Suspension modeling

[xXDavidCXx]

https://whitelineperformance.com/products/kca472-roll-centre-bump-steer-correction-kit

Above is the only adjustable ball joint product that I know of with mk7 listed fit. People have fitted mk6 adjustable ball joints to the cars, but there are clearance concern. Its my understanding that the ball joint has clearance issues with aftermarket control arms and BBK. Frankly, this does not look like a quality product and it looks prone to slippage if you are generating grip. I would love to be proven wrong though

These are not SCCA class legal, so I won't be analyzing the suspension with them installed.

 

[xXDavidCXx]

From what I can find, it looks like the Ohlins R+T are also setup with stiffer front than rear (399/285). @xXDavidCXx what was the rationale for adding such stiff rear springs vs stiffer rear bar (or do you have both), class limitations?

I have both, but as I've already mentioned in this thread, the stiffer rear springs allow for more forward weight transfer, helps with unsticking the rear (oversteer) and for allowing the rear to react to bumps quickly compared to the front, so bumps do not unsettle the suspension.

The lot I autocross on is relatively bumpy and high grip.

 

[Oversteermybagel]

These are not SCCA class legal, so I won't be analyzing the suspension with them installed.

Apologies for derailing your thread. Also here is some more source of derailment. Here are some interesting stats I found when I looked for that picture of the mk7 GTi LMS/TCR LCA I found some interesting facts about the dedicated race car. Take this with a grain of salt because these cars are quite a bit lighter and also have heavily modified arms (are the rear motion ratios even the same!?) and such so we don't really know how analogous this info is. They have aero. Also they run racing slicks not street tires. It comes installed with 628/571 springs...

http://imgur.com/a/BEGQgX5

I guess what I am trying to get at is that -3deg front static camber probably isn't nearly enough to prevent the car from going into positive camber on roll without some insane measures going into roll reduction

 

[xXDavidCXx]

Apologies for derailing your thread. Also here is some more source of derailment. Here are some interesting stats I found when I looked for that picture of the mk7 GTi LMS/TCR LCA I found some interesting facts about the dedicated race car. Take this with a grain of salt because these cars are quite a bit lighter and also have heavily modified arms (are the rear motion ratios even the same!?) and such so we don't really know how analogous this info is. They have aero. Also they run racing slicks not street tires. It comes installed with 628/571 springs...

http://imgur.com/a/BEGQgX5

I guess what I am trying to get at is that -3deg front static camber probably isn't nearly enough to prevent the car from going into positive camber on roll without some insane measures going into roll reduction

Corner weights play significantly into the ride frequencies so we won't be able to make a direct comparison.

 

[greekspec2]

I don’t auto cross but my Golf R KW Clubsports run the same spring rates as the TCR race car setup for rain or Nurburgring.
571
456

 

[tpellegr]

I have both, but as I've already mentioned in this thread, the stiffer rear springs allow for more forward weight transfer, helps with unsticking the rear (oversteer) and for allowing the rear to react to bumps quickly compared to the front, so bumps do not unsettle the suspension.

The lot I autocross on is relatively bumpy and high grip.

I understand you stiffened the rear springs to add more roll resistance in the rear and aid in oversteer, but stiffening the rear bar would accomplish the same thing, correct? Are there tradeoffs you considered when choosing one method over the other for your setup? It is my understanding that the disadvantage of using a stiffer bar to increase roll resistance is that it minimizes the benefits of having an independent rear suspension (since both sides are effectively connected to some degree), while the disadvantage of using stiffer springs is that it could cause tires to unload too much over bumps and possibly lose contact with the racing surface (on a smooth surface I would think this is less of a concern). With such a high spring rate (and on such a bumpy surface as you mentioned), I would be concerned about the latter, especially since, as we've seen, most other setups have lower rates in the rear.

 

[tpellegr]

I don’t auto cross but my Golf R KW Clubsports run the same spring rates as the TCR race car setup for rain or Nurburgring.
571
456

View attachment 194619View attachment 194620View attachment 194621

I'm guessing the reason most track and street setups have less stiff rear spring rates is for bump compliance. For an autocross where you are primarily taking low speed corners on a surface that is relatively flat, it is probably less of a concern?
Edit: also as david mentioned, comparing spring rates on two completely different cars (with different suspension geometry/components) is not as helpful as wheel rate.

 

[Oversteermybagel]

I understand you stiffened the rear springs to add more roll resistance in the rear and aid in oversteer, but stiffening the rear bar would accomplish the same thing, correct? Are there tradeoffs you considered when choosing one method over the other for your setup? It is my understanding that the disadvantage of using a stiffer bar to increase roll resistance is that it minimizes the benefits of having an independent rear suspension (since both sides are effectively connected to some degree), while the disadvantage of using stiffer springs is that it could cause tires to unload too much over bumps and possibly lose contact with the racing surface (on a smooth surface I would think this is less of a concern). With such a high spring rate (and on such a bumpy surface as you mentioned), I would be concerned about the latter, especially since, as we've seen, most other setups have lower rates in the rear.

Right so say you take the 400lb/in spring we were talking about before and you have 2 degrees of roll which might put you through 0.5 inches of travel. Making up reasonable numbers here this would have to be modeled. This gives you 200lb of roll resistance.

Now a 22x2mm roll bar, depending on how its mounted, might provide on the order 100lb of roll resistance per degree of roll.

Its reasonable to say that in this case that half of the roll stiffness might come the bars and half the springs. Now street tires can only take so much wheel rate. In this case we need the roll bars to make up the difference.

 

[bfury5]

I understand you stiffened the rear springs to add more roll resistance in the rear and aid in oversteer, but stiffening the rear bar would accomplish the same thing, correct? Are there tradeoffs you considered when choosing one method over the other for your setup? It is my understanding that the disadvantage of using a stiffer bar to increase roll resistance is that it minimizes the benefits of having an independent rear suspension (since both sides are effectively connected to some degree), while the disadvantage of using stiffer springs is that it could cause tires to unload too much over bumps and possibly lose contact with the racing surface (on a smooth surface I would think this is less of a concern). With such a high spring rate (and on such a bumpy surface as you mentioned), I would be concerned about the latter, especially since, as we've seen, most other setups have lower rates in the rear.

I run my rear bar as stiff as possible and it still wasn't enough, hence needing stiffer rear springs. Even if a stiffer bar were available, a stiff bar removes a lot of suspension compliance across the axle, which can upset the car over large bumps. Stiffer springs and proper valving to match can replicate the effect of a stiff sway bar without some of the downsides.

You absolutely have to worry about springs being too stiff and unloading the tires, but I personally haven't had that issue with my springrates for autox. Again, if my car was 100% track driving I'd probably be slightly softer in the rear.In autox you're basically trying to force the natural tendencies of a chassis at lower speed so you need to exaggerate all of the handling characteristics. On the track, the load/ unload of the chassis and suspension is usually a little bit slower (and at higher speed) so you wouldn't want it as snappy.

I'm guessing the reason most track and street setups have less stiff rear spring rates is for bump compliance. For an autocross where you are primarily taking low speed corners on a surface that is relatively flat, it is probably less of a concern?
Edit: also as david mentioned, comparing spring rates on two completely different cars (with different suspension geometry/components) is not as helpful as wheel rate.

I'd say it's partly bump but also safe. As lots of us has done the math regarding ride frequencies and with personal experience, when the rear end gets stiff enough compared to the front it can be a handful (OK at low speed autox, not preferable for safe track driving)

 

[tpellegr]

I run my rear bar as stiff as possible and it still wasn't enough, hence needing stiffer rear springs. Even if a stiffer bar were available, a stiff bar removes a lot of suspension compliance across the axle, which can upset the car over large bumps. Stiffer springs and proper valving to match can replicate the effect of a stiff sway bar without some of the downsides.

You absolutely have to worry about springs being too stiff and unloading the tires, but I personally haven't had that issue with my springrates for autox. Again, if my car was 100% track driving I'd probably be slightly softer in the rear.In autox you're basically trying to force the natural tendencies of a chassis at lower speed so you need to exaggerate all of the handling characteristics. On the track, the load/ unload of the chassis and suspension is usually a little bit slower (and at higher speed) so you wouldn't want it as snappy.


I'd say it's partly bump but also safe. As lots of us has done the math regarding ride frequencies and with personal experience, when the rear end gets stiff enough compared to the front it can be a handful (OK at low speed autox, not preferable for safe track driving)

Thanks Brian, well said. Your comment about the stiffer bars reducing compliance across the axle is what I was getting at, but was having a hard time explaining. That makes sense to me. Just wanted to make sure I wasn't misunderstanding.

I'm thinking about altering my setup a bit. Will likely add a front bar on stiff setting to try and reduce camber gain in the corners (which I'm guessing is pretty substantial with the stock springs/dampers and based on the model David setup) and keep the rear bar as is or move to a less stiff setting. Worth experimenting a bit in the real world to see what happens. Maybe during an autocross event this time and not at a time trial event.

 

[Oversteermybagel]

Thanks Brian, well said. Your comment about the stiffer bars reducing compliance across the axle is what I was getting at, but was having a hard time explaining. That makes sense to me. Just wanted to make sure I wasn't misunderstanding.

I'm thinking about altering my setup a bit. Will likely add a front bar on stiff setting to try and reduce camber gain in the corners (which I'm guessing is pretty substantial with the stock springs/dampers and based on the model David setup) and keep the rear bar as is or move to a less stiff setting. Worth experimenting a bit in the real world to see what happens. Maybe during an autocross event this time and not at a time trial event.

I'd bet you'd notice one end of the car getting too slide-y before the car starts to feel like a torsen beam/solid axle conversion and getting super upset by bumps it didn't hit with that wheel. Because while you are reducing roll (increasing grip) you are also robbing the happy tire of grip--especially at low roll cornering. So for a front bar, you could very well end up with a situation where at corner entry or corner exit the car behaves in a way where it pushes but have achieved more grip at higher g-forces elsewhere in the corner by reducing camber loss

 

[xXDavidCXx]

I ran in STH for an entire season on stock springs and shocks, but with the largest FSB I could find (Hochkis) and the 034 rear bar on full stiff. I ran solo nationals like this and placed mid-pack, but beat my local competition.

I had great grip, but, the rear was very planted, so an overall understeer at the limit car.

When I added 400 lb springs front and rear with the Ohlins the next year, I had all sorts of issues with massive understeer. It took me an entire season of shock setting, different springs, different swaybars and progressively less rear toe in, to get to a place where the car started to rotate. And then my RE71r's were heat cycled out. I also had a hard time beating the local competition.

If I'd went stock shocks and springs route again, I would immediately add 1/8 inch total rear toe out, with all bars on stiff, with as much front camber as you could get.

 

[xXDavidCXx]

Thanks Brian, well said. Your comment about the stiffer bars reducing compliance across the axle is what I was getting at, but was having a hard time explaining. That makes sense to me. Just wanted to make sure I wasn't misunderstanding.

I'm thinking about altering my setup a bit. Will likely add a front bar on stiff setting to try and reduce camber gain in the corners (which I'm guessing is pretty substantial with the stock springs/dampers and based on the model David setup) and keep the rear bar as is or move to a less stiff setting. Worth experimenting a bit in the real world to see what happens. Maybe during an autocross event this time and not at a time trial event.

With the available bars and spring rates mentioned in this thread, and considering the weight of the GTI, I don't think any of your compliance concerns will come into play.

 

[Oversteermybagel]

I ran in STH for an entire season on stock springs and shocks, but with the largest FSB I could find (Hochkis) and the 034 rear bar on full stiff. I ran solo nationals like this and placed mid-pack, but beat my local competition.

I had great grip, but, the rear was very planted, so an overall understeer at the limit car.

When I added 400 lb springs front and rear with the Ohlins the next year, I had all sorts of issues with massive understeer. It took me an entire season of shock setting, different springs, different swaybars and progressively less rear toe in, to get to a place where the car started to rotate. And then my RE71r's were heat cycled out. I also had a hard time beating the local competition.

If I'd went stock shocks and springs route again, I would immediately add 1/8 inch total rear toe out, with all bars on stiff, with as much front camber as you could get.

In the end did you find running Ohlins to be a significant step in your adventure to find time or a distraction?

 

[xXDavidCXx]

In the end did you find running Ohlins to be a significant step in your adventure to find time or a distraction?

Probably a necessary distraction.

One cannot succeed at the next level without understanding the physics behind what is required.

I'm trying to add data to what I think I now know into the public sphere, backed up with kinematic data, with the suspension design as a starting point.

This data will ultimately not be complete without the addition of spring/bar rates and ride heights, which will come nexxt.