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?

 

[JeffGTV]

REPLY TO
nkwheelguy (Automotive) May 27, 2004
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

HEY TIM:

I had a bit of a hard time interpreting this post, so let me see if I get this right.

OFFICIAL Wheel 'width' is the distance between the insides of the flanges, which is about 0.8 to 1.1" narrower than the UNOFFICIAL outside 'width' of the flanges that most people think of.

Next, offset is the distance from the wheel's natural centerline to the mounting backside of the boltholes pad, with NEGATIVE OFFSET meaning MORE distance from the unseen inboard flange to the unseen mounting pad than the LESSER distance from the seen outboard lugnut mounting pad to the seen outboard flange!

And, finally, the 'backspacing' is the distance from the tire mounting bead to the inboard mounting pad, so that BACKSPACING = 0.5000*(the bead-to-bead distance) - (offset measurement) ..... so that negative offset INCREASES Backspacing and positive offset DECREASES Backspacing .....

I am trying to get this straight so I can make an informed decision on the purchase of new 17" rims for my 94 LEGEND 6-SPEED with 6.5"x16"x(65)mm offset rims!

Dudes on EBAY seem NOT to know what they are talking about!

Thanks so much! Jeff Kay, Risk Analyst, Automotive Loans.

 

[Xwheelguy]

Jeff,

You have the terms right, but the positive/negative offset is a bit backwards. Please use the following image as a reference (this is a typical POSITIVE offset wheel):

Wheel width as defined by T&RA is labeled inner rim width, which as you can see EXCLUDES the flange(s). Keep in mind not all flanges measure the same, so if you desire fair accuracy for fitment, better measure them to be sure...certainly won't hurt anything (when performing calculations, etc.).

Now, you have the calculation of offset correct, but a positive offset is going to be FURTHER away from the centerline or inboard flange (or on the right side of the centerline as shown above). A negative offset will be to the left of the centerline as shown above. Zero offset wheels will lie right on the centerline.

Negative offset wheels appear to stick out really far when mounted onto a vehicle (abnormally far in my point of view). If you've ever seen the wire wheels that stick way out from the fenderlines (of a lot of teenager's cars), you know what I mean. The lower the offset number, the further out the wheel is going to sit compared to the well/fender.

You can of course do what you wish, but I would personally try to keep within a FEW inches of the stock wheel offset. This isn't going to give you any guarantees of ALL clearances though. Will just keep the wheel from sitting in too far or sticking out too far.

Hope this helps out.

Tim Flater
Senior Designer
Enkei America, Inc.

http://www.enkei.com

 

[CadMan2005]

I use AutoCAD to design wheels with and I have made 3 full sets of wheels, that have been used on vehicles to date. But how hard is it to have a set made by a established wheel company? Instead of just using my local machine shop.

 

[Xwheelguy]

If I'm misinterpreting your question, I apologize....I'm assuming you don't mean to just sell your designs to wheel makers...you actually want custom wheels made for personal use, correct?

Never heard of such a thing being done, at least for a very very low number of wheels (like a one-off set of wheels). Here where I am employed, we want wheel programs with large volumes so we don't have to set up machinery on a daily basis. We don't make money when the machines aren't running.

Also, most wheel manufacturers cast their wheels versus machining them from billet, which is what the machine shops probably do for you, correct? So you'd have to be willing to pay for the tooling costs as well. I'd think it would be a completely different story if the wheel company actually bought your design & decided to manufacture it as an aftermarket part on a mass production basis.

Tim Flater
Senior Designer
Enkei America, Inc.

http://www.enkei.com

 

[CadMan2005]

The billet machined wheels that I have had cut have been very expensive so far. My goal was to have a one off set for myself but now that I have seen the expense of it all. The low numbers for one off sets are the thing that have led me to look into casting.

Well, how would you go about submitting/selling a possible wheel design for manufacture? I have no idea of the costs that are involved in casting. What are your guidelines for submission for wheel designs? Because I have been sitting on several design for months on end.

The whole thing that got me started on this track in the first place is that I couldn't find a wheel that I liked. So why not trying to have a concept realized.

Jerry

 

[GregLocock]

There's a couple of approaches.

One would be to go for sandcasting. We do this for prototypes, in our experience a sandcast wheel is not acceptable for production, because of porosity and so on. However, you could probably get a pattern made from wood for about $10000 (less if you did a lot of work yourself). Then you'd pay for casting and machining and painting on top. The true cost of that is largely setup time and the cost of aluminium.

For production wheels we use low pressure die cast parts. The patterns probably cost about 10 times as much, and need an expensive refurbishment every few thousand wheels.

When designing wheels be very aware that the big manufacturers employ vicious lawyers to enforce their copyright.



Cheers

Greg Locock

 

[patprimmer]

What is wrong with machining the centres or spider from say 2" thick plate or billet, then using spun rims.

This limits the design possibilities, but would certainly reduce costs.

You could end up with something like Centre Lines or Weld wheels

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[cadastrophe]

That's basically what BBS, Symmonds, Image, Kodiak etc do. The centres are either cast/machined or billet/machined. It means that just a few standard rim shells can be used to produce a multitude of sizes and offsets, which can be further fettled by front-mounting, sandwiching, or back mounting them.
It's terrific fun designing and making your own wheels for motorcycles and cars, and is usually much cheaper than purchasing big name aftermarket wheels.

 

[CadMan2005]

Well, I have to ask a few questions, since ya'll have given me so much info.

I know that it is possible to create a billet center and attach it to a standard wheel. Is it possible to cast the centers and then attach them to a standard rim shell, for a relatively inexpensive cost?

What specs do you use for two-piece wheel construction, got any suggestions?

I have several wheels that I have used for reference for producing so far, but what programs you use to design with? Because there are several platforms out there, but I export out of AutoCAD for the designs that I have produced for machining.

Thank you for providing me with so much valuable information.

 

[cadastrophe]

I use ACAD for drawing up wheels too. I'm not aware of any dedicated wheel designing software.
I normally make two and three-piece rims from .190 6061 which are first spun, then heat treated, then trued and the spigot hole cut and then a final heat treat.
Centres can easily be sand cast in the back yard using MDF patterns cut out with a router.