Handling issues - need adjustment advice Options

[slivel]

I found the attached chart that shows the stock spring specs.

Can you post http link to that chart? It is interesting and I don't think I've ever seen that.

I'm not fully sure I believe those rates. At 50 lb/in it would take 10 inches of spring pre-load to support the approximately 500lbs of corner weight that a 914 will have. Seems to me they would have to be very long springs to get the pre-load needed to support the vehicle.

I was going to assemble my springs tonight but I'm thinking maybe I need to do a rate measurement just as a rationality check to this chart.

It might be right, not sure. I'm not familiar with the stock springs on the 914/4 but if the chart is right and the free length of the spring is over 18 inches, that allows for a lot of preload. The motion ratio of the rear suspension (stock) is about 1.296 and is beyond the pivot point which gives a leverage advantage so the spring rate would be multiplied by 1.296 - I think.

For most of my racing I had 350 lb/in linear springs on the rear. Now for the street I have stacked springs that give a progressive rate that starts at 150 and goes to 270 at full compression. Attached is the spreadsheet that I used when doing my development.
 Suspension_Worksheet.a.xls ( 33.5k ) Number of downloads: 47

[slivel]

Here is another spreadsheet just in case anyone is interested in progressive springs.  Spring_rates.xls ( 35k ) Number of downloads: 46

[Shivers]

Thank you

[Brian Fuerbach]

I found the attached chart that shows the stock spring specs.

Can you post http link to that chart? It is interesting and I don't think I've ever seen that.

I'm not fully sure I believe those rates. At 50 lb/in it would take 10 inches of spring pre-load to support the approximately 500lbs of corner weight that a 914 will have. Seems to me they would have to be very long springs to get the pre-load needed to support the vehicle.

I was going to assemble my springs tonight but I'm thinking maybe I need to do a rate measurement just as a rationality check to this chart.

It might be right, not sure. I'm not familiar with the stock springs on the 914/4 but if the chart is right and the free length of the spring is over 18 inches, that allows for a lot of preload. The motion ratio of the rear suspension (stock) is about 1.296 and is beyond the pivot point which gives a leverage advantage so the spring rate would be multiplied by 1.296 - I think.

For most of my racing I had 350 lb/in linear springs on the rear. Now for the street I have stacked springs that give a progressive rate that starts at 150 and goes to 270 at full compression. Attached is the spreadsheet that I used when doing my development.
 Suspension_Worksheet.a.xls ( 33.5k ) Number of downloads: 47

Impressive (IMG:style_emoticons/default/beerchug.gif)

[Superhawk996]

So I did a quick rationality check tonight using the highly scientific bathroom scale and tape measure.

168 lbs at 1.75" travel = 96 lbs/in for the 914Rubber 100lb/in spring. Pretty close.

quick design comparison using an online spring calculator
https://www.thespringstore.com/spring-calculator.html

Design #1 close to 914Rubber
9.6mm wire diameter
105.5mm OD
380 mm free length (15 inches)
6 effective coils
Result 90.795 lb/in (sorry for mixed units -- Imperial units for ease of comparision)

Design #2 close to published stock data and the chart data above
9.6mm wire diameter
105.5 mm OD
470mm free length (18.5 inches)
10 effective coils
Result 54.477 lb/in

Design #3 published data from factory manual
9.6mm wire diameter
95mm mean OD -- this works out about the same 105.5 mm OD I measured directly
460 mm free length (18.1 inches)
10 effective coils
Result 54.477 lb/in

(IMG:style_emoticons/default/idea.gif) So I was wrong assuming that the chart data seemed low at 50's ish lbs/in rate.

That is super interesting since I've always been lead to believe that stock springs were in the 90-100 lb range. I guess I'm guilty of propagating mythology.

Anyone have an OEM stock spring to confirm free length? Super interesting information that I should have verified for myself long ago!

[mepstein]

I always heard stock was 60-70 and 100 was an upgrade for performance.

[Brian Fuerbach]

So I did a quick rationality check tonight using the highly scientific bathroom scale and tape measure.

168 lbs at 1.75" travel = 96 lbs/in for the 914Rubber 100lb/in spring. Pretty close.

quick design comparison using an online spring calculator
https://www.thespringstore.com/spring-calculator.html

Design #1 close to 914Rubber
9.6mm wire diameter
105.5mm OD
380 mm free length (15 inches)
6 effective coils
Result 90.795 lb/in (sorry for mixed units -- Imperial units for ease of comparision)

Design #2 close to published stock data and the chart data above
9.6mm wire diameter
105.5 mm OD
470mm free length (18.5 inches)
10 effective coils
Result 54.477 lb/in

Design #3 published data from factory manual
9.6mm wire diameter
95mm mean OD -- this works out about the same 105.5 mm OD I measured directly
460 mm free length (18.1 inches)
10 effective coils
Result 54.477 lb/in

(IMG:style_emoticons/default/idea.gif) So I was wrong assuming that the chart data seemed low at 50's ish lbs/in rate.

That is super interesting since I've always been lead to believe that stock springs were in the 90-100 lb range. I guess I'm guilty of propagating mythology.

Anyone have an OEM stock spring to confirm free length? Super interesting information that I should have verified for myself long ago!

Thanks for sharing your measurements. I am really interested to know what my springs are but not enough to to pull one out.

So if the 914rubber springs are 9.6mm wire and 96lb and my springs are 10.4mm diameter, would the larger wire automatically translate to a higher pound rating?

[Superhawk996]

So if the 914rubber springs are 9.6mm wire and 96lb and my springs are 10.4mm diameter, would the larger wire automatically translate to a higher pound rating?

Yes larger wire = higher rate but don't forget that you have coating thickness on the wire. I'd assume the powder coat on springs is at least 0.5 - 1.0 mm thick.

If you can see the coils, count them and them if you're around 8 coils, I'd say you're probably running 100 lb/in springs

Assuming you're around 100 lb/in spring rate, to take us back to your original understeer behavior that you don't like, your choices are add rear bar or increase rear spring rate to decrease steady state understeer assuming you stay with the 19mm front bar.

I ran 140's back in the day but I also gave up a good bit of ride quality and back then my car was biased toward oversteer when running stock torsion bars in the front and no anti-roll bars, 185 tires.

[slivel]

This thread may have played out but I just love to explore the complexities of chassis and suspension. I think suspension and chassis engineers must be some of the most gifted in the auto world. In a previous career in telecom we said the same thing about antenna designers.
Here's a thought experiment: Many do not realize that tires have a spring rate. If you change out your stock wheels and tires, say 15" wheels and 60 ratio sidewalls for 16" wheels with 50 ratio sidewalls you have effectively increased the overall spring rate of the suspension. When the tire encounters a bump, the suspension takes up some of the shock but not all. The tire deforms and the sidewall absorbs some of the shock too. Low profile tires with short sidewalls typically have a higher spring rate. So the higher performance of low profile tires is due in part to increased spring rate, though tread design, compound, and carcass are also a big part.