i had were falling apart and i didn't really want another used one.

these rotors are for 90's Chevy 2 ton tilt cab trucks so they should be adequate for my 6400.

i have spent lots of time looking for rotors, i found 7 that had a center hole close to the same size so i had some sleeves made for the center hole. the bolt pattern was the same on all 7 and it,s not that far off, i could just make the holes a Little bit bigger then they would match.the only difference was the diameter and the depth the different diameters are 11.1, 11.54, 12.26, 14.29. i have the 14.29" ones now.

the machine shop wasn't worried about drilling new holes, they were worried about the hub breaking because of more stopping power.

If the hubs breaking are the danger I'd check into a couple of things.

First talk to a mechanical engineer about strengthening the hub. Perhaps an engineer can suggest a way to increase the strength enough to make the machine shop happy.

Second have the hubs magnifluxed to make sure they are not cracked.

What was the GVRW of that 90's 2-ton. Was it 16,000 or more?

I'm not sure about the GVRW of the 2 ton, but i will check.

i didn't think about having the hubs magnifluxed, I'll get that done too.

I'm not sure where I'm going to find that mechanical engineer.

Well its a little west from Wenatchee, but there are a lot of them at Boeing... Might also check the engineering departments at our various Universities. Or you might look at the yellow pages. Or you might find a forum where they congregate...

Hello There,

I actually happen to be a mechanical engineer and I have been doing some quick back of the envelope calculations here.

Assumptions I made:
65mph ~150kmph at a maximum.
Total weight is ~26000lbs (12k trailer, 8k truck, 6k load on truck) ~13000kg
Total decelleration due to braking occurs in 1 second, which is a worse case scenario.

I figure that under these conditions the truck would be experiencing a force of 546,000N.

Now I'm guessing that the diameter of the bolts that connect the rotors to the hubs are 1/2 inch in diameter and that they are made of structural steel.

I determine that each bolt can handle ~30,000N.
Giving that you have 12 bolts that means that we have a total yield stregnth of 360,000N.

This is less than what I figure the truck is experiencing under the conditions above. So just by this it would seem that they will not hold up.

However, this is assuming that the rear brakes are not working and that you have no trailer brakes. Also, I don't see any truck decellerating this fast.

So there you go. Now its time to put your engineering cap on and make a decision. How much of the force will be absorbed by the rear drums and the trailer brakes? How fast will you be asking your truck to slow down?

Now just for fun if you had 24 bolts total you would have 720,000N capacity and an estimated 560,000N maximum load. Now, even under the worst case scenario, you actually have about a20-30% foctor of safety, i.e. you system is overbuilt by that much. Now go back and look at decelleration and additional braking and your actual FOS is probably 100%, which means that in the end there is a reason those rotors have 12 bolts.

Good Luck,
54 Monster

do you mean 12 bolts per side or 6 bolts per side?

i will be using the rear brakes too, and the trailer brakes.

i am pretty sure I'll never have 24,000GVW, or at least not with this setup.

would it be strong enough if i had 16,000 LBS max?

how about the hub breaking? it is about 7/16" thick where the rotor mounts.

here's some pictures
http://community.webshots.com/album/566266796krtyLu

Hey 54 Monster thanks for jumping in here. I always like having some real data to work with!

The one assumption you used that I'm not comfortable with is the 1 second time to stop. That is really worst case, most likely hitting an unmovable object kind of time.

How does time affect the calculation? What if it were 5 seconds or 10 seconds?

Are the hub and rotor both cast iron? Any way to weld them together after the redrilling of the studs? Understandably, they would be unservicable after being worn but that would take many miles,
Scott

I think welding a cast iron hub to a cast iron rotor would take more than an engineer, it would take a magician..a very good magician.

Stuart

Welding the two together would likely cause more problems than just leaving them alone.

Have you checked the fit of the hub and disc on the spindle and clearance with the steering arms and linkage? How about the caliper bracket, is there enough room to make and fit one?

Hi guys,

when I said 12 bolts above for your maximum yield stregnth I am saying that you right now have 6 bolts on each side, i.e. a total of twelve.

The last little paragraph is just to give you an idea of the maximum yield stregnth that you would have if you had 12 bolt hubs connecting to the 12 bolt rotor.

I would definitely not weld the two together. It can be done if you heat each part to about 400 degrees and weld in small sections of about .5in in length. However, your penetration will be minimal and you would be asking for a lot of problems.

I don't see the hub breaking. The thing that tends to break in any system is the weakest part and bolts here are being torqued so just from geometry I would say that this is the weakest link.

Now, to determine force is simple. F= mass * acceleration.

I always do things in SI units so the units of mass are kg and the units of velocity are meters/second.

To determine acceleration take the velocity and divide it by the amount of time (in seconds) that you expect to go from max speed to zero speed.

For example, above 150kmph =~42 meters/second. I assumed stoppong time of 1 second. That gives acceleration = 42 m/(s^2)

I took mass times accel/ and got the Force = 546,000N.

So if you stop in 5 sec it would be 42/5~ 8.3 m/(s^2), which gives F= 107,000 N.

So stopping times matters a lot, however, as an engineer trying to give advice about a mechanical system breaking I would go with the worst case scenario and evaluate that.

I know that adding trailer brakes and rear brakes will absorb some of the load, the question is however, to again evaluate the worst case. When you do you realize that the engineers in Detroit designed the modern rotrs to bolt to hubs with 12 bolts because of higher highway speeds and increased mass.

Hope that helps,
54Monster

i was checking the fit today and found out the rotor was too deep, so i got a shallower one but it wasn't as big in diameter it is 12.26" instead of 14.29", but has the same center hole diameter as the bigger one. i already figured out the caliper bracket.

so should i put 12 bolts on each side or will 6 be fine? should i be using grade 8 bolts?

The same torque that the disc will see, which is what you are figuring the number and strength of the bolts that hold them on for.
Is the same torque that the caliper brackets will see.
So, how many bolts of what size and at what radius from center hold the caliper brackets on?
Wouldn't be good if the disc stuck to the hub just fine only to find out the caliper brackets won't stay on the spindles.

Rethinking the whole project, the P30 disc brakes, same as a 3500HD, have 13" rotors, and already come engineered and assembled to work safely and reliably. At this point I think the front axle swap is still a better solution.

Grigg

I have to agree with Grigg on this one. His points have yet to be proven wrong and considering he has used it in a practical matter.. I.E. his own 6400.

Not to mention a complete front axle from a salvaged P30 will probably cost only as much as you are already putting into you "custom" system; you migth be better off doing the swap instead of the fab.

well i have been looking for a P30 axle but i haven't found any with the 5/10 lug pattern for sale.

I can see one from my front yard. But my neighbor might notice if his motorhome were suddenly sitting on blocks...

I can see one from my yard too, came from an ex Snap-On truck..
There out there, keep looking.
Try a wanted ad on http://www.4btswaps.com/ These folks often scrap step vans for the engine, the axles are generally unwanted and reasonably priced. Remember now, not all step vans have the desirable axle, so you'll still have to look for the right one.

Grigg

I have found plenty of 5/10 P30 stepvans but they are all still running or too far for me to consider driving to get them.

I am still considering the Freightliner or International option. I helped a friend of mine put his 83 Suburban on a Freightliner chassis. So the work wouldn't be much different other than the front axle narrowing.