Keyword

TOPIC:

CC-01s: RV, Unimog, Bedford TM, Valp, Ford CMP, Series 3 Land Rover, SD Revopak 9 years 9 months ago #29217

:laugh: The canvas and small flowers are tip top Jonny :y: :y: :y:

Please Log in to join the conversation.

CC-01s: RV, Unimog, Bedford TM, Valp, Ford CMP, Series 3 Land Rover, SD Revopak 9 years 9 months ago #29229

Once I make & fit stronger lower arms to my CC-01/Blackfoot RV I'll need to fit stronger springs too - but this raises a question: how to measure springs, compare them & assess their suitability so I'm doing something a bit more scientific than going to my spares pile, picking out a spring, squeezing it, thinking "ooh, that feels harder" :whistle: & giving it a try.

There are some obvious measurements I could make with the naked eye and a vernier gauge, such as free length, fully compressed length, inner diameter, outer diameter, coil thickness, number of coils & so on.

For example, I picked out three obviously different springs:

A:
free length: 28.25mm
compressed length: 9.64mm
internal bore: 12.7mm
coil thickness: 1.24mm
number of coils: 8
weight: 4g

B:
free length: 48mm
compressed length: 11.2mm
internal bore: 14.3mm
coil thickness: 1.08mm
number of coils: 11
weight: 4g

C:
free length: 82mm
compressed length: 25.6mm (and the risk of having someones eye out)
internal bore: 14mm
coil thickness: 1.5mm
number of coils: 18 (linear spacing apart from doubling ring touching at each end)
weight: 12g

From experience I'd guess that A is from a small road car (M series)?, B is from a scale off roader (CC-01) or a front spring off a 2WD buggy with short front dampers (super hornet, etc), and C is from a heavy buggy or big wheeler with long shocks (Bigwig, etc), ans so (to a certain extent) I can judge their suitabilty - too short, too soft, and too long respecitively.

I can also do the subjective squeeze test, that gives medium, soft & hard as "results", which I can judge as being a bit too soft, far too soft & a bit too hard for my needs.

Assumption set 1 (internet myth/received wisdom): Tamiya springs are a linear, single rate - i.e. they're not progressive as the coil spacing or thickness doesn't change, there's no evidence or talk of different heat treatments on one end, and that where a a shock has a collar, using it to shorten or lengthen the spring affects (it's said) the ride height rather than the spring rate (even if Tamiya refers to adding/removing spacers as making the suspension hard, normal or soft).

Assumption set 2 (mine): for arguments sake an RC car weighs 1.6kg, so 400g per corner, and intended use determines the number of times this weight the spring must support; guessing here, maybe 4 times for fast road use, 3 times for scale off road, 6 for heavy buggies?

Assumption set 3 (following on from 1 & 2): that I could measure this at home on a basic set of scales by pushing down on the spring and see a constant weight from the point at which the spring starts compressing, right up to the point it reaches full compression & the weight reading is limited only by the force I can apply ... therefore expecting to see on the close order of 1600g, 1200g and 3200g on my samples.

The results:
A: 1g to 2300g (to out of range)
B: 1g to 1200g (to out of range)
C: 9g to 4300g (to out of range)

So, we can see that combining my erroneous assumption (even if there was some basis in fact on the maximum load) with internet myth leads to bogus theory. NSS :whistle:


My questions then: clearly my method was really no more informative than guessing the load rating by squeezing, are there any easy home method that works better?
And (bearing in mind that I can use Google and Wikipedia as well as anyone else), does anyone have any information on compression spring physics specifically relating to RC cars?

:)

Please Log in to join the conversation.

Last edit: by Jonny Retro.

CC-01s: RV, Unimog, Bedford TM, Valp, Ford CMP, Series 3 Land Rover, SD Revopak 9 years 9 months ago #29230

very interesting question. In first year science at uni we measured spring compression (and other motion) using a spring mounted horizontally above a ruler. One end was anchored securely and the other end was attached to a small cart on wheels that can move very freely (lowest friction possible). A known weight was place on the cart and then pulled back a specified distance from natural rest and the movement recorded. we used long springs, so it was slow and easier to measure. You can then compare different springs with same weights and pull back distances. This is probably the hardest to do, as its the response over time that you really want from a spring and an rc car spring is very short and won't pull back very far. From memory (this was many, many years ago) we recorded the first bounce back distance, the number of oscillations and time, but we were only doing simple calculations.

What would be better is if you could attach a pen or something and have a scrolling paper next to it, to record the motion. I've seen a similar setup at a tire place to measure the quality of a shock absorber, but I can't imagine it would be an easy setup though as the movement would be small in this case.

Maybe you can do a setup with a laser tape measure and have that recorded somehow. Either way, I think you want to measure a time and distance response from the spring after a weight is applied to be able to compare a spring to any other spring.

Please Log in to join the conversation.

CC-01s: RV, Unimog, Bedford TM, Valp, Ford CMP, Series 3 Land Rover, SD Revopak 9 years 9 months ago #29233

Stack weights on the spring and see how far it compresses.

Edit: Also, Shock collars do not (as you say) increase the spring rate. They will make the suspension harder though, since the force increases as a spring is compressed. The collars preload the spring so increase the initial force to compress it.

I can sell you a programmable electric actuator with force and position feedback that you could use to test them. Start at a mere £1000 (+VAT and delivery)

Please Log in to join the conversation.

Last edit: by Martin Bell.

CC-01s: RV, Unimog, Bedford TM, Valp, Ford CMP, Series 3 Land Rover, SD Revopak 9 years 9 months ago #29234

With a scale and a drillstand its quite easy to measure and calculate springrates or put them in a graphic. (as these are linear for most springs)
Other method is calculation but as Johhny says Google and Wiki make smart minds ;)

On assumtion 2 you also have to take into account the levarge created due to the postion of the spring and lenght of the suspension arms.

you could combine springs for a different charactaristic (axial does that)

Please Log in to join the conversation.

CC-01s: RV, Unimog, Bedford TM, Valp, Ford CMP, Series 3 Land Rover, SD Revopak 9 years 9 months ago #29235

I remember the academic book on the subject (dynamics) being very thick - and nearly everybody flunking it. :whistle:
Driving the car, it wouldn't just be about spring load but I think oscillation comes into play (won't link to the wiki page, lol).
Just throwing it out there - the higher it jumps after compression, the more energy it can store...
With that, I'll further refrain from starting to spew mindless theory.

Please Log in to join the conversation.

CC-01s: RV, Unimog, Bedford TM, Valp, Ford CMP, Series 3 Land Rover, SD Revopak 9 years 9 months ago #29236

Thought this was generally informative :

www.dailymotion...ng-sp_tech

Please Log in to join the conversation.

CC-01s: RV, Unimog, Bedford TM, Valp, Ford CMP, Series 3 Land Rover, SD Revopak 9 years 9 months ago #29242

Spray paint completed on the Blackfoot body ... time was I would have thought that was shiny enough (note reflections of lights on side) but I do want to improve it (see fuzzy reflections = orange peel on bonnet & roof) before doing the detail painting.

Also started work on the RV cab interior ... it would have made sense to have a plain "floor" just below window height & use the original driver, but I want to get a Sand Rover driver in there, so need the "floor" to be as low as possible. Not quite sure how I'm going to disguise all the boxing in though :whistle:

Dash is more a more complicated underlying shape than first appears (curved top, angles on front); filler became unworkable really quickly in the heat :(








Attachments:
The following user(s) Liked this: larbut, stingray-63

Please Log in to join the conversation.

Last edit: by Jonny Retro.

CC-01s: RV, Unimog, Bedford TM, Valp, Ford CMP, Series 3 Land Rover, SD Revopak 9 years 9 months ago #29251

Dunno if anybody else watched it but one thing about it is strange...
If you have a preload of 0.5 inch and need 5 pounds of force for that, then the spring rate is obviously 10.
So why would you measure it again?

That video starts to somehow become quite annoying after a while, I have to say...

Please Log in to join the conversation.

Last edit: by Edou.

CC-01s: RV, Unimog, Bedford TM, Valp, Ford CMP, Series 3 Land Rover, SD Revopak 9 years 9 months ago #29252

Edou, I did watch a bit of your video & although it was interesting, it didn't really answer the questions I had ... plus it started bringing back memories of GCSE Physics (14-16y, mostly maths with some experimentation with chemicals, + some with force, vacuum & electricity) and why I didn't enjoy A-level Physics (even more/deeper maths, lots of theory with practically no experimentation) ... enough moaning from me, perhaps I was being naive in hoping that there would be some simple resource telling me what spring I needed for a given weight at a wheel, but so far it looks a lot more complicated than that; method would have to be something like:

1- measure all spring dimensions

2- work out spring rate (home set up - drill press, kitchen scales & ruler)

3- find weight at each wheel (four sets of kitchen scales)

4- factor in (more research needed* :whistle:) shock angle & distance from fulcrum etc to work out force needed to hold each corner up

5- select spring on basis of all the above; there would probably be several nominally "right" answers at this point, e.g. a soft, long spring (low spring rate x lots of excess length = preload needed); a harder, shorter spring; or a hard spring barely longer than the free shock length (high spring rate x little excess length = preload)


At this point it's obvious that the issue of suspension sagging on the bench can be solved (* with more work re moments of force calculations), it tells us nothing about what would be best for the car in motion, the 3 "options" above are an obvious Goldilocks scenario, too soft (wallowy, falls over), too hard (skitters & bounces), and "just right" (maybe).

From this point you rely on educated guesswork/experience (in which case, why not stick with that from the start?) or you need to know the forces involved & throw a whole load more maths at it ... TBH unless I have a Eureka! moment I think I'd rather spend time building the RV than worrying about precisely which springs to use for "best" handling ... besides which RC springs are sold by car or are specific to the shock they come fitted on, they're not sold by size or spring weight.


:)

Please Log in to join the conversation.

Moderators: caprinutstingray-63AndyAus
Time to create page: 0.175 seconds
Cookies are required to make this site work. If you continue to use this site you permit us to use cookies.