Tire Size Question for 1.8L 1975 Options

[Aayala]

Hello all,

I was wondering if anyone could provide some feedback or recommendation on purchasing some tires and wheels for my 1975 1.8L 914.

I wanted to upgrade the wheels on my 914 to a 5 lug conversion to fit some Fuchs on it.

Question is...

1. Is it worth it to convert to the 5 lug and pretty simple to do?

2. I have someone selling their wheels and tires for a good price but the wheel sizes are 15x6 (front) and 15x8 (rear). Will this fit the stock 914 narrow body?? or will I have to for sure add fender flares to fit these?

thank you in advance!

[Pyetookh]

I´d like to replace my 4 lugs baby Fuchs for something a bit wider.


Unfortunately I can´t find any wheels 6x15 or 7x15 at 4x130. (IMG:style_emoticons/default/headbang.gif)

Therefore I decided to go for conversion plates 22mm thick to go from 4x130 to 4x100.



And use Japan Racing JR3 7x15 4x100 ET25 White 67.1 wheels.


What do you think about such combo, please?


I may like to go on 205/60 R15 front and 225/50/15 rear. Or maybe 205/60/15 all 4 corners.
This will require wider fenders

Or maybe Japan Racing JR3 7x15 4x100 ET40 plus 22mm spacer and try to squeeze them into narrow body.

Any thoughts are welcome (IMG:style_emoticons/default/confused24.gif)

[Superhawk996]

What do you think about such combo, please?

I may like to go on 205/60 R15 front and 225/50/15 rear.

Wheel spacers add significant scrub radius to the front wheel steering geometry increasing steering effort. This is in addition to any effort increase from increasing tire width.

It usually doesn’t work out to add the same width spacers at both front and rear. Changing front and rear unequally affects vehicle handling depending on which track width is wider front vs rear. Even if you can change front and rear equally, the change in track width still affects vehicle handling and suspension set up one way or another.

Wheel spacers increase stress and wear on wheel bearings.

I’m not really a fan of spacers especially just to accomplish lug pattern change but I’d bet you haven’t considered any of this.

The tire stagger that you are proposing will increase understeer. Is that really what you’re trying to accomplish with the change to the vehicle handling?

[Pyetookh]

What do you think about such combo, please?

I may like to go on 205/60 R15 front and 225/50/15 rear.

Wheel spacers add significant scrub radius to the front wheel steering geometry increasing steering effort. This is in addition to any effort increase from increasing tire width.

It usually doesn’t work out to add the same width spacers at both front and rear. Changing front and rear unequally affects vehicle handling depending on which track width is wider front vs rear. Even if you can change front and rear equally, the change in track width still affects vehicle handling and suspension set up one way or another.

Wheel spacers increase stress and wear on wheel bearings.

I’m not really a fan of spacers especially just to accomplish lug pattern change but I’d bet you haven’t considered any of this.

The tire stagger that you are proposing will increase understeer. Is that really what you’re trying to accomplish with the change to the vehicle handling?

These are all good points.

But wider track and GT flares were manufactured by factory so I believe they had a good reason for it. It is also very common mod, It can´t be all wrong. (IMG:style_emoticons/default/stirthepot.gif)

I also believe that you can compensate for conversion spacers thickness with higher wheel ET number,

[Superhawk996]

These are all good points.

But wider track and GT flares were manufactured by factory so I believe they had a good reason for it. It is also very common mod, It can´t be all wrong. (IMG:style_emoticons/default/stirthepot.gif)

I also believe that you can compensate for conversion spacers thickness with higher wheel ET number,

Fair points as well.

To the first: what is good for racing isn’t always good for the street. If you know how to properly tune a vehicle suspension for your desired outcome you’re ahead of the game. If you’re blindly following what someone did 50 years ago you might end up less than pleased with the changes.

To the second - you are correct assuming you can find an offset increase equal to the spacer. But at the end of the day, you’ve added unsprung weight and inertia.

Not trying to deter you per se. Just make sure you understand what you’re doing and what the trade offs will be.

[Pyetookh]

These are all good points.

But wider track and GT flares were manufactured by factory so I believe they had a good reason for it. It is also very common mod, It can´t be all wrong. (IMG:style_emoticons/default/stirthepot.gif)

I also believe that you can compensate for conversion spacers thickness with higher wheel ET number,

Fair points as well.

To the first: what is good for racing isn’t always good for the street. If you know how to properly tune a vehicle suspension for your desired outcome you’re ahead of the game. If you’re blindly following what someone did 50 years ago you might end up less than pleased with the changes.

To the second - you are correct assuming you can find an offset increase equal to the spacer. But at the end of the day, you’ve added unsprung weight and inertia.

Not trying to deter you per se. Just make sure you understand what you’re doing and what the trade offs will be.

Truth is that I don´t know what I´m doing and that´s why I joined this thread to learn a bit more from more experienced members. My car is non-runner and I haven´t ever driven P914.
On the other hand we have more options than factory some 55 years ago so there may be some options how to make these cars to look and handle a bit better.

Ultimately this is a sports car and I have lots of winding roads nearby so original skinny 155 tires are no option for me despite being outcome of Porsche engineers (IMG:style_emoticons/default/evilgrin.gif)
I will not compete with my car but really like enthusiastic driving.

My car will also get Inner longitudal stifner kit (BTW. Can anyone provide a drawing of the parts, please? Shipping these from US to Europe will cost more that the value of the parts plus custom duty (IMG:style_emoticons/default/ar15.gif) )

I think I will go for 22mm conversion spacers, 15x7 ET40 wheels, 205/60 tires and will make my fenders to fit. There are even more tire options in 195/60.

Do you think this is acceptable compromise? (IMG:style_emoticons/default/idea.gif)

[Superhawk996]

Truth is that I don´t know what I´m doing and that´s why I joined this thread to learn a bit more from more experienced members. My car is non-runner and I haven´t ever driven P914.
On the other hand we have more options than factory some 55 years ago so there may be some options how to make these cars to look and handle a bit better.

Ultimately this is a sports car and I have lots of winding roads nearby so original skinny 155 tires are no option for me despite being outcome of Porsche engineers (IMG:style_emoticons/default/evilgrin.gif)
I will not compete with my car but really like enthusiastic driving.

My car will also get Inner longitudal stifner kit (BTW. Can anyone provide a drawing of the parts, please? Shipping these from US to Europe will cost more that the value of the parts plus custom duty (IMG:style_emoticons/default/ar15.gif) )

I think I will go for 22mm conversion spacers, 15x7 ET40 wheels, 205/60 tires and will make my fenders to fit. There are even more tire options in 195/60.

Do you think this is acceptable compromise? (IMG:style_emoticons/default/idea.gif)

Forgive me for the length of the response. It is not directed at you in any sort of personal way. I'm writing it for the benefit of those that forget how all the little "upgrades" can actually be a degradation.

Since you have never driven a 914 it would be wise to drive it first before deciding that it needs significant upgrades.

The 914's handling (even in near stock form on modern rubber that will fit) when driven at the limit is already beyond the capability of most drivers. I can provide the data to back up this statement if necessary but I'll defer that unless there is truly interest.

This is a bit harsh to say but it is true. I say that as an automotive engineer with 30 years experience that includes having participated in successful Le Mans effort a few decades ago. That includes having personally raced both cars and motorcycles myself and knowing what can be done safely on public roads. Yes, I have driven in Italy, Switzerland, Germany, etc., and I'm familiar with the roads in Europe.

With respect to chassis stiffening. The typical chassis stiffening kits are a great way to add lots of extra weight.

There is very little data on this site to actually provide data on the actual stiffness increase is vs. the massive weight penalty you will incur. Stiffening is one thing for racing at Le Mans, doing a 3.6L powertrain swap, or going club racing on wide super sticky rubber whether it benefits a car that is supposed to be light weight and nimble is another.

Why do you think chassis stiffening is worth the weight gain for a 1.8L with very limited power?

I can tell you with certainty that it is very tough to engineer a 2000 lb vehicle.

Weight creep is real. Just a few pounds for wheel spacers (don't forget the lugs & studs). Then there is going to be approximately 20 lbs (1% of the vehicle weight!!) for the typical plate steel chassis stiffening kit. Don't forget the weight of the new larger wheels to accommodate the wider rubber you are proposing. And of course the new larger tires you've proposed are much heaver and have much more inertia than the original 155 or 165 width tires. Those wider wheels and tires will easily be another 1% weight addition. These seemingly little 1% here 1% there weight additions add up and are lot of extra mass and inertia for the poor 1.8L to have to accelerate.

Then comes the horespower upgrades you'll need to make up for all the weight that was added . . . which in one form or another almost always . . . you guessed it . . . adds yet more weight. The list just keeps growing, brakes, oil coolers, etc.

With respect to wheel offset. The original offset is 40mm on the 5.5" steel wheels. So the 23mm referenced for 5 lug Fuchs already has a scrub radius increase built in that degrades steering effort. I will agree most people find this to be OK but keep in mind, that entails rolling fenders and it isn't uncommon to get some inner wheel well rubbing too depending on chassis build variation . . . that is with wheels and tires smaller than you want.

The moral of the story being there are lots of things that urban mythology and internet lore will tell you are good things and "must do" but they are rarely done knowing the full set of tradeoffs that go along for the ride.

As always . . . its your car. Do what makes you happy. Don't follow internet advice (including mine) blindly.