The Bilstein strut option - shock guru help requested

[spiggs]

I have mentioned in other threads that a Bilstein strut insert for the front of an early VW Rabbit will fit in a stock strut tube. Since insert options are few for the X1/9 I thought it would be beneficial to start a thread about using these and their suitability for the job. I'll start with specs and valving since I want to make sure these are up to the task before doing any fab work. Below is the spec sheet from Bilstein giving all critical dimensions.

As with almost all Bilstein shocks/struts these can be revalved and rebuilt. I have read that these particular units are linear only or perhaps can be made slightly digressive but have yet to confirm with Bilstein. Below are the dyno sheets from Bilstein and a graph I made converting the metric units to english.

Now this is where I need help from those who know shocks. I am going to use these in my budget GP build following the Steve H suspension setup philosophy. Since I am sure the spring rates for this (350-450R 550-650F) are far beyond what the stock valving is designed for I need advice on what to tell Bilstein for a revalve. Of course if these are wholly unsuited for the purpose I also need to know that to cut my losses. I think it would also be beneficial to those looking at street/performance options from mild to wild to hear how these would work with other recommended spring/swaybar setups. So to those who understand the above charts or have other relevant info I ask that you please chime in.

Vince

 

[SpidArman]

I'm confused about spring rates....

I thought the stock setup was that the front springs rates were lighter than rear rates. Why when going to performance specs are the fronts heavier than the rears?

thanks,

Arman

 

[abec]

I thought the stock setup was that the front springs rates were lighter than rear rates. Why when going to performance specs are the fronts heavier than the rears?

thanks,

Arman

Stock is set up softer fronts then rears. I wondered this at first also, but it makes a lot of sense to put the heavier springs up front from a performance aspect.

There are a few reasons to have higher spring rates up front.

1st) If you equally spring front and rear, the weight in the rear will cause the back end to slide out first. So by adding spring rates up front you will neutralize this and balance the car out.

2nd) on braking higher front spring rates will prevent forward weight transfer and help keep the rear end planted.

 

[SpidArman]

hmmm, sooooo

for a "spirited" lowered road car, should the stock bia or performance bias be used?

Arman

 

[abec]

for a "spirited" lowered road car, should the stock bia or performance bias be used?

Arman

I would go with a more performance based spring set up for any car you want to feel sporty, just not a full race tune for street cars as they tend to be extremely harsh.

I just posted a cause and effect style tuning guide that I got from the MR2 forums. http://xwebforums.com/forum/index.php?posts/27098#post27098/ hopefully that helps you decide where your car is for how you drive it and what you can do to improve its handling. Steve H has a few good threads on the topic as well.

 

[CALMOTION]

VW struts

I was talking with Chris O and he mentioned using VW struts also on his autocross X. Maybe you can ping him and get some information.

 

[Chris Obert]

I was talking with Chris O and he mentioned using VW struts also on his autocross X. Maybe you can ping him and get some information.

The strut tubes Mark Plaia designed use the VW Rabbit rear inserts.

On my autocross car I use the Koni racing two way adjustable inserts, and I am still trying to understand them. Currently my worn out tires and lack of tire budget limit my playing with strut settings, so I give rides and have fun. The spring rates Mark came upon for this car are NOT something you would run on the street, or a rouph track (Candlestick will get the rear wheels in the air and the tach "jumps").

Mark's specification for his tubes is the Tokiko 5 way adjustable inserts, and everyone that has installed them is happy with the results. Mark's calculated spring rates & ride heights seem to be great for where he lives, but for our terrible California roads the rates are too strong and cause a very jaring ride. My local clients have also requested higher ride heights to give the bump stops more clearance. When I get around to building my street X with the Plaia suspension California road conditions at that point will dictate what spring rates & ride height I will go with.

If you want to hire Mark, as I have done for my client's and my own cars, he will run the calculations and make suggestions for starting points for what I consider to be a very reasonable fee.

 

[Steve Hoelscher]

Some thoughts

Looking at the graphs the stock dampening will likely be too soft for the rates I would suggest. As I am not familiar with the Bilstein dampening strategies, I would simply tell the engineers the corner weights and spring rates and let them figure the necessary rebound.

I would note that my experience with VW Rabbit front struts is that the compression dampening is way too stiff for the X1/9. Now I haven't tested the Rabbit Bilsteins but Koni, Tokico, KYB, etc... were all too stiff. You will want the compression dampening to remain where it is or soften it up some.

I would also suggest that instead of finding an insert that fits the X strut, I would look for a popular racecar, one that has lots of performance/race inserts available for it, and adapt that housing to fit the X. The cost to mod the housings is likely less than adapting or revalving inserts. And once you have the housings done, then you have lots of options available so when your budget will permit it you can easily upgrade your inserts.

I also noted the spring rates you listed. 350/450 rear and 550/650 front. I would suggest you go no lower than 600 front and 450 rear.

 

[Steve Hoelscher]

Stock vs Performance spring rates

Why when going to performance specs are the fronts heavier than the rears?

Arman, it is important to note that this isn't a "performance" application. It is a true "competition" car. There is a difference.

Yes, the stock setup has softer front springs. But that is a compromise for ride quality. The front end of the car is lighter and the rear is heavier. To achieve a comfortable ride, each end of the car must be sprung according to the weight it carries. Also, in order to give the car a smooth ride at crusing speed, the front end of the car must be sprung softer than the rear relative to the weight on each axle. The ultimate handling balance at the car limit of adhesion is a secondary consideration.

A performance setup typically is intended to give improved handling by reducing body roll and improving transient response. The bias is now tilted more toward peformance than ride quality so stiffer springs and an anti-roll bar may be used.

A true competition setup completely ignors ride quality in favor of handling balance at the limit. In the above example you questioned, the springs are the only means of body roll control as no anti-sway bars are used. Because of the lack of swaybars, to achieve a proper handling balance, the front axle must have more roll resistance than the rear. This offsets the car's natural rear weight bias. So stiffer front springs are used to bias the front/rear roll ratio to the front axle.

The resulting ride quality (or lack there of) must be ignored.

 

[Steve Hoelscher]

I just posted a cause and effect style tuning guide that I got from the MR2 forums. http://xwebforums.com/forum/index.php?posts/27098#post27098/ hopefully that helps you decide where your car is for how you drive it and what you can do to improve its handling. Steve H has a few good threads on the topic as well.

Interesting that you posted the guide from the MR2 forum. If you go to the autocross forum you will find my complete write-up on car setup using an STS class Mk1 MR2 as the subject car. However, most of the subject matter came from the development of my Championship winning X1/9. http://www.mr2oc.com/forumdisplay.php?f=45

 

[spiggs]

...
If you want to hire Mark, as I have done for my client's and my own cars, he will run the calculations and make suggestions for starting points for what I consider to be a very reasonable fee.

Thanks Chris. Sounds like a great option but I don't have any budget for that. My goal here is to first get a baseline reliable car to run and go from there. I also like to use it as a learning tool. I'll certainly keep Mark's service in mind though.

 

[spiggs]

Looking at the graphs the stock dampening will likely be too soft for the rates I would suggest. As I am not familiar with the Bilstein dampening strategies, I would simply tell the engineers the corner weights and spring rates and let them figure the necessary rebound.

Interesting. I thought a shock dyno report would give me the information needed to compare any shock to any other shock. However you mention dampening strategies, could you elaborate?

For corner weight will I be close enough by using the class minimum weight split by the 40% front 60% rear and equal per side or do I really need to get the car corner weighted in ready to go trim?

I would note that my experience with VW Rabbit front struts is that the compression dampening is way too stiff for the X1/9. Now I haven't tested the Rabbit Bilsteins but Koni, Tokico, KYB, etc... were all too stiff. You will want the compression dampening to remain where it is or soften it up some.

The Bilstein P30-0032-M0 insert is a fairly popular option to adapt, or at least used to be. I know the 240z, early BMW 3 series, and early RX7 crowd have used them. I used them before on a 128 and X1/9 after I bought a pair of X1/9 struts that had them already installed. My experience is that they rode more comfortable than the KYB inserts of the time (late 80's) with no sacrifice in handling which I am hoping points to less compression dampening. Overall with FAZA Stage 2 springs I think they did a good job as delivered, but this was strictly street.

I would also suggest that instead of finding an insert that fits the X strut, I would look for a popular racecar, one that has lots of performance/race inserts available for it, and adapt that housing to fit the X. The cost to mod the housings is likely less than adapting or revalving inserts. And once you have the housings done, then you have lots of options available so when your budget will permit it you can easily upgrade your inserts.

My plan is to use the Bilsteins and as time permits build a set of strut tubes for more universal inserts like the Koni 8611. I have seen the G-Force units but even though I think they are very reasonably priced $2600 is out of my range. For now all I need to get the Bilsteins going is a $300 revalve and springs. I did think of finding other struts to adapt but in the end whatever I found would still require work to use the high spring rates and then more work to switch over to race inserts. FWIW front struts from a 3rd gen MR2 are about the right length and koni sport inserts are readily available. But again they really would not handle the high rates without work. Maybe I am approaching this wrong so I am open to suggestions.

I also noted the spring rates you listed. 350/450 rear and 550/650 front. I would suggest you go no lower than 600 front and 450 rear.

Is this then the starting point you would suggest 600F 450R or is it still on the low side?

Thanks for your input.

Vince