simon's learning what to do with the polo thread

**simon k** · 02-10-2018, 01:13 PM

Originally Posted by Sydneykid

Quite a lot of over thinking going on, so let me try and simplify it.

thanks Gary - over thinking is normal for me, as is going around in circles trying to solve the same problem the same way and expecting a different result...

Originally Posted by Sydneykid

The top spring seat sits on the shock shaft, above the shock body. As such it doesn't need a bearing, it moves (angularity) with the shock shaft. You just need a solid spacer between the spherical (in the strut top) and the spring seat to allow for the angularity changes, so the spring seat doesn't hit the strut top (that would break the shock shaft).

I was thinking a bearing between the spring seat and the spherical bearing to allow the spring to rotate with the shock body rather than twist or grind on the seat or make the spherical bearing rotate. Same as this setup - I gather a shim does the same thing, but over-engineering goes with over-thinking... I'll leave that idea out

Originally Posted by Sydneykid

If the spring rate is, say, 7kg/mm (~ 400 lbs/inch) and the movement ratio is 0.9 and the leverage ratio is the same 0.9, then the effective rate (at the wheel/tyre) is 400 x 0.9 x 0.9 = ~325 lbs per inch. If the front (one side) weighs ~840 lbs (380 kgs) then the spring will compress 840/325 = 2.6" (66 mm) just to hold the car up.

Rule of thumb, we aim for around a minimum of 100 mm of travel (on the front of an FWD race car), usually 70 mm bump (compression) and 30 mm rebound (extension). A combination road and track car should be a fair bit more than that, say 130 mm and 40 mm. The bump (compression) measurement is before bump stop contact.

Cool - this answers some questions - my spring compression calculation was on the right track, but I was leaving out the effective/wheel rate. A 250lb spring ends up with 87mm compression just to hold the car up. With a 200mm spring that only leaves 113mm of bump - I don't know how much I have now, but there is a certain bump on my way home from work where the tyre scrapes on one of the screws holding the mudguard liner... I'd guess at 60-70mm of compression at the tyre, so 80% of that at the damper. I think I really do need to measure how much travel the damper has at the ride height

However, with that second diagram showing damper compression with my jack under the balljoint there's only 90mm of damper travel anyway... I don't think 130mm of bump travel is even possible without > 20kg springs and 6" of clearance between the tyre and mudguard!

Originally Posted by Sydneykid

Inverted Bilsteins have internal bump stops. Take the shock body out of the strut and measure the bump stop length, commonly 50 mm or 75 mm.

understood - the instructions that came with the Bilsteins said to use the stock bump stop, so I think I installed that. I also used the dust guard, so I can't actually see it... regretting not taking some photos while I was putting them together

Originally Posted by Sydneykid

You shouldn't have to use helper/tender springs on the front of a FWD car, there is enough weight (from trapped) to compress the spring to ride height. The rear is of course a different story as there is less weight to compress the spring and the spring rate should be much the same as the front.

Common FWD front spring free height is 7" or 8", occasionally 10". Looking at your diagrams you would need to move the bottom spring seat up around 70 mm to keep an 8" spring trapped (or use a spacer). Coil bind height is a spec provided by the spring manufacturer. It varies according to free height (obviously) but it also varies according to the number of coils and the wire diameter. FWIW, a 400 lb spring for example can be designed with less coils and thinner wire to give more travel or thicker wire and more coils to give a lower frequency. That's the spring designers choice depending on the objective. Obviously there is limit as to how few and how thin as that affects the spring durability which is also dependant on the material used and the winding method. High quality spring steel, cold wound is more durable and hence can be designed with more travel than lower quality spring steel hot wound.

Hope that helped.
Cheers
Gary

It does help, but gives me more questions I guess... a mate has offered me some 2.25" x 5kg/280lb springs, not sure what length but I'll grab them anyway, apparently one of the members in our local club has a spring tester he doesn't mind people using...

**sambb** · 02-10-2018, 02:50 PM

remember simon I've got some 7in (180mm) 7kg springs you can use if you get stuck. Now that I'm running that front rate on a stock FARB its actually quite streetable.

For what its worth my 200mm springs are compressing to 155mm fitted (seat to seat) = 45mm compressed.

**Sydneykid** · 03-10-2018, 10:51 AM

Originally Posted by simon k

understood - the instructions that came with the Bilsteins said to use the stock bump stop, so I think I installed that. I also used the dust guard, so I can't actually see it... regretting not taking some photos while I was putting them together

I'm not sure that the OE bump stops would fit inside the strut tube. On an OE non inverted shock the bump stop sits on the shock shaft, at the top, so they tend to be larger in diameter than the bumps stops designed to fit inside strut tubes. The rear Bilsteins being non inverted could use the OE bump stops (if they have them) and the OE dust covers.

Cheers
Gary

**Sydneykid** · 03-10-2018, 10:52 AM

FWIW, 111 mm at the shock is 111/0.9 = 123 mm travel at the wheel.

Cheers
Gary

**simon k** · 03-10-2018, 11:03 AM

Originally Posted by Sydneykid

I'm not sure that the OE bump stops would fit inside the strut tube. On an OE non inverted shock the bump stop sits on the shock shaft, at the top, so they tend to be larger in diameter than the bumps stops designed to fit inside strut tubes. The rear Bilsteins being non inverted could use the OE bump stops (if they have them) and the OE dust covers.

Cheers
Gary

ah - yeah, you're right. If I used them, I would have put them at the top of the shaft... so thinking about it that way it's probably impossible for me to have used them...

**simon k** · 13-10-2018, 05:53 PM

put in my SEAT intercooler today, all ready to go but I'm one hose clamp short

I had to take a little break though, because I dropped it on my big toe!

of course I was wearing my best safety-thongs...

simon's learning what to do with the polo thread-20181013_151129_1539416852875_resized-jpg

hurts like a mother-f*cker...

**MattSA9N3** · 13-10-2018, 06:49 PM

Ouch, plenty of swearing I'm assuming?

**simon k** · 27-10-2018, 08:27 PM

I've been mucking around the last few weeks machining my rear plates to give a bit more camber and put a couple of mm toe out into it. I CADed it all up and worked out the difference in thickness the plates needed to be at each mounting bolt

The 3/8" plates are installed in the car, I figured that if I used one of the 1/2" plates it'd give me plenty of room to make mistakes. The 3/8" plates don't give a lot of room for error. Turned out that was a good move because it did take me 3 goes to get the angles right... first I had them all upside down so I would have had massive toe in and positive camber, then I had the toe right but the camber was upside down, then I got it right...

So, first I made a machining boss to hold the plates, it's just a bit of solid round bar with a plate welded to it. I drilled and tapped a hole in the bar and drilled holes in the plate before welding.

simon's learning what to do with the polo thread-rear-plates-2-jpg

Then I machined the face of the plate flat

simon's learning what to do with the polo thread-rear-plates-1-jpg

and mounted one of the plates to it to drill a hole in the centre of the plates, not necessary, but easier than doing it on the drill press

simon's learning what to do with the polo thread-rear-plates-3-jpg

(spinny action shot)

simon's learning what to do with the polo thread-rear-plates-4-jpg

so then the plate can mount on the machining boss using the centre bolt

simon's learning what to do with the polo thread-rear-plates-6-jpg

next I welded some little bits of rod into the holes to locate the plate, and mount spacer shims (as shown - shims cut out shim stock and drilled)

simon's learning what to do with the polo thread-rear-plates-7-jpg

Spacer shims and measurements (in mm) shown for each hole. Hard to get the shims to exactly the right thickness but it's close, within ~0.03mm. I aimed to get them all a little bit thicker than the correct measurement, so all of the errors were at least in the same direction

simon's learning what to do with the polo thread-rear-plates-9-jpg

so then I mounted the plate, with the shims making it sit at the angle I wanted

simon's learning what to do with the polo thread-rear-plates-8-jpg

then machine it flat - here's the finished product for the left hand side against the original. It ended up about 9mm thick, down from 12.5

simon's learning what to do with the polo thread-rear-plates-10-jpg

the knob in the middle where the bolt was holding it to the boss fits neatly inside the stub axle

I installed the left hand side this afternoon, then took the plate from there and machined it to suit the right hand side... Machining and installing it only takes half an hour, just a bit of ****ing around up front. Was correct first time on the right hand side

I don't know what the camber is, but it looks even. I ended up with less toe than I wanted, it's more-or-less straight ahead, 0.1° on the left hand side and 0.02° on the right, totaling 0.6mm. Though that's just measuring with a stringline, the error margin is probably more than that....