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/sh...?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???