Compression and Offsets

Gold TX Sleeper · backyard mechanic Joined: 05 Feb 2002 Posts: 509

1. I have a standard G200, What would be a rough estimate of the compression ratio if i fitted an 1800 head with standard gasket? And are the valves bigger in the G200? theoretically they would be, But i'd like to know for certain.

2. How do you measure wheel offsets? And why is too small or too big an offset a problem?

Cheers,
Jasyn...

RedTG · tinkerer Joined: 04 Feb 2002 Posts: 136

My only advice will be to go to a mag wheel specialists, i hate that offset stuff and negative stuff, just give me big wheels for small price is the go

LOL

Racegem · backyard mechanic Joined: 30 Jan 2002 Posts: 987

Hey Jasyn,
In answer to question 2, start by imagining the wheel in front of you, front on. The centreline of the width of the rim is zero offset. If the hub face, where the wheel bolts up, is closer to inside of the car, that is negative offset. If it's closer to the outside of the wheel, that's positive. Measurements are in MM. Basically what that means is too much positive offset, the tyres start to rub on the chassis at full lock (Camira rims). Too much negative offset, the tyres start to rub on the guards during suspension travel. The wider the rim and tyre, the more critical the offset becomes.

Hope this helps,

Yonnee.

Hamal · tinkerer Joined: 14 Dec 2001 Posts: 64

Also, if the offset is lower (towards the negative) than the original wheels, then the wheeltrack (measured between the centre of the rear/front wheels, ie: zero offset) is lowered. This is a bad thing as the rear wheels are closer together meaning less stability than the original factory car.

Transport Authorities will not accept that, here in WA you are not allowed to lower the wheeltrack at all and I imagine it would be the same around the nation.

In WA however, you are allowed to increase the wheeltrack by an inch and no more. This means you can have an offset half an inch larger than factory.

A little off topic now, what is the regulations in other states concerning flexability with the wheel track? Also here in WA you are not allowed to fit any wheels wider than 7 inches, unless a wider tyre was fitted as standard. In that situation you are not allowed to go over the standard.

Racegem · backyard mechanic Joined: 30 Jan 2002 Posts: 987

I'm not sure what you mean about the wider tyre, but here in Vic. you can go two inches wider than largest factory for that model but no more than one inch track increase. Diameter I don't think matters as long as the "rolling" diameter doesn't change by more than 10%.
Weren't there CDT cars with 13 X 8's? Imagine....10's on a Gem!!!
How do you guys in W.A. go with wheel tubs? Cause that would increase the overall width outside tyres for stability, but decrease rear track width with the wider tyre being kept inside the guards!

Yonnee.

Hamal · tinkerer Joined: 14 Dec 2001 Posts: 64

http://www.transport.wa.gov.au/licensing/myvehicle/safe/modifications/tyre.html

This is where I got my information from. There may be 'exceptions' to the rule, especially where there is cause for debate.

The part that worried me though was this :

In any event, rim width shall not exceed 177.8 mm (7") unless specified by the manufacturer as standard equipment for the vehicle in question. No increase in rim width is permitted for vehicles fitted with original equipment rims in excess of 177.8 mm (7") width.

However that said the 7 inch wheels on Drago Ostric's Gemi look wide enough (barely) to keep me happy. I still have some confusion about tyre/wheel sizes though. I thought the first number (ie: 215's on Drago's) was the actual width of the tyre, it seems to fit fine on the 7inch wheels though?

I'd like it if someone could explain that to me!

Racegem · backyard mechanic Joined: 30 Jan 2002 Posts: 987

The rim width and trye size are two different things. 7" is the width of the rim, 215 is the size of the tyre fitted to that rim usually measured in MM across the outside of the tread. You could even fit 225 tyres to a 7" rim. I explained tyre sizes in a post a week or two ago.

The second paragraph mean that if the vehicle was fitted with 13 x 8's as a factory option, these are legal but you can't go wider.

Yonnee.

Gold TX Sleeper · backyard mechanic Joined: 05 Feb 2002 Posts: 509

So does that mean the guy's with big-ass deep dish rims on their HQ's and Torana's are running a lot of negative offset ? I think i'm getting the picture now... Thanks alot peoples.
Jasyn...

Racegem · backyard mechanic Joined: 30 Jan 2002 Posts: 987

Usually J, it depends on the rim. If you picture a four inch wide rim with two inches to the out side, it has zero offset (its in the middle). An eight inch rim with four inches to the out side looks alot deeper but still has zero offset cause it's still in the middle.
The old trick with HR's and LC - LJ's was to unstitch the weld around the centre of the rim, turn it round (reverse it) and weld it back in the same spot, hence a positive offset rim turned into a negative offset rim and filled out the guards a bit more. Not entirely roadworthy cause it widened the track at both ends.

Yonnee.