Difference between shock coils on 2008 Caterham vs 2012 Brunton Stalker

[Anaximander]

I was looking at an ad on the For Sale forum at a 2008 Caterham Superlight R400 (drool) and noticed that the front Bilstein shocks (Caterham #75517) were either 150# or 170# shocks. On my 2012 Classic Brunton Stalker, I have begun to autocross the car and the stock configuration for the front shocks is a 500# coil over the GAZ shocks. In autox mode, the recommended coil is a 600# coil which I just installed along with upgrading the rear 250# coils to 300#. On a blat after installing the upgraded coils, there was a noticeable difference in the handling of the Stalker with a more taught (little lean) but still not unpleasant ride. I can't wait to hopefully autocross the car this coming Sunday.

 

I know the suspension geometry is different for the two cars along with likely different front end weights but I was sort of surprised at the significant differences in coil stiffness between our two cars?

Edited June 13, 2017 by Astro Bob

[Davemk1]

I don't know the particulars on the spring rates of these two cars but there are certainly many reasons it might be this way. The most obvious is that the Stalker no doubt weighs much more (500 lbs?) and this of course would necessitate stiffer springs.

 

The other reason could be that the leverage ratio of the two different suspension designs is likely much different. The angle that the shock sits from vertical can be a rough indicator of the leverage ratio. Where the shock attaches to the A arm compared to the tire contact patch of the tire is another (if the shock attaches far inboard the spring needs to be stiffer to give the same effective spring rate). And of course is the suspension uses a rocker arm set up all bets are off.

 

Typically the spring rates are chosen to give the car a given amount of static sag (measured as a percentage of shock travel) so that the wheel can move both up and down from its "home" position. If the springs are too stiff the car will sit with the shocks topped out (or nearly so) which means that if the ground drops away (or the car unweights) the tire can't reach down for the surface and will become unweighted and traction will be lost.

 

At the same time if the springs are too stiff the wheel will not be able to move up enough when it encounters a bump and the tire will become overloaded and traction will be lost.

 

I have no idea if the spring rates you have fitted will make the car autocross better or worse.....that said it's easy to over-spring a car and have it feel fast (quick turn-in, very little lean....etc) but ultimately have the car end up being more difficult to control at the limit and slower against the clock. Typically speaking having springs that are a bit softer than optimal will make for a car that is slower than one fitted with springs that are a bit stiffer than optimal.

 

You'll know if the springs are too stiff if the car feels disconnected from the surface....it will feel "skippy" like a stone skipping across the water. It will still feel fast.....in fact it might feel very fast due to the quick turn in and lack of lean or brake dive but if it's not connected it will actually be slower. If the car feels faster but is actually slower it could be that your spring rates have been made too high.

 

dave

[Anaximander]

Dave, Wonderfully articulate explanation. I had been told by a representative of the manufacturer that historically, the increase in spring weight improved autox performance. I indeed used the compression ring on the front springs to set the ride level such that the A-arms were parallel to the ground. The rear compression rings for the coils were set to a historically predetermined spot that has proven to be effective in increasing rear-end performance with the shock angles being different than the fronts and different demand characteristics for the rear shocks in terms of performance. I don't know what the weighting strategy is of a Caterham R400 but I know that the Classic Stalker has about a 54% bias to the rear. Although being a total rooky in autox, I do find that when the Stalker gets out of shape from say too much throttle that just a slight let-off really straightens out the car unlike some vehicles like a rear-engine Porche where the rear-end weight bias is significantly greater and reportedly leads to the rear wanting to swap ends with the front of the car in a more pronounced way.

 

The Classic Stalker weighs about 1450 pounds which I suspect is not a full 500# greater than the Caterham and more like about 350# difference? I am just at the very beginning stages of learning about chassis geometry, corner weighting, etc. and as a newb found the difference in spring values to be dramatic between my current car and the Superlight R400. Thanks again for your keen explanation and I look forward to any others who have insights into why the big difference in spring values?

Edited June 13, 2017 by Astro Bob

[Davemk1]

One thing to keep in mind......stiffer is not always faster. If stiffer was always faster then the logical conclusion would be to do away with the springs and have the suspension be solid with zero wheel travel. Since this clearly won't be the answer and at the same time super-soft springs aren't the answer this means that we have a Goldilocks situation where one end is too soft and the other is too hard and the one is the middle is just right. Finding that one in the middle is much harder than it might seem and is the Holy Grail.

 

Lastly - autocross is not road racing. The two types of cars require MUCH different set ups. Road racing tends to have very stiff set ups and all too often Solo guys tend to look at what they have done and copy it. This will seldom end up working well for solo. Road race cars are driven on lots of steady state situations where the car takes a set and the car is balanced there for longer periods of time. Transitions in road racing are not that fast.....Solo on the other hand is a 99% super-fast transition and steady state corners are very rare. So Solo cars need to be set up so that they transition well. Often this means that both springs and shocks will be softer than the road race set up so that the car can take a set in one corner before the driver asks it to turn the other direction.

 

There's lots of good reading out there on this and it's very cool when you get the set up right...or closer to right....and the car just sticks and flows.

 

Do you know the weight of your car? Do you have the 6 or the 8?

 

dave

[Anaximander]

Dave, I have the GM 3.8L Supercharged V6 which is fairly heavy from what I have been told. Apparently, a GM LS1 V8 engine is lighter than my V6? One of challenges for a newbie is that my driving skills in Solo events are rather limited although I used to race go carts as a kid and have lots of experience with high performance sport bikes and aircraft. I am still experiencing a fairly steep learning curve. I was a little cautious to change anything but was assured that the setup I changed to was preferred by others with similar cars/engines that have been doing Solo events for many years. I have only run 2 Solo events with the original 500#/250# springs on the car and hopefully will have an opportunity this coming weekend to determine if the change has much change in performance. What is additionally problematic was that when I was changing the springs, I noticed that the left rear coil was not even compressed which caused great concern in terms of initial suspension setup. Furthermore, I reset all of the GAZ shock preload settings to the recommended values and did not expend further energy to determine what they had been set to originally because the car had not been set up by the original builder for autocross. I have a pretty good handle on what my Avon A11 compound slicks are doing on the surface I am running on so should be able to get a bit of a handle on whether or not the slightly stiffer springs and proper preload settings have moved me forward or backward. Hopefully, the weather that has been rainy in South Georgia/North Florida will be clear this coming Sunday so I can see where I stand. The manufacturer's representative (Glenn Minehart), had mentioned a story about how profoundly important the preload settings were for the shocks when he related a story about one of his customers that reluctantly and cautiously changed them to the final setting only to find that the final recommended values decreased his time significantly.

Edited June 13, 2017 by Astro Bob

[Davemk1]

Cool - have fun with the learning!

 

 

dave

[Anaximander]

I called Scott Minehart of Stalker cars this morning on an unrelated matter and asked him about the coil values that I had just changed to. He advised that with the 600#/300# combination, he was able to place 3rd in the Solo Nationals in E-Modified with this shock setup on a Classic Stalker. He also mentioned that Dennis Brunton had used the same coils over the GAZ shocks to win road races. Apparently, for the Classic Stalker, the spring weights have utility in different kinds of genre which was a bit surprising to me but helped me to realize why his father had recommended the increased spring values. I still can't wait to try them out on course to see how the changes in handling will affect my performance. I am trying to act like a sponge and take in all of the knowledge I can find about how to be faster/better and improve my skill set for a unique form of driving (SCCA Solo).