Does a drum to disc conversion increase or decrease wheel offset?

[Josh69]

For Front and Rear? How much per side front and rear? What is the cause of this?

I searched, but couldn't really find a consensus.

[6'9"Witha69]

Front, not really. Rear yes. But that was for me using Wilwood stuff. With the dozens of ways to upgrade brakes it is impossible to make a blanket statement. That is why there is not and will not be consensus on the matter.

[TonyL]

your wheel offset? It wont. your wheel's offset remains the same weather they are on the car or off. (sorry. I couln't resist.)

actually it depends on the brakes. the thickness of the "hat" can range from 1/8 to 1/4 and that moves you out that far. I've had a friend who converted to willwoods have to change wheels due to rubbing.

[Josh69]

I didn't realize the thickness of the disc hat was that much thicker than the drum but I see it come up now and then.

Let's assume a GM type 1-piece rotor, like a C4 or C5 unit, or even the stock style front disc set-up.

I'm trying to determine if I need to do my brakes first, or if I can order the wheels I want and run spacers with my stock drums until my budget allows for the brakes.

[Josh69]

Front, not really. Rear yes. But that was for me using Wilwood stuff. With the dozens of ways to upgrade brakes it is impossible to make a blanket statement. That is why there is not and will not be consensus on the matter.

So what you are saying is that all rotors have a different hat thickness? I would guess aluminum hats are thicker, but wouldn't the average 11-13" stock style iron rotor be about the same material thickness?

I would assume the drum on my car has to be about as thick as a 'typical' rotor hat, is it not?

[Damn True]

I would get the brake stuff on there first to ensure your measurements are 100% accurate.

[Josh69]

It's primarily a street car including daily and spirited driving, sometimes in heavy traffic. I am undecided on leaving the rear drums, or converting the rear someday also as budget allows.

So I need some real answers. If I order my wheels based on a range of typical change and use a spacer in the rear to take up the difference until my decisions is made, that would be nice to know.

[beefy]

The "REAL" answer is to get your brakes on first... THEN Measure your needed offset (you need to know how WIDE you want the wheels first of course) and then order your wheels accordingly.

Getting the wheels first is nothing short of putting the cart before the horse in this case...


NOW, if you were planning to build the chassis around the wheels like you see on TV (not your average car builder) then get your wheels and THEN get your Plasma cutter, tubing bender and welder out, but based on your previous response, I have a feeling that's not the case.

[Josh69]

Any info besides opinions?

[Damn True]

The answer has been given to you. Three times in fact.

The answer is, "There is no concrete answer."

If you want to be certain of the dimensions, you'll have to do the brakes first.

[beefy]

Any info besides opinions?

Seriously dude... are you paying attention here?

You've gotten the answer....

It's NOT a matter of opinion... There is NO solid answer to your VERY vague question.


If you can give us some NUMBERS maybe we could help ya, but I have a feeling if you new how to get the "numbers" we need you wouldn't be asking in the first place.

based on the information that you Gave us YES... is the answer.... A disc brake conversion will INCREASE or DECREASE your wheel offset.

[6'9"Witha69]

You can lead a horse to water . . .

[Josh69]

I have little experience with drum to disc conversions, and the car is not currently in the same city as me, so I can't take measurements. That doesn't mean I can't, or won't before moving forward.

I was hoping that there was some 'uniform' thickness of a typical GM rotor, versus a typical stock F-body 9" GM drum, that alone should yield the difference, and if that doesn't solely account for the change in offset, I wanted to know what does. TonyL hit on the hat thickness, so that helped.

Is it also affected by the hub, or the spindle, is it just in the thickness change due to the rotor hat? That isn't vague to me.....

[Josh69]

Beefy, since you seem so 'knowledgeable', I share with you my idea, and you tell me if I'm completely off base.

1) If I knew that the hat thickness was the only dimension that would change on my rearend track width, I could measure the thickness of my drum at the mounting surface, and I could get a measurement of the hat thickness of the disc hat that I intend to use. If there is a significant difference, greater than +/- .125", than I would use a spacer and/or decide if it was feasible.

2) If I knew that a disc spindle vs a drum spindle was the cause, I would consider whether to machine my drum spindles, or use a kit that required disc spindles

3) If I knew the hub was a cause, I would also consider whether or not to go with a kit that I can machine down my hub's outer diameter and reuse, or a kit with a disc hub.

Make sense?

[6'9"Witha69]

To answer
#1: Yes, in the rear the thickness of the hat v. the drum will be the deciding factor.
#2: The spindles are the same other than the upper bracket boss. The Disc version is thicker than the drum. THis only affects caliper bracket mounting, not rotor.
#3: All the different kits use different types of hubs. This is where there can be no concensus. Some may increase tw while others decrease and some keep it relatively the same. As you say, < +-0.125" won't make much difference at all unless you are shoving 9.5"+ wide 17-18" rims up front and wanting it to work the best.

[beefy]

Beefy, since you seem so 'knowledgeable', I share with you my idea, and you tell me if I'm completely off base.

1) If I knew that the hat thickness was the only dimension that would change on my rearend track width, I could measure the thickness of my drum at the mounting surface, and I could get a measurement of the hat thickness of the disc hat that I intend to use. If there is a significant difference, greater than +/- .125", than I would use a spacer and/or decide if it was feasible.

2) If I knew that a disc spindle vs a drum spindle was the cause, I would consider whether to machine my drum spindles, or use a kit that required disc spindles

3) If I knew the hub was a cause, I would also consider whether or not to go with a kit that I can machine down my hub's outer diameter and reuse, or a kit with a disc hub.

Make sense?

1) yes... if you KNOW that the only change you are making is replacing a drum with a disc... then YES... the only change in the offset is the difference in thicknesses of those two components.

2) IF you must replace the spindle to swap from drums to discs (not always the case) then you would have to account SOMEHOW for the difference if there is one. Remember, it could be that the new spindle causes a negative (or inward) change in offset requiring you to actually use a SPACER rather than cuting down the hub.


3) IF you are changing the hub (which is NOT always required in a disc conversion) then you would have to account for the difference there as well.

The bottom line is this: First off, you didn't provide ANY of this information in your original post and frankly, even this information is STILL very vague.

We still have no idea what kind of car you are referring to OR what type of conversion you are attempting... So I'm not sure why you seem to be so confrontational about this.

It's clear that people here WANT to help you figure it out, but you're not helping us to help you.

[6'9"Witha69]

1) yes... if you KNOW that the only change you are making is replacing a drum with a disc... then YES... the only change in the offset is the difference in thicknesses of those two components.

2) IF you must replace the spindle to swap from drums to discs (not always the case) then you would have to account SOMEHOW for the difference if there is one. Remember, it could be that the new spindle causes a negative (or inward) change in offset requiring you to actually use a SPACER rather than cuting down the hub.


3) IF you are changing the hub (which is NOT always required in a disc conversion) then you would have to account for the difference there as well.

The bottom line is this: First off, you didn't provide ANY of this information in your original post and frankly, even this information is STILL very vague.

We still have no idea what kind of car you are referring to OR what type of conversion you are attempting... So I'm not sure why you seem to be so confrontational about this.

It's clear that people here WANT to help you figure it out, but you're not helping us to help you.I am working off the assumtion it is for his 69 Firebird.

[Josh69]

To answer
#1: Yes, in the rear the thickness of the hat v. the drum will be the deciding factor.
#2: The spindles are the same other than the upper bracket boss. The Disc version is thicker than the drum. THis only affects caliper bracket mounting, not rotor.
#3: All the different kits use different types of hubs. This is where there can be no concensus. Some may increase tw while others decrease and some keep it relatively the same. As you say, < +-0.125" won't make much difference at all unless you are shoving 9.5"+ wide 17-18" rims up front and wanting it to work the best.

Thank you, that is the type of info I was looking for. I knew about the upper boss being different, but wasn't sure if the bearing mounting flange was inboard/outboard relative to a drum spindle. I'm primarily looking at kits that re-use the drum hub (most C4/C5 conversion kits). If I understand correctly, if I re-use my drum hub and spindle, the only difference will be in the thickness of the hat vs drum on the front as well?

Sorry if my original question wasn't clear to all, I'll try to phrase them more specifically....I guess I figured the general info was applicable to most situations.

Beefy, I'm not trying to be confrontational, but your questioning my abilities didn't exactly come off as being very helpful. The car is listed in my sig....I assumed that people would make that deduction, it's a 1969 Pontiac Firebird.

[TonyL]

Yeah. Same for front. Depending on the kit.

[1FstChevy]

I'm in somewhat of the same pickle, since I want to do a front and rear C5 brake conversion but I'm doubting those brakes will fit inside my current 15" Torq Thrust D's... Now I plan on getting the 18" Torq Thrust Classic II which is the exact same model & size wheel that StreetKing has (https://www.pro-touring.com/forum/showthread.php?t=7958&page=4) +/- .25" in backspacing.

So.... Since I can't afford to do both the wheels and brakes at the same time, I guess I'm asking if it would be a decent plan to just figure out what width the C5 brakes & rotors would be, and then buy my American Racing wheels off of those measurements and then simply modify or spacer the rear drums and correct the front if necesary until the new disc brakes are in place???

[Josh69]

Based on my current info, the Touring Classics kit rotor is .300" thick. I'm not sure which one I need, so I would verify if you need the hybrid or C5 kit for your app and get that measurement directly from them.

Considering that it uses the stock drum brake spindle and drum hub, which is machined down to fit inside the rotor hat, the only affect to your track width will be the difference between that measurement, and the thickness of your drum.

I think that theoretically, if you take that difference into consideration, and make sure that the wheel spokes clear the caliper (using the template on their site), you should be fine.

We should be able to either make a spacer if the discs increase the track width to take up the difference until we get discs on, or if it's nominal, just live with it. If the discs actually decrease track width, then you will have to determine how much room you will have to move the wheel inboard. You can start with a slight bias towards the fender lip with adequate clearace and know that it will move inboard once the discs are on if that is the case.

I am planning on using 18x8" wheels up front, with a 255 width tire at the maximum, so I should have plenty of room.

[nowukno]

I'm in somewhat of the same pickle, since I want to do a front and rear C5 brake conversion but I'm doubting those brakes will fit inside my current 15" Torq Thrust D's... Now I plan on getting the 18" Torq Thrust Classic II which is the exact same model & size wheel that StreetKing has (https://www.pro-touring.com/forum/showthread.php?t=7958&page=4) +/- .25" in backspacing.

So.... Since I can't afford to do both the wheels and brakes at the same time, I guess I'm asking if it would be a decent plan to just figure out what width the C5 brakes & rotors would be, and then buy my American Racing wheels off of those measurements and then simply modify or spacer the rear drums and correct the front if necesary until the new disc brakes are in place???

Sent Ya' a PM

[1FstChevy]

Alright, now this might be going more off-topic but its somewhat inter related, Would it be a safe bet to say that those same 18x8s and 18x10s I'm looking at would clear the C5 Z51 rotor/caliper setup listed here
( http://www.nastyz28.com/forum/showthread.php?t=74546 )
After reading around a bit they say that they won't clear some 17" wheels so I'm assuming that an 18" would be more than enough clearance???
Thanks!

[Josh69]

That's a question that I have too, but can't answer. Maybe give Touring Classics a call, and let us know what they say.

I'm considering the same wheels as you in 18", so I'm very curious also. If they don't know, I'd call American Racing and see if the 17" and 18" wheel centers are the same or if they are different...I know some wheel mfgr's use the same centers, and just a larger rim hoop with a bigger step up in it.

[1FstChevy]

Who are Touring classics anyway??? I guess they can supply the rotors for the Z51 setup? Since I'm doing the modification with the Kore3 components, then a 3rd gen hub and the C5 Z06 calipers with the larger abutments to accomodate the slightly larger rotors (hence Z51).

Perhaps I'm a little confused since I'm working back and forth between 2 Camaro forums here, but have you looked into simply the C5 modification? or were you looking to do the Z51, since I'm still trying to locate someone who sells the rotors and abutments for all 4 wheels at a decent price. Also... if you check out the link ( http://www.nastyz28.com/forum/showthread.php?t=74546 ) it would be nice to find those black coated abutments shown in the photo, unless you think that guy may have justed painted the zinc coated metal himself??

--------

Okay well I just checked out Touring Classics so I know what your talking about but thats a little too much for my pocket book, besides their caliper design sucks compared to the look of the black or red claipers with Corvette stamped in them, but I guess thats of course if you want that on your Camaro.

My goal is to do all 4 wheels with the Z51 for about $1,000 and I'm pretty sure it can be done, mostly just looks like I'd have to shop around a little.

Although I'm still confused with the whole +/- offset thing, it deffinitly helps if your looking at the same wheels I am, but I'd think going from an intergal Hub/Rotor thats on there from the factory to a machined down Hub from a 3rd Gen plus a new rotor it would be thicker and therefore increase wheel offset! but then again I haven't taken any measurements so perhaps I'm speaking too soon.

Oh, and if your looking to get the same wheels, I have connections with this guy in Oklahoma who can get you the exact same set that I'm looking at and that StreetKing has for less than $1200.00 delivered to your door.

[1FstChevy]

Okay... well I spoke to the wheel guy and he said even with the switching brakes situation he said the difference would probably be so minimal that he would still recommend the 18x8 with 4.5BS and 18x10 with 5.75BS.

Now I haven't given American Racing a call yet to determine a good to go clearance but hopefully I can get around to that before they close up shop.

I'm hoping to figure this out soon since with the way I'm doing it I've got to have the wheels first before the big brakes, and obviously it would be a bad idea to order the fancy custom offset American Racing wheels without knowing first, but if they come in incriments of .25" BS I'd think it would at least be narrowed down between either 4.5/4.75 and 5.5/5.75 for whatever brake setup, of course you have a 1st Gen though so that would probably be a little different?

[Josh69]

I have a Firebird, so mine might be altogether different due to the 'wide trac' from the 60's Pontiacs, and my rear axle is a BOP unit rather than Chevy. So I'm taking any speak of offset on this forum with a grain of salt for my application until I measure it since most have Camaros.

I've decided to get the brakes on first and buy a wheel mate so I can nail everything once it's on the car. I am rolling the heck out of my quarters before paint for clearance. I am interested in those wheels, or the AR500's....still have to decide. I'll keep you in mind if I need to utilize your contact.

I will most likely take the caliper and powder coat it the color of my choice, and stencil PONTIAC on it, or I may end up with wilwoods....it's all up in the air for now. My car should be painted within the next couple of weeks, so once I start to assemble it, I'll make a decision from there.

[ponchopwr70]

I just ordered wilwood brakes for my lemans. I have the bop rear end. I think the offset is very small 1/8 or 1/16 not sure. On the fronts there is a change in offset I think a 1/4 according to willwood. But I ordered the 13" 6 piston front and 12" 4 piston rear. My front tires need to come out a little any way. I have 17x8 with 4.75 front and 17x9.5 with 5.5 rear. I'm not expecting any problems (knock on wood).

[1FstChevy]

Hmm... well I'd consider setting the car up for a few weeks so I could do the brakes first and then go off of those measurements for the wheels, except I don't have it in a garage at the moment so it would stay outside with no wheels.... ; ( and then once I order the wheels it would be about another 3 weeks since they are custom order... so maybe a month!? and thats if i could swap over the brakes without any complications in a week?

BTW ponchopwr70... Those sound like some serious brakes, I've heard rave reviews about the Z51 C5 setup with the 2 piston fronts and 1 in the rear so I can only hope that your 6/4 piston set up stops that mother on a dime.

I'm thinking the best alternative for me would just be to measure all the brake components that are going onto the car in advance so I'll know the difference that way my car won't be inoperable forever... and I would'nt mind a 1/10th here or there but if my front wheels came out 1/4in or something like that to the degree that they almost exceeded the back wheels then I might be concerned...

Also... I know many people have done the C5 conversion on here... I'm interested to know if anyone whos done the Z51 setup knows of a place to get quality drilled rotors @ a decent price, I think they are the 13.4in rotors???

[TheMonkey]

Okay... well I spoke to the wheel guy and he said even with the switching brakes situation he said the difference would probably be so minimal that he would still recommend the 18x8 with 4.5BS and 18x10 with 5.75BS.
....

i would be very cautious about just listening to what a wheel guy says.

consider installing the brake conversion first and measuring your scrub radius, and determine your offset that way.

simply referencing back to stock scrub radius may not be ideal, as scrub radius was not as crucial with skinny tall tires from the 60's. with wide radials on 18" rims, proper scrub is waaay more important.

here's a discussion on a method of measuring scrub radius and why it's important. consensus seemed to be 1/2" to 2" SR.

https://www.pro-touring.com/forum/showthread.php?t=29872&highlight=scrub

don't count on just getting lucky, and don't count on what other posters say will work.