Wheel parameters 6

[KahnQuistador]

Can someone tell me how to convert wheel backspace in inches to wheel offset in millimeters?

The guys at the tire shop didn't know.

I need for a 7.5 inch wheel width with a backspace measurement of 5.062 inches, what's the proper offset in millimeters?

 

[GregLocock]

I haven't come across the term backspace before, but it sounds as though it is the depth of the mounting face relative to the inner rim. Which part of the rim do you use as a datum? If you measure it directly using the extreme edge of the wheel weight flange then you need to measure the flage width relative to the inner wall of the flange, which is where the 7 1/2 inches is measured.

The offset is the position of the mounting face relative to the nominal centreline of the wheel.

Cheers

Greg Locock

 

[KahnQuistador]

Thanks for the reply.

I agree with your supposition about backspacing so I did the math and came up with a backspace of 5.062" being equal to about 34mm offset.

The wheel I want to replace has "Alfa Romeo 5 1/2 J x 14" and "campanatura 38" on it. I took the latter to be the offset.

Does that mean I could replace the 5 1/2 x 14, 38 offset wheel with a 7 1/2 x 16, 34 offset wheel?

Seems every time a put that question out locally, is gets a "yes you can, no you can't" response.

 

[GregLocock]

campanatura seems to mean camber (based on a brief web search).

There are several problems with fitting wider wheels, offset is only one of them. The wheel or tyre may hit the eyebrow, or the body, or the suspension, at some combination of jounce and steer. So there is no hard and fast rule, sadly.




Cheers

Greg Locock

 

[patprimmer]

One rule I like to follow, especially on front wheels is to keep the track reasonably close to stock as this affects scrub radius.

To much scrub radius causes any pulling in the breakes to be greatly exadurated, and kick back from potholes to be extreme.

The 5.5 X 14s have about 4.1" backspace and the 7.5 X 16s have about 5.25" backspace.

When going from the 5.5 to the 7.5's, you will move the inside bead in about 1.15", and the outside bead out about .75", and the scrub radius out about .2"

Of course tyre size will have a major impact on what will fit

Regards
pat

 

[KahnQuistador]

The plot thickens.

I measured the distance from the outer edge of the inside of the wheel to where the wheel contacts the hub. It measures exactly 4.625". If the backspace is supposed to be 4.1", I must be measuring from the wrong place. True?

I also found campantura related to camber. If you go to

http://babelfish.altavista.com/babelfish/tr

you find it defined as “camber angle”. I found it related to camber at

http://www.surace.it/winner.html

which contains English and Italian text. Another site for Alfa Romeo wheels at

http://www.afra.it/afraccer.htm

uses campanatura implying offset. Reference the 5 spoke wheel for the Alfa Spider at 6 J x 14 campanatura 35.

I'm beginning to think that the meaning may be defined by the context in which it is used. A camber measurement of 38 degrees would be a bit extreme and I see no reason to put camber information on a wheel.

I do know that the 7.5 x 16 wheel with 5.062" offset fits the car with no problems using 205/50 VR 16 tires.

And I thought a simple transfer function would exist!

Thank you for the input, gentlemen.

Bryan Foster

 

[mrcch]

To give you an easy explianation of these terms check out the following link

http://www.budnik.com/techInfo.asp

You can see the backside is the distance from the rear flange (furtherest out point) to the mounting pad. Offset is calculated from the centerline of the wheel to the mounting pad of the wheel. It can be figured out as an estimate if you figure the rear flange is approx. .450" thick. Do the math

Wheel Width = 7.5"
Backsdide = 5.062"

7.5" / 2 = 3.75" (to find center of wheel)
3.75" + .45" = 4.2" (adding the flange thickness, this gives you the distance to the center of wheel from the rear flange)
5.062" - 4.2" = .862" (this is the offset in inches, now convert to millimeter)
.862" x 25.4 = 22mm final offset

 

[KahnQuistador]

The flange measurement is the missing link! That would make Pat's 4.1 inch data correct using my measurements.

I'll order the 7 1/2 inch wheels with an offset of 22 mm.

Now, if one of our Italian friends would give us the answer to the "campanatura" question...

Thanks to all,

Bryan

 

[Xwheelguy]

Mrrch has it right. Being a wheel designer, the way to calculate wheel offset is done as follows:

1. Measure the inner rim width with a set of inside vernier calipers. Make sure you are on the flattest surfaces & not hitting any radii. The inner rim width is the distance from the backside of each flange, or the distance between the bead seats. Take this number and divide it by 2.

2. Measure the inner flange width with vernier calipers, making sure not to contact any radii on the flanges. Subtract this result from the calculation done in step 1.

3. Measure the depth of the mounting surface from the inner flange (top of where the balance weights are clipped) using a depth caliper & 10mm flat bar. Subtract this measurement from the result in step 2 & now you should have your offset. If you are measuring in MM, round to the nearest whole MM, as typically there are no target offsets in tenths of a MM.

Now, to make things more confusing, there ARE negative offset rims and positive offset rims. Negative offset rims would be the type that stick way out when mounted on a vehicle.

Hope this helps someone out!


Tim Flater
Senior Designer
Enkei America, Inc.

http://www.enkei.com

 

[Joest]

Don't forget that a 7.5" wheel is really 8.5" wide. 7.5" refers to the distance between the sealing beads not the overall wheel width. Backspacing is measured by placing a straight edge across the back of the wheel and measuring to the hub mounting surface. For example:

17 X 9 wheel w/ 24mm offset
Backspacing is 5.94"

(9+1)/2 = 5
5+(24/25.4) = 5.94"

-Joest

 

[NormPeterson]

"Don't forget that a 7.5" wheel is really 8.5" wide."

I think that's what Tim's steps 2 and 3 cover. His procedure is a little more involved, but it does get you around the possible situation where the inner and outer flanges are unequal/unsymmetrical (like OE 17" Maxima wheels, for example).

I'd always figured that the flanges for alloy wheels were about 0.5" thick overall including any lip (and that "steelies" are slightly less), but that judgement is based on an extremely small sample size. My thanks to you and mrcch for providing me with some confirmation.

Norm

 

[Xwheelguy]

Joest,

You are totally estimating about overall rim widths, which WILL lead to incorrect calculations (albeit by a few millimeters, but at times every millmeter counts when you're talking clearance of the tires & surrounding parts).

Let's take your example, a 7.5 inch wide rim. For sake of argument, we'll assume the rim is US T&RA compliant (BTW, most rims are measured in metric, not english). A 7 inch wide rim is nominally 190.5±2.0 mm . The MINIMUM but compliant width of EITHER flange is 11.0 mm, but the flanges could be bigger as well....I've seen them as big as 17 mm (close to 11/16") which COULD aid in tire clearance.

You stated that a 7.5" rim is really 8.5", but that's not always the case nor is it a very good assumption since few flanges are 12.7 mm (most are bigger but some can be smaller as well). That will depend on the actual width of EACH flange. A 7.5" rim could really be well over 8.5" inches OR well under and still be compliant to T&RA. Guesstimating on flange width might get you into trouble if your tires are going to be close on the inside...if you've got the tools, why not measure the flange and be SURE? Those extra wide rims might just fit, afterall.

I hope you don't take offense to me pointing you out, I just wanted to explain it in detail so folks would know accurate information. I did not wish to single you out as being unqualified, so please understand where I'm coming from here.

Now, to clear up some 'common' terms. Offset is the distance from the centerline of the wheel's WIDTH to the wheel's mounting surface. Mount surface depth is the distance from the highest point on the inboard flange to the wheel's mounting surface.

I've never heard the term backspacing used by anyone in the wheel industry, but I HAVE heard it used by folks in the aftermarket industry who usually don't know what they're talking about....(they usually use it to mean OFFSET)....not their fault, but it's a good way to spread incorrect information & confuse consumers who research fitment before they buy aftermarket rims. I'm not saying the term backspacing is NEVER used in the world, just that in 15 years in solely the wheel industry, I've never heard it when dealing with GM, Honda, Toyota, Mitsubishi, Nissan or Mazda. Just wanted to share my knowledge in case anyone ever walks into a wheel manufacturer's office and tries to talk the talk ;o)

Would it surprise anyone to know that there are wheels out there that have molded lettering that states the wheel is something like 38.0 mm offset but is really 37 or 39? Well, there are. Best bet is to do the measuring & calculations as best you can & remove as much doubt as possible.

Hope this helps out some folks out there.

Tim Flater
Senior Designer
Enkei America, Inc.

http://www.enkei.com

 

[Xwheelguy]

I also made a slight mistake in the calculation of a wheel's offset. Should you use a 10 mm flat bar & depth caliper, don't forget to subtract the 10 mm from the flat bar's thickness to get the true mount depth.

Doing it word for word in my first post, one would have gotten an offset that is 10 mm bigger than it actually is.

Tim Flater
Senior Designer
Enkei America, Inc.

http://www.enkei.com

 

[NormPeterson]

Thanks for the additional information, Tim. It does seem that at the parts counter/technician level that the relation between width, offset and "backspacing" is not universally well understood. I personally know of at least one otherwise knowledgeable dealership shop tech who is convinced that the nominal width is a meaningless number (for advertising purposes only) and that wheel width is overall width.

I've also seen wheel designations that include the letters "J", "JJ", and "JK". As I understand it, those letters have something to do with the flange profile, but do they also affect the overall flange thickness any?

I have a little experience with fitting wheels with very close inside clearances (to the inside face of the flange but not its lip in one case). In some cases even your minimum wheel clearance might not show up with the car at rest (e.g. relative upper control arm movement under bump + steer conditions).

Was "A 7.5 inch wide rim is nominally 190.5±2.0 mm ." intended?

Norm

 

[Xwheelguy]

Yeah....you caught my typo...thanks. According to Tire & Rim Association, a nominal 7.5" rim is 190.5±2.0 mm. No, it doesn't come out exactly when you convert it to inches, but that may be a tire/wheel fitment thing...who knows? Normally, what I would do is target nominal & then make the tolerance tighter, to avoid a large floating area for the offset. But that's just me...the auto makers usually leave the rim profiles & target dimensions up to us. We just have to make sure our rim will fit on the car, fit the tire & balance weights, while maintaining T&RA or JATMA compliance on rim dimensions. BTW, JATMA is for the most part the Japanese version of T&RA.

As far as rim designations (J, JJ, JK, T, B, etc.) those are just different rim profile types. From my experience, the J ISO is the most common. But that's not to say amongst ALL wheel makers either...just those that I've dealt with. There could be a multitude of dimensions that are different between the different rim designations. However, they all serve a particular purpose out in the real world, but those purposes are not clearly defined by the T&RA manual (at least in my eyes). Basically, the rim section of the T&RA manual has a title (rim style) then a profile of the rim with nominal, gage, max & min dimensions on it. Then there might be a couple of charts detailing a particular dimension that may vary in size (like rim diameter) and it will calculate those dimensions from like 14 inches to 22 inches. There really is not any detailed explanation for what can & cannot be done regarding a rim design. As long as you meet the shown dimensions, you are OK.

Flange widths (or thicknesses), as far as I have seen, are purely customer (auto maker) driven. They are shown as a minimum in every T&RA rim profile that I've seen. I've yet to see a maximum dimension. Most customers have a standard flange width they prefer to use, but that doesn't mean each customer uses the exact same flange width. I would tend to think that they have determined a favorable flange thickness based on pot hole testing; where they drive a car with aluminum wheels over simulated pot holes then check wheel damage and wheel runout. Obviously, you don't want a cracked rim and if you do happen to hit a major pot hole, you don't want the wheel to be out of round or off center very much or you will feel the effects through noise & vibrations. Flange widths are a factor in this type of situation, and it's all up to the customer as to what sort of performance they desire. Most flanges that I've seen are between 15 and 13 mm thick and are called MC type flanges. If you were to go under that type of flange width, then you get into a different design called an AW flange, which uses a different type of balance weight.

Being a wheel designer, when I hear rim width, I don't think of the overall width of the rim. I think of the fitment to the tire. Just like with the diameter...an 18 inch wheel is anywhere from 31 to 37 mm BIGGER in overall diameter. Basically, when you're talking rim size designations, you leave out the flanges. I know that seems odd, but it's all relative to where the tires seat versus the measured size of the wheel.

Tim Flater
Senior Designer
Enkei America, Inc.

http://www.enkei.com

 

[GregLocock]

The flange design is poorly specified in the T&RA book, that is, there are many flange profiles that will fit in the defined dimensions.

Sadly this means that it is easy to define a flange which does not take a standard wheelweight. It is very easy when developing a wheel for a specific pothole or chuckhole test (and kerb impact, in Australia) to end up with a profile that is too thick. This plastically bends the clip on the wheelweight, then the weights fall off on the rolling road and hit the operators and you end up with an op health and safety issue.

This is one of the reasons why we've gone for stick on wheelweights - to get a low profile tyre through kerb impact you need thick flanges (and good heat treatment).




Cheers

Greg Locock

 

[GregLocock]

The wheel boutique guy is wrong. The offset is independent of the rim width.

I wouldn't worry too much about changing the offset at the rear, within reasonable bounds.

Changing the offset at the front will directly alter your scrub radius. As you increase the scrub radius you will lose steering 'precision feel', but should reduce parking effort. You may get more pull under braking, which may or may not be a good thing.

Many modern cars use scrub radius' in the range -20 to +20 mm. However, some cars handle reasonably well with a scrub radius of +100 mm .

If the change is of the order of 5-10mm, as is likely, I wouldn't sweat it. Changing from positive to negative might be of some concern, but I can't see why.

Cheers

Greg Locock

 

[sunracer]

Greg,
Thanks for filling in the blanks for me. The offset change I'm looking at will probbaly be from +55mm in front to around +40mm and fromt +60mm in the rear to around +45mm.

The only thing I can think of when I think about messing with the factory designed offsets are the cars you see where people have moved from a serious positive to a significant negative and the wheels hang outside the fenders. This is, of course, a gross exageration of my situation but I wanted to get a sanity check before I proceeded.

It sounds like I should fret too much over a small change of ~15mm in offset in this situation.

Thanks again,
Scott

 

[GregLocock]

Sounds fine to me. Let us know if it isn't!

Cheers

Greg Locock

 

[NormPeterson]

It's possible that you might end up tinkering with your static toe settings, as torques about the steering axis (and the associated compliance effects) will be somewhat different.

Norm