Learning Suspension tuning

Keckster

Well-Known Member
I recently signed up for HPAcademy and started working hard on their suspension tuning courses. With a lot of measuring and crunching numbers I feel like I have missed something since the stock springs from DF are almost double what I'm calculating. I've only calculated for the front so far as the numbers seem wildly off. Lets type it out for sanity and double check

Step 1) Determine desired suspension frequency

-Based on the following chart provided by HPAcademy I chose to use 1.5Hz as the target frequency. I have little experience on what frequency I'm looking for but based on the other options I believe this one makes the most sense as the goblin is definitely high performance street but not far enough to be considered a low down force car. If anyone has any insight of a frequency they would choose please let me know.

38629



Step 2) Measure weight at front/rear

-My front build weight may not be fully accurate as I weighed it just before winter and made a few small changes but should be in the ball park.

-Front weight: 628.6 lbs.
-Corner weight: 628.6lbs/2 = 314.3 lbs.



Step 3) Measure Unsprung weight

-Front Unsprung weight: 89.5 lbs.



Step 4) Calculate Sprung weight

-At this point I also converted to SI units to stay consistent with the course calculations without have to convert constantly

-Front sprung weight: 314.3 lbs. - 89.5 lbs. = 236.7 lbs.
- 236.7 lbs. = 107.37 kg.



Step 4) Measure Motion ratio

-I did find a thread where Lonny posted that the front motion ratio is .65in. damper travel per 1in. suspension travel which would come out to a motion ratio of about 1.54​
-When I did my own measurements I found a motion ratio of about 1.73​


Step 5) Spring rate(K) Math

K wheel = M Sprung corner * 4π^2 * F^2
K wheel = 107.37 kg * 4π^2 * 1.5^2 = 9537.3 N/m * 1000 = 9.54 N/mm
K spring = K wheel * MR^2
K spring = 9.54 N/mm * 1.54^2 = 22.62 N/mm / .175 N/mm = 129.25 lbs./in. (Lonny MR)
K spring = 9.54 N/mm * 1.73^2 = 28.6 N/mm / .175 N/mm = 163.43 lbs./in. (measured MR)
Step 6) Purchase new springs

Based on the calculations I found need somewhere between a 130 to 160 lbs./in. spring rate for the front end of the goblin. Obviously there are other driving factors than just the static weight but based on my studies into motorsports this appears to be a proper starting point. I see some seasoned goblin racers running upwards of 500 lbs in the front and lowering the rear spring rate to around 300 lbs. I would love to hear some reasoning into any calculated decisions in this department as I learn more.​
 

Desert Sasqwatch

Goblin Guru
Other factors play into the choice of springs - anti-dive (and anti-squat & anti-lift in the rear), roll center/control arm instant centers, available suspension travel, anti-swaybar (if used), tire pressure/TW rating and likely some I'm not thinking of right now. I'm pretty certain, based on the experiences of many, many Goblin builders driving on the street and the track that springs under 200lbs are really too soft and the front end will bottom out quite often - which can damage the QA1 shocks. Calculations on paper can provide guidance for designing and a good starting point, but only real world testing can determine how far in the park the equations can get. Many of us have put pencil to paper and have lots of predicted results, but given the stock front suspension geometry requires stiffer springs. No argument intended with this response, just wanting to provide the experiences of those who have already been down this road.
 
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Keckster

Well-Known Member
thank you for the insight @Desert Sasqwatch i definitely appreciate the advice! Do you run the stock springs from df or have you done some experiments of your own! Would love to learn more
 

Desert Sasqwatch

Goblin Guru
I am not that far into my build yet - the information regarding spring rates and driving experiences come from the numerous inputs from the forum suspension section, build logs and the almost weekly inputs from the many owners how autocross their Goblins. Springs rates from 225 up through 500 have been tried and reported for success - or not so much - and being shared among the many racers has been proven to work. Many have settled on the 350-400 lb springs for the front to keep body roll to a minimum and avoid bottoming out during braking in the rough stuff. Don't get me wrong, the softer springs are desired to keep tires in contact with the pavement and some changes to the suspension geometry from stock would help to better utilize softer springs - namely improvements in anti-dive and anti-lift, plus optimizing the roll center - would help minimize the preloading of the spring during braking and turn-in.
 

Keckster

Well-Known Member
After reading through your comment Sasqwatch I went and measured the front suspension geometry of the goblin and found that both our a arms are mounted parallel to each other from a side view. that would mean there is no instant center? I feel like this may be a big problem as to why we get so much dive on braking. I'm gonna do some more studying and see what kind of changes I am capable of making. ideas welcomed!
 

Gtstorey

Goblin Guru
Maybe assemble the car first as designed and then decide what changes need to be made, unless building it is the goal, not driving it.
 

Desert Sasqwatch

Goblin Guru
Then please, read through my build log - the last 5-6 pages - which has some information about the process I followed to build in anti-dive into the existing frame. There are limitations to what can be changed and still have sufficient clearance to mount the control arms. PM me if you have any specific questions.
 

GoblinGuyZ

Well-Known Member
If anything I find most persons playing with suspension tend to almost always go stiffer in the front, some suggesting as much as 500lb springs and then quite soft in the rear particularly if you are running the base model sway bar. I have on 350 at the front now and think it is a tad on the softer side so may play with a 400 or even 450 which should also help give the rear a bit better braking bias. Right now I have my wilwood proportioning valve almost all the way favoring rear bias and my front still locks up before the rear. The back is indeed quite stiff and for acceleration grip I don't think it squats quite enough so will probably run slightly softer there as well.

As DS said, paper theory is one thing but there are so many variables I dont think you can really rely on that I would much rather rely on the collective input of real world tests and reviews others have given and then tailor to suit your needs and style.
 

Desert Sasqwatch

Goblin Guru
Keep in mind that stiffer/softer springs can have a significant affect on the understeer/oversteer in the chassis setup. Slight understeer to neutral steering is the desired balance for the majority of drivers - even though oversteer may seem desirable, it can be a problem in some situations (getting caught in the rain, maneuvering during panic stops, etc.).

In the front, adding stiffer springs induces understeer, if the rear springs are not also stiffened or the swaybar is not added or also stiffened to help maintain chassis balance. On the flip side, if someone does add the swaybar to the rear suspension to help limit rear bodyroll, then the front springs rate should be increased to compensate. It is a balancing act and some trial and error is typically required to get it close to correct - no two Goblins are the same.
 

GoblinGuyZ

Well-Known Member
Keep in mind that stiffer/softer springs can have a significant affect on the understeer/oversteer in the chassis setup. Slight understeer to neutral steering is the desired balance for the majority of drivers - even though oversteer may seem desirable, it can be a problem in some situations (getting caught in the rain, maneuvering during panic stops, etc.).

In the front, adding stiffer springs induces understeer, if the rear springs are not also stiffened or the swaybar is not added or also stiffened to help maintain chassis balance. On the flip side, if someone does add the swaybar to the rear suspension to help limit rear bodyroll, then the front springs rate should be increased to compensate. It is a balancing act and some trial and error is typically required to get it close to correct - no two Goblins are the same.
good points. On the goblin oversteer can somewhat be induced by steering input or throttle anyway. I would much rather have slight natural understeer
 

Keckster

Well-Known Member
definitely agree on slight understeer being preferred. I ran the SS sway bar last year and it caused the rear end to be way too loose mid corner. down graded to the NA SS bar which is 22mm rather than the 24, will be testing the difference. I'm gonna be focusing on tuning the whole package but found that the arm angles is the biggest issue as of now.
 

Desert Sasqwatch

Goblin Guru
The base model NA bar is 18mm and has been reported to be a good compromise between track and road handling, plus the front spring rate can be reduced somewhat as there is less oversteer induced by this swaybar. Just something to try if you get the chance. ;)
 

Josh's09

Well-Known Member
I am not that far into my build yet - the information regarding spring rates and driving experiences come from the numerous inputs from the forum suspension section, build logs and the almost weekly inputs from the many owners how autocross their Goblins. Springs rates from 225 up through 500 have been tried and reported for success - or not so much - and being shared among the many racers has been proven to work. Many have settled on the 350-400 lb springs for the front to keep body roll to a minimum and avoid bottoming out during braking in the rough stuff. Don't get me wrong, the softer springs are desired to keep tires in contact with the pavement and some changes to the suspension geometry from stock would help to better utilize softer springs - namely improvements in anti-dive and anti-lift, plus optimizing the roll center - would help minimize the preloading of the spring during braking and turn-in.

Super noob question here, but how do I measure the springs tension on the goblin? I feel like on the front, I just tightened them quite a bit, but I have no clue on if its too much or too little. I am working to install the rear now, should I just install them and figure all that out later? Or is it something I should do now? (Responses with few/no technical terms appreciated)
 
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Josh's09

Well-Known Member
Update, I rewatched the front suspensions and saw the coilover was just tightened until there was no slack in it. Still wondering how to measure the spring tension/rate.
 

ATMironov

Well-Known Member
For what its worth, I'm starting out with 550lb Front, 450lb Rear springs, 3/4" front adjustable sway bar, and 18mm rear non adjustable sway bar.
The car can theoretically make up to 450lbs of down force at 100mph, and almost 900lbs at 140mph.
The car is purpose build for the race track. It will see minimal street driving.

This is just a starting point, I'm sure I'm going to be tweaking suspension for a long time.
 
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escapepilot

Goblin Guru
Update, I rewatched the front suspensions and saw the coilover was just tightened until there was no slack in it. Still wondering how to measure the spring tension/rate.
Your spring rate is probably a constant such as 300 lb/in of compression. The rate is a characteristic of the spring and you can't change it, only preload it by tightening down the retaining rings. There are variable rate springs, but that is beyond the scope of this response....lol.

As Gts said, you just preload them until the lower A-arms are horizontal when it's done.
 
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