Anyone familiar with trailing arm geometry can help with this idea?

I have an audi tt. 

it's a mk1 quattro. so rwd.
These cars are super nimble.  but it is actually a bit too nimble. 
I want to try extending the wheelbase a bit.
But I don't know enough about how the rear geometry works.
I would abviously be setting the alignment properly afterwards.

But do you guys have any idea about the suspension arm angles or anything else I might not be thinking of?
i only want to add maybe 1-2cm.

i attached a pic of how I plan on accomplishing this.

imagine the arrows being 1cm of adjustment. All other mounting points have flex joints of some sort, so they should allow for the trailing arm to move forward a bit. I'm just curious about the way the geometry functions, if the change in the arm angles and things would cause any issue?

Okay. Let's pause for a second. You just need more caster angle. You don't actually need an increased wheelbase and mk1 TTs are actually prone to understeer at the limit. Sure you can change spring rates and stiffen up sway bars and there's a little something called lift off oversteer, but let's not make it out to be a 930. 

And the answer what you'd like to do is to improve your caster angle for higher speed stability and there traditionally ways of doing it with changing wheelbase. I'll add at -.25 (in) of toe on each side of the rear would also likely be highly beneficial. What are your current alignment specs? 

Please pick this up, it's a wonderful book and a must read for those wanting to set up a car properly.

In reply to Captdownshift (Forum Supporter) :

Hi. 
I'm currently very tiny toe in F/R  almost neutral really.
stock caster specs, and 2deg neg camber.

But I am fully aware of the benefits of additional caster, and I am totally on board. But that's the front only.  Or are you trying to say that I can play with increased caster in the rear?
On my car rear caster adjustment isn't even a thing people do.

Also (and my main point) I fully plan on maximizing caster and perfecting toe/camber and everything. But the wheelbase increase with be in ADDITION to all that.  
IE I'm hoping for an extra benefit of stability on top of all the alignment trickery.

Mainly just wondering if pushing the wheels at the back outwards would cause anything weird with the rear control arms.  I'm just not knowledgeable about how all the geometry works back there so I was hoping someone here would know.

P.S. I do have camber adjustable rear control arms, so camber/toe won't be an issue.

So get it onto an alignment rack, and get your true actual current settings so that we can start there. If you have a pyrometer, also go into a large vacant lot and do 3-4 circles in each direction then take the temperature measurements across the widths of all 4 tires and report back. 

You need to think of suspension geometry and alignment settings as science, because they are. Which means you need to establish a baseline, and then you were just one variable at a time. That way you know what that variable did with regards to making a difference whether it be an improvement or not. If you throw E36 M3 at the wall to see what sticks, You're just going to continually throw E36 M3 and money at the wall and you're never likely to get the optimal setup.

Have you ordered the book yet?  

I also want to add, as I didn't have the time to this morning, that I want to commend you on a few things. First of all being that you've found a platform that you enjoy, that you can develop and that it's capabilities can grow with you as you get more seat time. Take the time to learn and refine the nuances to get the platform to do and perform how you would like it to be able to. The second thing that I really want to commend you on is the fact that you are trying to get the rear end settled down a bit. So many people are obsessed with eliminating understeer to the point that they develop someone unpredictable oversteer, as if it's some sort of advantage. 

I'm confident that you will be able to get the handling dynamics dialed in via alignment settings, probably a solid coilover setup (I'd talk to Chris at redshift) and potentially sway bars without a change in wheelbase being needed. It will be a process, but you'll end up with settings that are suited to your driving style and enable you to have the most confidence in the setup, and you'll know why and how it works how it does, when you're all said and done. 

I see no reason to believe that your wheelbase is the problem, which means that increasing it by a mere 1-2cm is certainly not the solution..

Once the alignment has been validated to have not major issues, my next step would be confirming that none of the aftermarket parts that have been added are to blame. I'm not sure what else you've done other than the cheap coilovers,  but would be suspect here. Even most coilover rates are front biased on rates, so my gut reaction is the damping and/or travel. A cheap and easy thing to trial if rate tuning might be part of the issue would be disconnecting the rear sway bar.

Oh. So the mk1 TT is just known for its snap oversteer and twitchy(though agile) handling. 

Was actually a lawsuit and recall of the first version cuz a bunch of people died.

The lawsuit was due to areo at triple digit speeds causing the backend to get light and actually had nothing to do with suspension or handling.

 https://www.nytimes.com/2000/02/20/automobiles/audi-offers-tt-fix-after-5-deaths.html

Please use factual attributes with regards to your vehicle cuz it'll actually help correct the concerns instead of painting it to be something that it's not and then trying to correct something that's not an actual issue. Being truthful with yourself will help you get the project where you want it to be. Again I commend you for loving the platform, That is absolutely awesome and we should all love the platform that we have as much. But don't project things that aren't even half truths, this isn't the place for it. From statements regarding it being rear-wheel drive bias, comparable to an E46, to twitchy and oversteery, just stop. Honesty with yourself will help you accurately evaluate the platform as it is and develop a plan and course of action to improve the platform into what you want it to become. 

In reply to Captdownshift (Forum Supporter) :

Well hey I appreciate the support!!