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Best tyres for Jabs

Postby diablo » Wed Sep 30, 2015 2:15 pm

I have been searching for a new set of tyres for my SK and it looks like the chi=oice is very limited. The two tyre brands that seem to be recommended by Sky-Craft are:

1. Vrendestein 4.00-4 4PR or 6PR
2. Trellborg 4.00-4 4PR or 6PR

I was surprised to find that the specifications are quite low for the above tyres. Although they are marketed by sky-craft and sold as the ideal tyres for strip or hard runways flying, on checking specification for both the makes and rating of tyres, they seem to have a maximum operating speed specifies as 25 to 40 kph (around 13-22 kts). Bearing in mind that the Jabiru SK has a full flap stall speed of around 42 kts and the UL slightly lower, and considering the fact that in some cases you may want to come in faster due to a flapless or 1st notch flap setting, I am at odds how such a tyre as specified by sky-craft could be suitable for the aircraft.

Both tyres have a maximum load specified as 150Kg which would put them above the limit of a 430kg SK and well above for any other aircraft that lands on its main wheels.

I searched further afield and found the following tyres, but could not source in the UK.

http://www.junkers-profly.de/Reifen-Schlaeuche/Reifen/Sava-Reifen:::59_60_113.html

They are made for aeroplanes and specifications are much better. The SAVA 4.00-4 B11 66L are rated to 120 kph and have a load rating of 300kg (150kg on the above mentioned tyres). I believe that they are suitable and adequate for the task and should hold up in a heavier or faster than usual landings.

Cost is better than sky-craft's recommended tyre and specification are well beyond the flying wheel barrow tyre specifications ... On emailing the LAA I have received the following comment :

"Thanks for your input, however it is not unusual to find homebuilts with tyres that appear to be way under-rated in terms of speed rating, even to the extent of some aircraft types having been cleared for decades with wheelbarrow tyres – aircraft seem to get away with It, which I’ve always assumed must be because they don’t have to make long journeys on their wheels. I assume the speed rating is to do with continuous use, and probably the actual performance depends on the load applied to them as well. I think if they suffer from punctures its more likely to do with the number of plies rather than the speed rating as such. But it’s an interesting point you make"

I like to play safe and considering the importance of a tire (regardless how long it is on the ground for) which is what the aircraft depends on when taking off and landing , I think it is well worth spending 35 euros on a tyre that is made for the job rather than 50 quid on something that is made to push rubble to the skip and not made to spin at 70 odd MPH !
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Re: Best tyres for Jabs

Postby Steevo » Wed Sep 30, 2015 3:12 pm

For me, it's the inner tubes I worry about the most. Since owning my Jab, I have had a total of 4 tyre (inner tube) failures. 3 of them with me flying and one with a colleague who flew the aircracf back from maintenance for me. The actual loss of an inner tube/tyre is actually a fairly serious event on a Jab. On all the occassions, it has resulted in total loss of steering/control. One of the events was quite serious when the front tyre was flat upon landing and this caused the Jab to veer severly to the left with it only narrowly missing a Cessna waiting to cross the runway.

When the mains go, not only does it cause the the plane to go where it wants, it also results in very limited braking.
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Re: Best tyres for Jabs

Postby pilottrev » Wed Sep 30, 2015 4:05 pm

Trelleborg T991 tyres were recommended for the Shadow variants both microlight and streaks, I had them on my previous steed and a friend of mine uses them on his school Eurostar and they stand up to his students landings, they are the 6 ply variants ,they are not cheap but as previously stated a flat tyre can lead to disaster.
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Re: Best tyres for Jabs

Postby diablo » Wed Sep 30, 2015 5:06 pm

I still think we are cutting corners by fitting these tyres. The tech specs for the T991 are :

T991 6PR Farming speed 30 kph and max weight of 190 kgs

still not suitable on both counts. All it would take is a hard landing to splatter it! The 4PR has a max weight rating of 150Kgs ...
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Re: Best tyres for Jabs

Postby Garyo » Wed Sep 30, 2015 7:26 pm

Like wise on punctures, 2 in a 3 week time slot, still cannot figure out why, nothing in the tyre or wheel to cause it.

I suppose on the tyres there will be a safety margin, so will be x 1.5, so if rated to 100 mph, then good for 150 etc same as weight wise, if good for 100kg then tested to be safe at 150 kgs.
Same applies to pressure holding equipment and lifting equipment, steel structures etc
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Re: Best tyres for Jabs

Postby diablo » Wed Sep 30, 2015 8:59 pm

My point is that the tyres sold by sky-craft (the Jabiru agent) are not fit for purpose as they have a maximum weight rating of 150 kg to 190 kg as well as a limitation on speed.

Considering that most aeroplanes land on the main wheels and the fact that the all up weight over-exceeds the recommended maximum load on tyres (a Jabiru SK all up weight is 430Kg and an SP is 470Kg ), it is not one, but multiple parameters that are being exceeded (max speed by a factor of 125% or more and max weight by 33% or more).

Simply put and with a little maths ...

Jabiru UL (middle of the road) Aircraft weight = 450Kg
Maximum allowable weight on Tyre = 150 kg (multiply by 2 as there are two tyres) =300kg

On a soft controlled landing there will be around 450 kg on mains or approximately 33% more weight than the maximum allowed weight. On a hard landing it could be a lot more. Add the speed factor as we are probably landing at two to three times the speed rating of the tyre and the possibility of aircraft being above MTOW (easy mistake to make) and in my books, it seems we could be well beyond the extra 100% margin. I think it is quite fortunate that there have not been far more accidents because of this. I feel it is a safety issue and It needs to be raised.

Perhaps this is a factor that impacts inner tube life as well (constant pinching and rubbing) as in many occasions there are no traces of fracture or sharp objects in tyre following a puncture.

Considering some of the areas that seem quite trivial but seem to become quite sticky issues with the LAA, I am perplexed as to why they have not picked up on this matter and issued any guidance.
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Re: Best tyres for Jabs

Postby Garyo » Thu Oct 01, 2015 8:09 am

Hmm, get your point, sometimes with the LAA is best to let sleeping dogs lie.. may be worth putting the point to Skycraft and Jabiru, see what they say to defend the ratings
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Re: Best tyres for Jabs

Postby Steevo » Thu Oct 01, 2015 9:40 am

I was trying to look up information on aircraft tyres. It does seem to be a common event in the aircraft world that the performance and weight of the aircraft regularly exceeds the tyre maximum performance so this does appear not to be limited to the Jab or indeed permit aircraft.

It seems that even in the commercial, jet powered aircraft world it is well known that on take-off, they do sometimes exceed the performance of the tyres. There is an article from Boeing on the subject.

I do wonder why this is tolerated. If it were a car, it would fail an MOT. I have never seen or heard of a tyre shredding on a light aircraft, but it does seem to be a very common event for inner tubes to go without any real explanation. I guess it could be an under performing tyre putting more pressure on the inner tube, or it could just be that the inner tubes themselves are just not up to the job. I would also think (relating to cars) that the performance of a tyre is much more crucial when the tyre is tubeless.
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Re: Best tyres for Jabs

Postby Bug » Sun Oct 25, 2015 6:28 am

It does seem crazy to fit wheelbarrow tyres on aircraft and statements like. "Nothing has gone wrong yet" are not very helpful. So what will it take, an injury, a fatality ?

Aircraft tyres should be fit for purpose and I would not look for a tyre and find a purpose to fit it (cheap, looks OK etc etc).

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Re: Best tyres for Jabs

Postby Steevo » Mon Oct 26, 2015 9:14 am

I agree the tyres seem under rated for the specification but the ratings on tyres (at least on a car) work on a sustained figure for an amount of time. Also on cars, the tyres are under pressure where as on a Jab, the tyres are not under pressure, the inner tube is and is a fairly low PSI. When a tyre goes on a car, it is usually blown out and very rarely shreds. I think the only way you would destroy a Jab tyre under normal conditions would be for it to go over something sharp and big and even then it is only likely to take a chunk out the tyre rather than destroy it. It would take a lot to shred the tyre even at the ratings currently on the Jabs. The inner tube takes the brunt of everything.

When I was training, the airfield I trained at was mainly students. There were tyre/inner tube failures every week and in every event that I was aware of, it was the inner tube that had gone and not the tyre.

The problem is that there does not seem to be a great choice of inner tubes for the Jab. Coupled with the fact that the Jab does not have much clearance around the tyre, it only takes a slight pinch and the inner tube is punctured.
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