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914World.com _ 914World Garage _ 18" wheels to much stress?

Posted by: retrotech Oct 2 2005, 11:40 PM

Any opinions, or experience with 18" wheels on street/track 914? Trailing arms reinforced, but thats all.
I am running 17's now. If the Boxster needed changes to run 18's, can a 914 dare?
I want to.

Posted by: Andyrew Oct 3 2005, 12:06 AM

My dad says Im not allowed to. he says that it will just cause big problems, and the force given on the suspension (let alone the wheels) on a car with such stiff suspension (like any 914 deserving 18's..) will be bad...

but my cause is cus I want to run 18x11's on the back.... lol

I think 17s on the front and 18's on the back would be ok, because the front end takes all the bangs...

andrew

Posted by: Mueller Oct 3 2005, 10:54 AM

the early 90's 911/964 had problems with larger diameter wheels, they also have a full Aluminum suspension.....in the mid-90's it was fixed with beefed up design.

The stock 914/911(pre-89) a-arm does have a possible inherent weakness where the sheetmetal is bent near the ball joints...I believe if one added a form fitting piece of metal to this area and welded it, it should be fine.

There are a few 914 race cars (and a few street, but not many) running 18's with loads far exceding those that you will likely encounter on the street and I have not heard of a failure.

Andrew, why would a car with 18's need a stiffer suspension than one with smaller dia wheels??

Posted by: groot Oct 3 2005, 10:59 AM

Increased wheel weight is a big reason the loads go through the roof with larger wheels.... combine that with shorter and stiffer sidewall and you get incredible loads.

One example... modern platform was looking to go from 18" to 19" wheels and the chuck hole loads went from ~30gs to over 50gs in the rear.... they had to redesign the rear suspension.

So, let's be kind and say your only increasing the loads by 50% over a 17" wheel, which is at least double the loads of a 15" wheel.... then you have a lowered car, which on some of these 914s leave precious little jounce travel (usually none) before you get into the jounce bumpers. Now imagine transmitting most of that load straight into your strut bearings... and strut towers... How long do you think that will last?

Posted by: goose2 Oct 3 2005, 11:00 AM

I'm not sure I understand why a larger diameter wheel would impart more stress, all other thing being equal.

Posted by: goose2 Oct 3 2005, 11:01 AM

OK...I get it....the above post came in while I was typing, thanks.

Posted by: groot Oct 3 2005, 11:05 AM

Mike,

With a ball joint on one end and bushings on the other.... I don't think the the a-arm is loaded any differently with larger wheels.

Posted by: Mueller Oct 3 2005, 11:09 AM

QUOTE (groot @ Oct 3 2005, 09:59 AM)
Increased wheel weight is a big reason the loads go through the roof with larger wheels.... combine that with shorter and stiffer sidewall and you get incredible loads.

One example... modern platform was looking to go from 18" to 19" wheels and the chuck hole loads went from ~30gs to over 50gs in the rear.... they had to redesign the rear suspension.

So, let's be kind and say your only increasing the loads by 50% over a 17" wheel, which is at least double the loads of a 15" wheel.... then you have a lowered car, which on some of these 914s leave precious little jounce travel (usually none) before you get into the jounce bumpers. Now imagine transmitting most of that load straight into your strut bearings... and strut towers... How long do you think that will last?

True...GM now specifies and sells specific 20" wheels and tires engineered for "aftermarket/upgrades", in some cases, they want you to install a plate/bracket in the wheel well to help prevent the wheel from coming into the cabin in an accident.

my 20" rims/tires of my truck are lighter than the factory rims and tires (and so is my wallet ) ...most of those flashy/ugly rims sold for trucks wiegh too damn much

Posted by: redshift Oct 3 2005, 11:11 AM

Yes, I have experienced hood nipples. (tower nuts thru the hood)

If you raise your spindles, less chance.. but still.

M

Posted by: Cap'n Krusty Oct 3 2005, 11:15 AM

An additional problem is you have to wear a mask so your friends won't recognize you and point at you and laugh ....................... The Cap'n

Posted by: Mueller Oct 3 2005, 11:42 AM

QUOTE (groot @ Oct 3 2005, 10:05 AM)
Mike,

With a ball joint on one end and bushings on the other.... I don't think the the a-arm is loaded any differently with larger wheels.

with the lower profile tires (which usually run higher air pressure), the end of a-arm can be subjected to harsher and more violent forces since you are "removing" rubber (from the tire/suspension) that helps absorb some of the shock.........now that i think about it, I'm kinda leaning towards not recommending it, but I'm not a paid professional so don't take my advice as gospel

Posted by: groot Oct 3 2005, 12:36 PM

I will conceded that increased braking loads will transmit higher forces with a higher grip tire, but otherwise I don't think the a-arm cares what size wheel you have because it pivots on either end.....

From a OEM perspective, we vehicle engineering types usually want smaller wheels (that fit around adequate brakes) on regular passenger cars, but marketing and styling always want larger wheels. Smaller wheels are generally lighter (when built to OEM standards) and deliver a better ride and produce much lower loading conditions which allow the car's suspension and body to last much better.