Spacers versus more rotating mass?

[kaivball]

What is the lesser evil?

Running 25mm spacers on 15” rims to avoid the big brake kit caliper

or

Using 17” rims to clear the caliper but adding 7.5lbs more rotating mass to each corner due to heavier rim and tire?

Kai

[e30davie]

IF the 17s are in fact 8lbs heavier.Id say your spacers and 15s will have a lower moment of inertia than 17s on their own and thus the 15" with spacers is "better" from a weight point of view.

From an engineering point of view spacers are sound. I think the issues come with the fact that there is 4 bolts that are hidden that must be kept tight. I personally don't like hidden bolts, especially on a race car or something that is going to see a lot of abuse. It is another failure point that can be avoided with the right wheel combination.

I would try my hardest to find 15s that fit without spacers.

[kaivball]

Thanks for the feedback.

I would have to find a light weight 15x7 4x100 with zero offset. Which doesn’t change the overall physics but at least all bolts are visible.

Right now the wheels have +25 offset and need a 25mm space or they will make contact with the caliper.

The wheel and tire package is ~33lbs (225/45/16 Direzza ZIII).

The 17x7 rim and tire combo weighs ~40lbs but clears the caliper and doesn’t require the 25mm spacer.

And it is for an endurance track car.

Kai

[e30davie]

What about 16s?

The closer you can keep the weight towards the centre of the wheel the better. Moment of inertia of a circle is heavily effected by radius. Ie a small bit of extra weight further out on the wheel can make more difference than a larger amount of weight towards the centre.

[kaivball]

What about 16s?



The closer you can keep the weight towards the centre of the wheel the better. Moment of inertia of a circle is heavily effected by radius. Ie a small bit of extra weight further out on the wheel can make more difference than a larger amount of weight towards the centre.

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NOTE: For use along with 17" or larger wheels with antiquate brake clearance, often times, a 10mm spacer may help a great deal

[nando]

IF the 17s are in fact 8lbs heavier.Id say your spacers and 15s will have a lower moment of inertia than 17s on their own and thus the 15" with spacers is "better" from a weight point of view.

From an engineering point of view spacers are sound. I think the issues come with the fact that there is 4 bolts that are hidden that must be kept tight. I personally don't like hidden bolts, especially on a race car or something that is going to see a lot of abuse. It is another failure point that can be avoided with the right wheel combination.

I would try my hardest to find 15s that fit without spacers.

spacers don't have hidden bolts. 5 lug adapters, do of course.

There's really nothing wrong with spacers. I've run them for years and years without issues - both hubcentric and non-hubcentric. But I would recommend only using them with a stud kit.

I'd pick the 15s, 8lbs per wheel is quite a lot.

[kaivball]

spacers don't have hidden bolts. 5 lug adapters, do of course.



There's really nothing wrong with spacers. I've run them for years and years without issues - both hubcentric and non-hubcentric. But I would recommend only using them with a stud kit.

I think the “hidden” bolts the poster is referring to is the condition when you are racing and have wheels on the car in the pits.

You can quickly torque the visible wheel nuts during a pit stop but can’t check the bolts holding the spacer to the hub.

Maybe the answer is to use blue loctite on the fasteners bolting the spacer to the hub.

[nando]

There's no bolts holding the spacer to the hub. You just use longer bolts (preferably hardened studs). As long as the spacer is hubcentric, it works just fine.

[kaivball]

There's no bolts holding the spacer to the hub. You just use longer bolts (preferably hardened studs). As long as the spacer is hubcentric, it works just fine.

Not recommended when you run 25mm or larger.

[nando]

I don't see why it would be a problem with quality studs.

[e30davie]

I assumed we were talking about spacers like this:

https://www.driftshop.com/4x100-hubcentric-wheel-spacers-20-40mm.html

(first google result)

I am not sure what is better, the "slip on" style where you just need longer wheel studs, or this bolt on style. Part of my likes the bolt on style as you arent relying on a longer stud, but then as mentioned before you have the 4 hidden bolts.

I suppose the slip on style is no different than if you have a really thick mounting section on the wheel.

[nando]

as long as it's hubcentric the length of the studs doesn't make any difference. they are only providing clamping force, they are not supporting the weight of the vehicle.

[E30SPDFRK]

But over 25mm, there is a lot of twist force being applied in shear to the studs, and almost all spacer companies recommend switching to bolt on spacers over 25mm.

[nando]

yeah, I don't buy that. Based on what? The friction between the hub and the wheel is going to stop any 'twist'.

The irony is spacers put the studs into double shear - so even if you doubled the force it would be the same in the end..

[JimmyP]

Race tires are cheaper in 15" than 17".
15"s feel great on E30s.

You can fit pretty decent sized brakes inside a 15" wheel (I have 11.75" x 1.25" rotors with 6 piston calipers inside 15" Koseis on my E30M3 race car).

I'd get the right sized wheel with the right offset, and if needed re-do the brakes to fit.

Bottom line, No way, not ever would I personally ever be running around a race track with anywhere near 1" thick spacers on my race car. Never. I think thats way too much energy, hung way too far out on a set of stud, plus all the leverage its putting on the hubs. None of the geometry works for me in the least.
Get yourself a set of ET22-25 15" wheels.

I would avoid 16"s at all cost. The selection of track tires in 16" that are a good height for an E30 is 0

Cheers
jimmy