Einpresstiefe

[k.russell]

it seemed almost everyone except myself was on cheap second hand slick tyres or wide Kumho's on Monday at Cadwell. i have begun to investigate fitting them to my car. my car is at the moment fitted with 6" fronts and 7" rear wheels with ET223/24 . the second hand rear slick are mostly 23/53 - 13 so need around a 8" or 8 1/2" rim. My car has the top mounted anti roll bar and so i need to retain around 125mm from the inner rim face and the hub mounting face. I have read on the recent CXR wheel bulk buy that using the same ET20 ish on the bigger wheels causes problems in this area. Does anyone have a 8 or 8 1/2 CXR rim that they can measure across the width, more difficult with tyre fitted, i can then work out the Einpresstiefe figure to clear my bits.

 

Kevin R

 

black(but sometimes orange)-ali HPC

here

 

Edited by - k.russell on 30 Jan 2011 15:29:17

[Pendennis]

Sorry, don't have any but in the past I bought a set of CXR's specifying the offset which Compomotive agreed to. From memory, when they arrived the offset was 2 or 3mm out, which maybe the problem experienced by others.

 

Why don't you order them ET18 to be safe?

I found Compomotive a very good company to deal with

 

With these wider wheels, are the ears an issue on some de-dions

 

I have an 8.5" SLR as well as a 8.5" Barnby rim if you want to have a look, but from memory they both measure ET21-ish

 

Want to rent an 18th century Farmhouse in Rural Somerset?

[k.russell]

using approximate sizes, if a 7" rim measures 202mm overall width and i need 125mm depth, this would give the ET25 figure.

ctr of 202mm wheel = 101 , 125 - 101 = ET24

 

i then assume that with an 8" rim approx 230mm wide to retain the 125mm depth i would need to go for approx ET10,

ctr of 230mm wheel = 115, 125 - 115 = ET 10

 

this would put all of the extra width outbourd of hub.

 

Kevin R

 

black(but sometimes orange)-ali HPC

here

 

Edited by - k.russell on 30 Jan 2011 15:31:05

[Jason Plato]

the cxr's have a slightly different ET measurement owing to the different design of rim bead or wheel edge on the inner rim versus the outer rim section.

 

The outer rim section has what is basically a false bead. This false bead has a decorative finish that changes the ET measurement. The decorative edge adds about 4 mm to the offset.

 

I believe the only safe way to specify CXR's is by using the back depth measurement as the bead on the rear of the rim is conventional

 

david

 

 

here is my Duratec R .... C7 TOP

Taffia AO

[Richard Price]

the cxr's have a slightly different ET measurement owing to the different design of rim bead or wheel edge on the inner rim versus the outer rim section.

 

The outer rim section has what is basically a false bead. This false bead has a decorative finish that changes the ET measurement. The decorative edge adds about 4 mm to the offset.

 

I believe the only safe way to specify CXR's is by using the back depth measurement as the bead on the rear of the rim is conventional

---

 

I think you'll find that its the "backspace" that looks different with CXR's rather that the ET value, as the ET value is the difference between the centre "wheel width", and the "mounting pad".

 

look here

 

 

 

[Jason Plato]

hmmmm

 

as a working example ( and bear in mind I relay dont care as my CXR's fit fine, this is for the benifit of others)

 

Using front wheels as an example.....

 

the backspace measurement for my 6" CXR's is the same as my barnbys were. The outer face of the wheel is about 5mm further from the mounting face than my barnbys. Therefore if you measure the wheel centerline without considering the relative position of the tyre bead vs false rim edge, you will end up with front wheels and tyres that rub on your wingstays if you order using the offset measurement.

 

hope this makes sense.

 

 

here is my Duratec R .... C7 TOP

Taffia AO

[MacP]

I am thinking of ordering some of these so want to understand Dave J's point on offset.

 

If I have understood correctly:

 

1. There is an additional 5mm of decorative "bead" compared to standard wheels so the Overall Width of the CXRs is 5mm greater.

 

2. The Specified Rim Width remains 6 inches (i.e. the 5mm of additional metal doesn't affect this).

 

3. Assuming that the Centreline measurement is based on the Overall Width (as per Compomotive's tech info section) the Centreline would be 2.5 mm (half of the 5mm) further out from the inner edge compared to a standard wheel.

 

4. This has the same effect as increasing the Offset by 2.5 mm - i.e. bringing the inside edge of the wheel in 2.5 mm.

 

5. For people with cycle wings this could result in the tyre/wheel rubbing against the stays if a standard offset is specified.

 

Would be grateful if anyone could confirm whether this is right.

 

Thanks

 

Edited by - MacP on 5 Oct 2010 21:10:11

[Shaun_E]

MacP - that is my understanding.

 

Kevin - do you really need 125mm backdepth on the rear? An 8.5" rim with that backdepth will probably stick out from the wheel arch. I run 235 Kumhos on 8.5" rims with 140mm backdepth (which is also what the Caterham 8" wheels are) and have the top mounted ARB but with the drop links repostioned inside the ARB. The tyres definitely "fill the arches"!

 

I have ordered 2 sets of wheels from Image in the past and only ever specified the back depths as that is the critical measurement. They seem happy to work with that and the wheels fit perfectly.

 

As Pendennis says, you may have to fettle your de-dion tube depending on which tube you have and the design of the wheel.

 

Yellow SL #32 - member of Drowned Rat Racing