33 replies to this topic

[DrDomm]

I searched the archives, and am shocked I can't find a thread about how to measure front and rear track. I believe it's supposed to be measured at the height of the hub, right? Is there a tool to make it possible? Am I missing something simple?

[Johnny D]

^{If you have a 25mm offset wheel, you pretty my much have it. 30 is smaller track width.
The SCCA rules state what it is and you can use spacers to get to the equiv of 25.
I think the tech measurement is running the cars tires through power and measuring width.
Read the rules and do a search here for wheel offset and it should all be explained.
J~}

[DrDomm]

I bought ET30 wheels previously just to be safe. I'm aware that some posters have said they can run ET25 wheels. The alternative is to add 5mm spacers, which I ordered. I don't want to use them if it puts me over the legal track...I doubt it will by looking at what other cars look like, but that isn't a precise method. The SCCA GCR outlines the method of how track is measured by Tech. It isn't the powder method. That's to make sure the contact patch doesn't extend past the fender. That's affected by camber. Track shouldn't be affected by camber, since it's measured at the height of the hub.

[Johnny D]

I'd run your 30's with the spacers.
Roll your fenders so they dont cut the tires.
Not sure how it is in NY but I've never been checked.
J~

[Jim Drago]

If you run 25mm offset wheels or less(>25), you will be legal.

Jim

[DrDomm]

If you run 25mm offset wheels or less(>25), you will be legal.

Jim

Ok. Great.

[davew]

If you run 25mm offset wheels or less(>25), you will be legal.

Jim

Note that 30mm offset is less than 25mm offset. 25mm wheels give you the widest legal track width. 30mm wheels will give you approximately 3/8" less track than the 25's

Dave

[Bench Racer]

I searched the archives, and am shocked I can't find a thread about how to measure front and rear track. I believe it's supposed to be measured at the height of the hub, right? Is there a tool to make it possible? Am I missing something simple?

2011 GCR-169

4. Measuring Track

Basically measure track width at hub horizontal centerline height at the rim radius front & rear of vertical centerline. Tech uses a dealy-bob like a toe checker. They measure from the inside of one rim to the outside of the other rim both front & rear, add the numbers & divide by two.

[DrDomm]

2011 GCR-169

4. Measuring Track

Basically measure track width at hub horizontal centerline height at the rim radius front & rear of vertical centerline. Tech uses a dealy-bob like a toe checker. They measure from the inside of one rim to the outside of the other rim both front & rear, add the numbers & divide by two.

Right, I understood how to do the calculations. I just have never seen how you measure from the outside of a rim to the inside of another with the tires and other parts in the way.

[Jim Drago]

They have a bar they use to measure. It is expands and contracts in the middle, with two 6-8 in tall ends on either side .

[Ron Alan]

They have a bar they use to measure. It is expands and contracts in the middle, with two 6-8 in tall ends on either side .

so I assume it has 2 settings(rear)for NA or NB cars 10mm apart...do you know this measurement Jim?. For the front also? I assume they go to the center of the rim/wheel correct? I mean hoosiers are wider than toyo's by a small amount aren't they? is this taken into account? 3 years in SM and I've never seen this checked...and it should be!

[Bench Racer]

I viewed the track width checked on an F production miata. IIRC the tech folks fabed their own checker. It might have been a modified toe srick. It's like a toe stick/measuring scale attached/with sliding offset probes at each end. The probes are offset to reach around the tire sidewall bulge when simo touching the inside of one rim/outside of other rim. Track dimensions are in the SM Class Specifications. End result is that track measurement is between the centerline of the tires.

[DrDomm]

The above process, as Jim says, is how it was described to me. I just wondered if there was a tool available to do it. Apparently not. It kinda makes me wonder about compliance when no one is measuring it, and just assumes 25mm offset rims are ok. Oh well, I've got more important priorities.

[Nsparks]

If you end up getting different offset wheels make sure you keep the sets separate. I have had my track measured a few times now because I mixed my sets up and had 3 25mm offset wheels with 1 30mm+ spacer. Steve DeVinney checked me at a regional race and Mike Collins checked me at the Nationals. Good job guys we need more "Out of the box" things checked at tech... So I can start cheating my restrictor plate again!

[cpdenis]

So what would be the offset recommendation for wheels. 30mm or 25mm. I understand 25 would be legal but what is the best tuning option or overall best wheel offset to use/purchase?
CP

[Bench Racer]

So what would be the offset recommendation for wheels. 30mm or 25mm. I understand 25 would be legal but what is the best tuning option or overall best wheel offset to use/purchase?
CP

IMHJ 30 mm rims (narrow track) with 5mm spacers (wider track).

[davew]

I have never been a fan of spacers. Buy 25mm wheels and forget about it.

When I was on the SMAC we wrote the rule so you would be legal with 25mm and no spacers. I measured about 20 cars, Drago measured some and Collins measured some.

Mixing offsets is not legal, read the GCR

Dave

[Sean - MiataCage]

Mazda built the Miata from the factory with the rear track wider than the front track to induce understeer so that when Grandma on the road panics and hits the brakes the car will push instead of spin. If you go with 25mm offset wheels all around then you are just making the situation the same but wider. I personally like to "square" up the car so that the front and rear track is the same (perfectly legal with spacers), or in some cases make the front wider so that it turns in better. If you go with 25's then legally you can not do this.

There are numerous theories on this and it is up to each driver and their preference, but the bottom line is that spacers are tuning aids. Just like air pressure and bar changes and ride height etc, etc, etc. Spacers as tuning aids are not going to be noticed by everyone, especially if they are not doing the prep needed to run up front. For my cars, they get a complete set up every race and in most cases differently for every track we run. With wheel spacers we can make minor adjustments easily and at least start life with a square chassis.

As with everything in SM, ask lots of questions, use common sense and logic and do what you think makes sense, not what someone on an internet forum tells you to do. If you are just starting out I highly suggest getting all of your wheels with the same offset so that there are no surprises in tech if you have a crew member accidentally grab the wrong wheel in a hurry.

Keep in mind as well that many wheels only come in (1) offset, so you might not be able to get the offset you want with every wheel design.

Thanks...... Sean

[Ron Alan]

Sean's post made me go back and read the GCR(no jokes Cooper, I do read!) The numbers given in the GCR refer to measurements from outside to outside on the hubs, not tires(1450/1465 or 75)...get this. In regards to using 25mm or 30mm, spacers, no spacers would fall under the category of checking tire tread which touches the ground under static conditions....to which the overall width/dimension shall not exceed that of a vertical plumb line drawn down from the the widest portion of the fender. "Compliance of this checked by rolling car through a powdery substance" So for sure I have never seen this and Jims description of a giant caliper would make sense if it is the width of the front or rear fenders...is this the case? Bottom line is the top of the tire is inside the fender while the camber sends the bottom of the wheel outward. So what seems to be really fuzzy here is camber, tire pressure, and the rubber on the tire can affect where it actually ends up touching the ground(not to mention must be on a dead flat surface). So what and where is this gauge measuring to and what is the dimension??? Is this a secret?

I first started wondering about this whole issue when I was just checking toe on a couple cars with toe plates...and came up with cars over 1" different(not toe guys...just total dimension)...same year cars. In this case i know how they both got where they were and was pretty confident one wasn't legal but really had nothing to go by short of breaking out my cocaine stash(for you jackson)

I think this rule could be written so much simpler. "25mm offset wheels max...no spacers, knuckles specific to the year of the car" When they check brake rotor diameter in impound they can check these easily when the wheel is off.

With regards to Sean's comment on spacers, I'm having a little dejavu...didn't this assertion get disputed? GCR says if spacers are used they have to be equal per axle. Is axle the correct terminology here or should it be hub? If you cant run different offset wheels to stagger or square the car, why would you be able to do it with spacers...what am I missing? If running 30mm up front(and 30mm in rear) with 7mm spacers squares the car, would only seem logical you would be able to do the same with different offset wheels if that was the goal? My point being I didn't think you can only run 2 spacers?

[DrDomm]

Yeah, I read that part about equal spacers per axle last night...grrr...

Hey Sean, Saferacer says they're waiting to get my spacers from MiataCage.