Tyre Pressures
Discussion
It's for a kit car and the manual doesn't specify tyre pressures. It will depend on weight, tyre diameter, width, profile and type of driving. I've set them to the pressures for the donor car but as it weighs less and the tyres are wider it won't be correct. I've also seen it said that you draw a chalk line across the width of the tyre roll it backwards and forwrds then see where the chalk has been rubbed off. If it's rubbed off the centre reduce pressure and increase if it's rubbed off the outer edges.
That little snippet (kit car) would have helped us reply more accurately.
That is different then.
Do the manufacturer not have any information available for what they would expect?
Alternatively, you could approach the tyre manufacturer with your tyre sizes & weight & weight distribution, they may be able to help.
Is there a similarly weighted "normal car" with the same size tyres to use as a starting point, Ariel Atom/Caterfield/etc. perhaps.
I am not sure about the temperature difference bit you say as that will pretty much be the same for any tyre over the same temperature range, so of no help.
I am not sure, but the chalk thing doesn't sound right, because for the edge of the tyre not to touch the road it would have to be enormously over-inflated, there will be a difference in contact pressure, but not a lack of pressure on the tread to miss the edges.
That is different then.
Do the manufacturer not have any information available for what they would expect?
Alternatively, you could approach the tyre manufacturer with your tyre sizes & weight & weight distribution, they may be able to help.
Is there a similarly weighted "normal car" with the same size tyres to use as a starting point, Ariel Atom/Caterfield/etc. perhaps.
I am not sure about the temperature difference bit you say as that will pretty much be the same for any tyre over the same temperature range, so of no help.
I am not sure, but the chalk thing doesn't sound right, because for the edge of the tyre not to touch the road it would have to be enormously over-inflated, there will be a difference in contact pressure, but not a lack of pressure on the tread to miss the edges.
Why would the fact it's a kitcar have any relevance? It's the method of using hot v cold pressures to establish the optimum I'm asking about.
Unfortunately the car is from the mid 80's and the manufacturer is no longer trading so I can't ask them. I could ask Vredestein but doubt the have any info.
I've checked the increase in pressure from hot to cold with the pressures I'm using now and the fronts increase by 1psi and the backs by 1.5psi Does this sound about right?
With the chalk method I assume the load distrbution over the contact patch is uniform when the pressure is correct and rubs the chalk off the centre if over-inflated and and off the outer edges if under inflated.
Unfortunately the car is from the mid 80's and the manufacturer is no longer trading so I can't ask them. I could ask Vredestein but doubt the have any info.
I've checked the increase in pressure from hot to cold with the pressures I'm using now and the fronts increase by 1psi and the backs by 1.5psi Does this sound about right?
With the chalk method I assume the load distrbution over the contact patch is uniform when the pressure is correct and rubs the chalk off the centre if over-inflated and and off the outer edges if under inflated.
Surely the increase will vary from car type to car type and tyre to tyres. Plus what is hot and what is cold. Cold in mid Jan is not the same as cold in August
If it helps, pressures on the Evora from March 2 degree C morning go up by 3 psi all round when the car has been driven fast for 20 mins or so.
If it helps, pressures on the Evora from March 2 degree C morning go up by 3 psi all round when the car has been driven fast for 20 mins or so.
Spydaman said:
Why would the fact it's a kitcar have any relevance? It's the method of using hot v cold pressures to establish the optimum I'm asking about.
Unfortunately the car is from the mid 80's and the manufacturer is no longer trading so I can't ask them. I could ask Vredestein but doubt the have any info.
I've checked the increase in pressure from hot to cold with the pressures I'm using now and the fronts increase by 1psi and the backs by 1.5psi Does this sound about right?
With the chalk method I assume the load distrbution over the contact patch is uniform when the pressure is correct and rubs the chalk off the centre if over-inflated and and off the outer edges if under inflated.
Because if you had said it was a kit car I wouldn't have said "check the manual....the end"Unfortunately the car is from the mid 80's and the manufacturer is no longer trading so I can't ask them. I could ask Vredestein but doubt the have any info.
I've checked the increase in pressure from hot to cold with the pressures I'm using now and the fronts increase by 1psi and the backs by 1.5psi Does this sound about right?
With the chalk method I assume the load distrbution over the contact patch is uniform when the pressure is correct and rubs the chalk off the centre if over-inflated and and off the outer edges if under inflated.
Vredestein will be able to tell you under what loading conditions (weights) you want what tyre pressures on what size of tyre. Whether they can be bothered may well be a different point.
1/1.5 psi for how cold to how hot? Under what load? For how long? from what initial pressure? etc. etc. Too many variables/unknowns/unknown effects for it to be anything other than meaningless. On my track day car 20 laps of Cadwell Park gives a rise of well over 8psi, on the road at 30 it is probably a lot less even after 2 hours or something.
Also, I'll bet if you now up the pressure by 10 psi, I'll bet it still only goes up by 1/1.5 psi. Where is the theory then.
Once again the chalk thing, think about it. If you look at the tyre at 30 psi & see how flat the top is, then put it to 40 psi, then try 50 psi & see what it looks like. Then look at the contact patch. I'll bet it all still touches the road. So will all be rubbed off, except the fact that the tyre isn't rubbing the road, it is rolling over it, so how is it going to tell you anything?
I am sorry to have to tell you that both theories hold no water at all, you need to find another way, and I suspect the only way to get anywhere near optimum will either be trial & error or research looking for similar cars on similar sized tyres.
Spydaman said:
Why would the fact it's a kitcar have any relevance? It's the method of using hot v cold pressures to establish the optimum I'm asking about.
Unfortunately the car is from the mid 80's and the manufacturer is no longer trading so I can't ask them. I could ask Vredestein but doubt the have any info.
I've checked the increase in pressure from hot to cold with the pressures I'm using now and the fronts increase by 1psi and the backs by 1.5psi Does this sound about right?
With the chalk method I assume the load distrbution over the contact patch is uniform when the pressure is correct and rubs the chalk off the centre if over-inflated and and off the outer edges if under inflated.
how hot...and how cold ?Unfortunately the car is from the mid 80's and the manufacturer is no longer trading so I can't ask them. I could ask Vredestein but doubt the have any info.
I've checked the increase in pressure from hot to cold with the pressures I'm using now and the fronts increase by 1psi and the backs by 1.5psi Does this sound about right?
With the chalk method I assume the load distrbution over the contact patch is uniform when the pressure is correct and rubs the chalk off the centre if over-inflated and and off the outer edges if under inflated.
Really..never worry about it, somewhere around 30psi is fine for most. If tyres wear or something doesnt feel right for your particular vehicle and tyres...try a different pressure. Simple as that.
http://www.pistonheads.com/gassing/topic.asp?h=0&a...
Did you read it here?
I guess you're asking for the Bonito? (Nice, BTW!
)
Start with the pressures for the Beetle and reduce them from there.
Do you know how much the car weighs compared to the Beetle?
Did you read it here?
I guess you're asking for the Bonito? (Nice, BTW!
)Start with the pressures for the Beetle and reduce them from there.
Do you know how much the car weighs compared to the Beetle?
spydaman,
I too think that this x degrees difference cold to hot is a myth.
The 'right' tyre pressure is the one at which all the tread is on the road as much of the time as possible.
Being on the road heats the tyre through the tread, but once the heat's in the tyre gas, you have no idea where it comes from.
You need to measure the tyre temperature across the tread, ideally with a needle probe, but a infrared gun will do it well.
Too much pressure - heat concentrated in the middle, and vice versa.
This can also tell you about your camber angle too, once you have the pressure sorted.
JOhn
I too think that this x degrees difference cold to hot is a myth.
The 'right' tyre pressure is the one at which all the tread is on the road as much of the time as possible.
Being on the road heats the tyre through the tread, but once the heat's in the tyre gas, you have no idea where it comes from.
You need to measure the tyre temperature across the tread, ideally with a needle probe, but a infrared gun will do it well.
Too much pressure - heat concentrated in the middle, and vice versa.
This can also tell you about your camber angle too, once you have the pressure sorted.
JOhn
Spydaman said:
That's the one! Yes it's for the Bonito. The Beetle weighs 870kg whereas the Bonito is about 700 so 20% less. The Beetle tyre pressures are 20F/28R so will try 16F/22R and see how it feels.
Sounds like a good starting point. TBH, I doubt you will notice too much difference. I've never tried the cold pressure plus 10% theory and can't be bothered to do the sums but it's probably not as daft as it sounds. (To do the calcs, it's worth remembering that you would need to use absolute temperatures (and pressures), so differences in ambient aren't that significant.)If your pressures are too low, they tyres will wear more on the edges and if over inflated will wear more in the middle of the tread. This is what the chalk method is aiming to spot - never tried it myself, but it might give you a clue.
If you find that the steering is heavy, bump the front pressures up a couple of psi and see what it's like. I wouldn't think the car would be too sensitive to pressures, but I guess you never know!
Good luck!
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