RC 101: Wheel Offset

[morrisey0]

I know I am getting deeper and deeper into the "old dog, new tricks" portion of life, but my mind isn't shot. Until things like this happen, and well, maybe it is.

Offset should be simple. It is the difference from wheel mounting plane to center of the wheel plane. I get this, and have no issue with this in the 1:1 world.

But I have these RC wheels, and they ain't jiving. I have a few sets of HPI wheels of various offsets I picked up from the 50% off bin at HTUSA, and I can't make their math work, and I want to get this right before ordering wheels for a build I am working on.

So for my example, I will use some HPI Racing 26mm wheels with 9mm as stamped on them. They mic out at ~26mm wide, so all good there, and the magic center distance I will use is 13mm. So, 13mm center, and 9mm offset, I need some measurement of 4mm. From the inside rim, I get ~6mm to the inside of the hex (what I would consider mounting plane) and ~10mm to the "outside" of the hex. I would think that 9mm +offset wheels should be 13-9mm so just 4mm from the back of the wheel, and these are obviously far from that. For my measurements, I am subtracting the 2mm of my straight edge. I am not getting anything that works out to 9mm offset. I have some 6mm offset wheels, and they are making just as much sense.

This has to be simple and I am just missing it, but I am lost here. Any help?

[THEYTOOKMYTHUMB]

If the Long Island Ice Tea doesn't help, then I sure can't...

[juicedcoupe]

I've heard about others getting abnormal measurements from HPI wheels as well. I was considering some a while back but decided against it.

I prefer the way RPM does, providing actual measurements from all valid points.

[XLR8]

I've had similar experiences myself and it can be frustrating.
I have touring car wheels from a wide variety of manufacturers (Kyosho, Tamiya, ?(china), etc.) and the offset number molded into the wheel never seems to make any sense. I reckon that everyone has developed their own method for establishing "offset".

[juicedcoupe]

I've had similar experiences myself and it can be frustrating.
I have touring car wheels from a wide variety of manufacturers (Kyosho, Tamiya, ?(china), etc.) and the offset number molded into the wheel never seems to make any sense. I reckon that everyone has developed their own method for establishing "offset".

Reminds me of work.

Every time someone says that something doesn't make sense, I ask them if they want to walk out to the plot limits (pipe rack converges there). I tell them that you can check the labels on every pipe leaving here, none will be labeled "sense". Thats because we don't make that here, anywhere.

[XLR8]

I've had similar experiences myself and it can be frustrating.
I have touring car wheels from a wide variety of manufacturers (Kyosho, Tamiya, ?(china), etc.) and the offset number molded into the wheel never seems to make any sense. I reckon that everyone has developed their own method for establishing "offset".

Reminds me of work.

Every time someone says that something doesn't make sense, I ask them if they want to walk out to the plot limits (pipe rack converges there). I tell them that you can check the labels on every pipe leaving here, none will be labeled "sense". Thats because we don't make that here, anywhere.

hahaha... Good point. In this case, they make wheels not sense.

[Coelacanth]

What is the measurement from the inside surface of the hex hub, where the wheel hex actually contacts the hex recess, and the outer rim of the wheel? That would be your positive offset measurement. Hopefully this pic might help my explanation. Of course, if the wheels aren't spec'd correctly by HPI, it's a moot point.

Capture.JPG (43.54 KiB) Viewed 2930 times

Capture.JPG (43.54 KiB) Viewed 2930 times

I know at one point years ago, I was trying to find some axles to mod an AYK Buffalo and I needed axles that were about 49mm long, give or take a millimeter or two, and found some HPI axles that were advertised, right on the header card, as being 49mm, if I recall correctly. The first set I bought had a TOTAL length of 42mm, not even just the axle length, so I was baffled by how HPI arrived at this measurement. Figuring it might be a mistake, I bought 2 more of the same item and they were all the same 42mm total length. Even HPI Support reps were dumbfounded.

[morrisey0]

If the Long Island Ice Tea doesn't help, then I sure can't...

It is kool-aid and vodka. I am not a complete drunk! Staying hydrated while drinking is the secret.

What is the measurement from the inside surface of the hex hub, where the wheel hex actually contacts the hex recess, and the outer rim of the wheel? That would be your positive offset measurement. Hopefully this pic might help my explanation.

Coelacanth, little difficult to see what measurement you are looking for, but I think it is the same as my last pic, which is 12mm - 2mm straight edge, so 10mm overall from back of wheel to "outside" of the hex.

What sucks is that I only bought these wheels super cheap to be measuring devices. These were going to be how I judged 6mm and 9mm offset wheels, so I could make decisions based on their total track width. And now it looks like they are pretty worthless. What is weird is that I threw some tires on the 9mm wheels and the total track is almost dead on what I want, but unless I order HPI wheels, I learned nothing because other 9mm offset wheels aren't going to fit.

[juicedcoupe]

Just checking, but did you zero the calipers with the depth gage. On my cheaper calipers, the jaws and depth gage don't zero together.

[THEYTOOKMYTHUMB]

Hah!

[Coelacanth]

Coelacanth, little difficult to see what measurement you are looking for, but I think it is the same as my last pic, which is 12mm - 2mm straight edge, so 10mm overall from back of wheel to "outside" of the hex.

What sucks is that I only bought these wheels super cheap to be measuring devices. These were going to be how I judged 6mm and 9mm offset wheels, so I could make decisions based on their total track width. And now it looks like they are pretty worthless. What is weird is that I threw some tires on the 9mm wheels and the total track is almost dead on what I want, but unless I order HPI wheels, I learned nothing because other 9mm offset wheels aren't going to fit.

My pic was showing a red line to indicate the inside surface of the hex recess, which is where I'd start measuring the offset, to the lip of the rim, where I'd stop measuring the offset. I used the second red vertical line to indicate the approximate length and direction of measurement.

You don't want to measure the upper edge of the hex recess because that's not what your hub mounts to, it mounts to the inner surface of the hex recess.

Could you mount some wheels with your body mounted and show how it looks now, and say what you're aiming for? That might help too.

[morrisey0]

Just checking, but did you zero the calipers with the depth gage. On my cheaper calipers, the jaws and depth gage don't zero together.

It is about .2 mil out.

My pic was showing a red line to indicate the inside surface of the hex recess, which is where I'd start measuring the offset, to the lip of the rim, where I'd stop measuring the offset. I used the second red vertical line to indicate the approximate length and direction of measurement.

You don't want to measure the upper edge of the hex recess because that's not what your hub mounts to, it mounts to the inner surface of the hex recess.

That is where my confusion is, and heck, maybe the industry wide confusion. From my research, it is literally half the people saying to measure to the "inner" portion of the hex, and the other half the "outer" part. Heck, I can see "inner" as meaning either side, just open to interpretation. So that is why I measured both and subtracted both from the 13mm centerline.

Wouldn't a direct measurement from a point to the inside lip plane be backspacing as opposed to offset?

Here are a couple of contradicting pics off the interwebs:

[Coelacanth]

Well, technically, a wheel with neutral, 0 offset is measured as if it were cut in half down the middle. You'd have the same amount of rim spacing on either side of the imaginary center line.

It can be tricky to calculate because, although it's easy to determine the imaginary center line, it's not easy to determine the actual mating surface of the inner hub. RC wheels have 12mm hex depressions, but 1:1 car wheels don't have that. A full-size car wheel bolts directly onto the hub face.

What measurement do you get when you measure the wheel width, divide it into half...which is your imaginary center line...and compare that measurement to the measurement I discussed, from the inside surface of the hex depression to the outside of the outer wheel rim? The difference would be your offset...positive offset if the wheel surface is mostly flat on the outside, or negative offset if the wheel surface has the old-school muscle-car deep-dish look.

[terry.sc]

Your problem is assuming an industry standard definition from the full size world is the same as in the r/c world.

In the r/c world the nearest you can get to an actual definition of zero offset is "whatever measurement Tamiya arbitrarily used when they made their first touring car back in 1991" So that's around 5-7mm from the outside of the wheel, I don't have a wheel to hand to check. The actual wheel width and the centre line is irrelevant to the measurement.

Then offset itself is different to the full size world, a positive offset on a full size wheel makes the track narrower, in r/c a positive offset makes the width wider. Remember 99.9% or r/c owners have no knowledge of the full size world so using a system that makes sense to them is more useful than copying the full size standards.

It's simple for most r/c drivers, bolt a set of zero offset wheels on and your car is the standard width, fit a set of +3mm wheels on and your chassis is now 6mm wider than standard. Fit +9mm wheels and your chassis is now 18mm wider. If you fit 31mm wide zero offset wheels on your chassis, the chassis will be overall the same width as using 26mm offset wheels, as the offset is measured from the front face.

[threesheds]

Coming late to this party but is offset important for racing class rules, stresses on other car parts or steering geometry or all three?
I was wondering if tyre alignment with steering kingpin was important.