Tyres and Molding

Discussion on Design, Build and Mechanical Theory.

Postby slurp on Wed May 09, 2007 2:02 pm

I'd say the softer compound would generally lead to better static friction but agree that the loss of traction and bounce can be atributed to tyre flex.

With tyre flex increasing with softer compounds this backs your observations that Shore A 20 was not only better for traction than Shore A 30 but also Shore A 10 due to the flex and bounce.


Put aside the bounce for the moment, when you consider that the frictional force is independant of contact area then the softer compound would be better as the contact compound.

Trying to draw understandable pictures or models is not easy. I've been thinking in terms of brush brissels, elastic bands even a series of flexable rulers running radial out from a hub. Where you are looking at the stored energy from the deformation and how that springs free when the limiting factors are reached (frictional forces to loading the tyre vs weight and spring like load building in the tyre).

What we are wanting to do is reduce the spring load that can be enduced in the tyre such that it does not release to give us the bounce. It might be effected by the depth of soft contact compound as well as the base material but the control of compound mixing becomes a large factor too. Oh, and not forgetting the control mechanism and smoothness of drive may also factor as a release trigger for these forces!


As you've said, you've had some fantastic results but there seems to be a practical limit to what you can repeat. Hmmm, that sounds like I'm knocking the investigation - I don't mean too.... just some may find it easier than others.

It all leads back to other ideas - think smart, it might bridge a technical short fall. As per other posts, we may do our best and still fail head to head but win if we get a different attack angle.

Now, just how is the fastest, stongest, smartest.... :wink:

regards,
Colin
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Postby slurp on Wed May 09, 2007 6:53 pm

Back to easy process, low cost, small quantity and easily available I popped into our local Halfords, ok other automotive parts stores will be cheaper... but they where open at 6:15PM.

They had Hermetite Instant Gasket £4.49 for 38g, but I can't see enough info to make me interested. Then there is a whole range of Loctite products... :D

5699 - Oxime silicone, grey sealant 45 to 75 Shore A at £4.99 for 99g
5920 - Oxime silicone, copper coloured gasket/sealant 23 to 38 Shore A at £4.99 for 99g
5926 - Acetoxy silicone, blue gasket >25 Shore A at £3.49 for 40ml.
595 - Acetoxy silicone , clear Potting/Coating/Sealing >/= 20 Shore A at £4.49 for 40ml
598 - Oxime silicone, black gasket 26 to 40 Shore A at £4.99 for 95g

I'm not certain on the differences between Oxime and Acetoxy Silicone but both use moisture to start the cure process. As such I guess they're fine for wheel coating as per earlier suggestions but the cure speed may be different for more closed molds. I think I might start with the 5926 and avoid the 5699 :wink:

regards,
colin
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Postby slurp on Tue Jul 03, 2007 10:58 pm

Image

First application of Loctite 5926 was a little thin, probably not doing much more than filling the gaps. It pealed out easily, I need to look at a thicker application and keying it to the surface more reliably. I really need to do more testing on this... tamiya tyres may not be the right item to apply to but it may be surface dust and contamination stopped effective bonding.

Image

Despite my earlier exclamation of What A Result! the lint rollers I used didn't hold-up to prolonged use with spinning wheels and stalled robot. You can see the sample of material from Insumountable is much thicker. If taking this route to tyres this will be the stuff:-

http://www.caraselledirect.com/zcart.mo ... hp?PID=106

regards,
colin
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Re: Tyres and Molding

Postby PeterW on Mon Jul 12, 2010 6:39 pm

I have been thinking of trying a new material for tyres and tracks.
Because we tend to run the cometitions starting head to head it all comes down to traction.
The sticky rubber I use would fail the pick up paper test if in new condition so I tend to run them dirty with much less grip.
After some searching and reading up on the internet I have found some two part Platinum Silicone rubber which does not require moisture to set and which has a shore value of A10 and quote unbelievable tear strength with 700% elongation at break. They also do another one at A2 but I thought that may be too soft. I have ordered a pack of the A10 (£40 inc P&P) and I will let you know if it is any good.
Almost forgot http://shop.shopatcuretime.co.uk/epages ... one_Rubber
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Re: Tyres and Molding

Postby PeterW on Tue Jul 13, 2010 7:49 pm

The material arrived next day and so far is looking good. To do a test mould I cut a rectangle out of a piece of 2mm plasticard and used elastic bands to hold it to a second piece with a layer of cling film between them.
I then mixed up a small amount of the rubber and filled the mould. It was slightly more viscous than I had hoped but still quite pourable.
see below mould parts with the piece of rubber I moulded.
http://i109.photobucket.com/albums/n66/ ... MG1051.jpg
Due to the viscosity and the relatively fast cure time the are still some very small bubbles in it but not enough to weaken it.
See moulded rubber streched to about 300%.
http://i109.photobucket.com/albums/n66/ ... MG1052.jpg
This was as far as I cared to stretch it at least until it has had a full 24hours to cure fully.
The rubber is not sticky but does appear to be quite grippy. I need to set up some sort of test bed where I can get some actual figures.
I probably need to get my self a spring balance like one of these that are push pull.
http://cgi.ebay.co.uk/DOLPHIN-PUSH-PULL ... 439ff791cd
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Re: Tyres and Molding

Postby slurp on Wed Jul 14, 2010 3:47 pm

Sounds interesting.

I understand the ratio of the mix can impact upon viscosity, cure time and hardness. It might be worth giving it a try. The tackiness may be related to mixing and cure which may go over time.

From previous employment I noted most of our liquid rubber was subjected to vacuum to help remove the bubbles. I'm not sure how easy this would be to replicate at home. If you can do it beware the initial expansion vs container volume!

The ballance is a great idea, mine where low cost online. Have you seen Rick Brooks Exert-O'Meter?

regards,
Colin
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Re: Tyres and Molding

Postby PeterW on Fri Jul 16, 2010 7:48 pm

I think with this rubber they don't recomend changeing the mix but they do sell aditives that make it less viscous and another that reduces the shore value but I believe they reduce the strength.
I have now moulded my first tyre which proved a bit tricky.
I machined a circular grove in a piece of fibre glass so the wheel was a tight fit around the inside edge and a plastic tube a tight fit inside the outer edge.
http://i109.photobucket.com/albums/n66/ ... MG1061.jpg
http://i109.photobucket.com/albums/n66/ ... MG1062.jpg
I covered the wheel spoked surface with tape to stop the rubber going inside the wheel.
The main problems were first getting the mould filled with no air gaps using a piece wire and then trying to get out the air bubbles which I failed to do.
The tyre wasn't too bad once I removed all the excess bits but bonding it to the wheel is proving difficult.
At the moment it is held on with double sided tape but that is not great.
http://i109.photobucket.com/albums/n66/ ... MG1064.jpg
I am now experimenting with silicone sealer as an adhesive but that needs an over night cure.
The scales arrived and on my arena which is a very matt finish both Insumountable and Stealth 2 took a front push of 600 to 620gms to move them.
http://i109.photobucket.com/albums/n66/ ... MG1059.jpg
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Re: Tyres and Molding

Postby PeterW on Sat Jul 17, 2010 9:10 am

Rather unexpected result the silicone sealer stuck to the tyre silicone rubber but not the wheel.
On closer inspection the wheel feels very slippery I guess it has a coating of silicone left after the nolding process.
I have cleaned the wheel with white spirit and sanded the surface so it is less shiney and am now trying again.
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Re: Tyres and Molding

Postby slurp on Sat Jul 17, 2010 9:56 am

PeterW wrote:I have cleaned the wheel with white spirit and sanded the surface so it is less shiney and am now trying again.


I'd go for the roughest paper you've got to give it a good key. I think the enclose moulds where you inject the silicone give you a better chance of pushing the material into the rough surface.

best regards,
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