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Vertigo35

Lost in TC spring rates ...

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Hi,

For a bit of context, I am currently rebuilding an HPI Sprint2 for racing at the local club. The stock shock springs on this car (black, 1.7mm steel, 7 coils) are incredibly stiff, presumably much too stiff for our very low grip race track. I tried to replace them with what I had on hand, that is the 53333 TC short springs that I use on my M-05, but from static testing they are too soft on the Sprint2, even the blue ones. So here I am looking for springs of intermediate stiffness...

I experimented a bit with a caliper and my electronic scale, and using this clever method I was able to quantify things a bit more https://youtu.be/vBmP-P9erBI

Results : The Tamiya blue spring needs about 1650g to be shortened by 10mm, while the HPI black spring needs 4500g, almost 3 times more ! 

 

To find springs with intermediate stiffness, preferably from Tamiya, I looked at this post https://www.tamiyaclub.com/forum/index.php?/topic/101612-information-on-tamiya-trf-42278-spring-rates/&do=findComment&comment=916741, and also at the petitRC listing https://site.petitrc.com/reglages/1-10thOnRoadSpringChartv2.pdf

The problem I have is that according to my testing, the 53333 blue spring has a rate of 165g/mm (and the HPI 450g/mm), thus I should look for springs somewhere in between, say around 250-300 g/mm. But in the petitRC list, the same Tamiya blue short springs "for mini" that I have are listed with a rate of 280g/mm, so values do not match at all, and therefore I don't know what new rate values I should be aiming at.

I think I have checked everything twice, but really can't figure out where I am doing a mistake. OK the method above is not super-precise, but should be the same principle as used by manufacturers ? Am I missing something ? Any help would be greatly appreciated ! 

 

 

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Thanks, good idea to buy a set of springs and experiment. Still, this set is made of 1.2mm steel, approximately equal to the 53333 springs, so I am not sure I would get the higher rate values I am looking for 🤔

Will try to find a set comprising 1.4-1.5mm steel springs and various number of coils. Preferably with known ratings, so that I can sort out this testing method mistery 😋

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Best option might be to just go along to that local club and ask a few racers if you can try some of their springs on the bench to see what seems vaguely right size and stiffness. Most will have a box full (I have most Xray touring car ones for example)

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What I did is precisely buy XRay 4S springs, rated c=2.5, 2.7 and 2.9.

Then tested the kitchen scale method, worked like a charm, I got 2550g, 2700g and 3100g for 10mm shortening, almost as expected.

This suggests that the 53333 tamiya springs are in fact much softer than most touring car springs, équivalent to c=1.2/1.5/1.65 in XRay rates (or 120/150/165 g/mm for red/yellow/blue springs respectively).

This softness is probably adapted to m-chassis, as on these cars the lower shock attachment is close to the A-arm tips. 

It remains that the values reported in the existing listings, and the idea that 53333 and 42168 have the same rates, might be wrong.

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I tend to agree, as I also have the yellow/black springs from the 54753 TT-02 CVA set, which I think are the same as 42168, and they are indeed as soft as the 53333 yellow.

 

20230118_205259.jpg

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The 42168 blue are rated at ~ 280g/mm on the tamiya spreadsheet

 https://www.tamiyaclub.com/forum/index.php?/topic/101612-information-on-tamiya-trf-42278-spring-rates/&do=findComment&comment=916741

But the presumably identical 53333 blue are much softer than XRay 2.7, as seen here (and the tamiya at 23mm is initially 3mm longer than the XRay). 

So something is not right... 🤔

 

 

 

20230118_205807.jpg

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I would only trust a chart from Tamiya Japan, not something that someone has put together at Tamiya USA (with good intentions no doubt).

I have a number of those springs and those rates are obviously wrong. You can calculate the rates quite easily, there are spring calculators in the web which give accurate results.

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ok, if we assume the tamiya USA sheet is wrong, then it could explain the results I measured. 

However the rates on petitRC listing (linked in the OP) for tamiya "short springs for mini" are also in the 220-280 g/mm range, so I don't get it.

There are tamiya "ProMod" springs in the petitRC list that are in the 115-150 g/mm range, which is closer to what I measure for 53333. Does someone know what these "ProMod" springs are ?

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The petit RC charts are also put together by people with good intentions, but good intentions doesn't mean you have a good method of determining spring rates.

The pro mod springs are for a very rare rear suspension conversion released for the trf414 (or maybe 415).

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