Tyre pressures on alloy wheels - less??

[Ruralman]

After a bit of help please. Had a look at a mates bike to try to help get it running right for her.
Its a Honda Magna 250 about 1995 - a V twin cruiser style bike that has solid alloy wheels front and rear. The suspension travel is a bit limited as per most small cruisers, and I'm pretty sure the front forks need the oil changed.
She has put new tyres on it and the pressures have been set at around 32psi. Without being rude enough to ask, her weight would be in the 60's Kgs. The tyres feel really hard to push on and even with my weight (100kg) sitting on it and pushing the forks up and down you see very little flexing in the sidewalls.
My question is, given that the alloy wheels have absolutely no flex (compared to spokes) what pressure should she, or could she, safely run the tyres at that would help maybe soak up a few of the small imperfections and loose stones on the road rather than just having the suspension have to cope with everything.
Unbeknown to her she had been riding with the back one at 10psi !!! - it still felt hard to touch apparently (I didn't see it then) but was rather vague to ride. Bike shop recommended getting it up to 32-36psi.
My feeling is that she should try it at around 23-25 psi at both ends but I'd appreciate some comment from others - so thanks if you can help.
Without taking note of the actual sizes the tyres are quite wide for the size of bike.

[Smokin]

Well I have 36 front and 42psi for the rear for my Continentals, I would have thought 36/38 for most bikes.

[NighthawkNZ:Me]

Well I have 36 front and 42psi for the rear

Thats what I usually runn mine at as well (and is what is recommended in me manual)

[Ruralman]

My question is - what about when you have rigid alloy wheels and relatively limited suspension - can you pull the tyre pressures back a bit to help take up some of the smaller road irregularities. And if we reduce the pressure how far can we go without causing other problems. Remember this is a cruiser bike so it won't be leaning over as far as the VTR or CBR you guys have.

[Smokin]

The CBR and VTR have alloy wheels and tubeless tyres, If you run the tyres too soft the sidewalls wont support the tyre properly and handling issues will arise, also the contact patch of the tyre can be reduced as well as poor braking performance.
you will also have a problem with tyre wear and overheating.
Dont worry about the suspension travel, it will cope just fine, the CBR has bugger all and it's limits are rarely found and it was made to cover ground a fair bit quicker than a cruiser.

[Speedy]

You should run 30-32psi in the front and 32-34 in the rear. the tyres are cross plys and have a hard wall / having a slightly firmer tyre is gna make it ride nicer than if u had less , theres nt much u can do about the suspension thou srry . i wrk in a bike shop and do tyres on thos all the time

[NighthawkNZ:Me]

My question is - what about when you have rigid alloy wheels and relatively limited suspension - can you pull the tyre pressures back a bit to help take up some of the smaller road irregularities. And if we reduce the pressure how far can we go without causing other problems. Remember this is a cruiser bike so it won't be leaning over as far as the VTR or CBR you guys have.

My XJ special (is a cruise styled bike) and had Alloy wheels and tubeless, crap rear suspension, and I use have 32 in the front and at least 34-36 (depending on load I was carrying) in the rear... if I went any less then I would notice that handling wasn't the best and became a bit iffy at times.

[Ruralman]

You should run 30-32psi in the front and 32-34 in the rear. the tyres are cross plys and have a hard wall / having a slightly firmer tyre is gna make it ride nicer than if u had less , theres nt much u can do about the suspension thou srry . i wrk in a bike shop and do tyres on thos all the time

Thanks speedy and you other guys as well - the front suspension doesn't feel quite right and I have suggested taking it into the shop to get the fork oil changed to see how it feels then (which will also mean oil level will be put right if its not) - so it sounds like the pressures should be left as they are now and get the suspension working as well as it can first.

[Motu]

Heh heh,another dirt biker who can't cope with the idea of running the high pressures the road bikes use these days.I've never used more than 30 psi on any bike,and I'm really reluctant to go over 25 psi,it just seems so wrong.

[NighthawkNZ:Me]

Thanks speedy and you other guys as well - the front suspension doesn't feel quite right and I have suggested taking it into the shop to get the fork oil changed to see how it feels then (which will also mean oil level will be put right if its not) - so it sounds like the pressures should be left as they are now and get the suspension working as well as it can first.

If the suspension is right then yeah it would all feel a bit iffy... once you get the suspension sorted it may be better... Also how old are the tyres...??

[XP@]

What is going to happen if you get them wrong?

Too high and the tire will become too rigid. This will make for a smaller contact patch with the road. Heat will be concentrated in this small patch, usually the centre, and where the rubber gets too hot it will wear.

Too low and the side walls will flex. This will make the sides of the tire hot and wear will take place at the side of the tire.

So if you compare the side rubber temp versus the center rubber temp they should be about the same, given that there is enough flex in the tire to distribute the heat evenly. With practice using your hand to feel the temp is generally enough to tell "low, high or about there".

Disclaimer: The physics behind this seems sound enough, and i have used it with a fair amount of accuracy myself I havent scientifically proven this method of testing. Although if anyone has access to accurate temperature recording devices I would be interested in proving it...

[Bonez]

Without taking note of the actual sizes the tyres are quite wide for the size of bike.

Sign of times, looks kawl etc etc. Personnally I think a lot of smaller bikes, and a fare few of the larger capacity bikes, are way over tyred. Different manufactures have different profiles, some are "rounder" than others. Considering going down a size on the back may possibly be the go on the next tyre change.

10 psi will feel vague all right. I'd personnaly start with the manufacturers specs and go from there. Raising or lowering the pressures a few psi till the rider is happy. You've noted the forks seem iffy, probably hasn't been looked at since '95, it'd pay to get them sorted.

Being a cruiser style bike its not really designed for rough roads, ergos are all wrong. But if riden sedately, being a cruiser and all, should handle most NZ roads quite well indeed.

Hope you get it sorted out and your friend is enjoying her bike . It's a good weekend for riding.

[DEATH_INC.]

I've run as low as 25psi in x-plys with no ill effects.

[Ruralman]

What is going to happen if you get them wrong?

Too high and the tire will become too rigid. This will make for a smaller contact patch with the road. Heat will be concentrated in this small patch, usually the centre, and where the rubber gets too hot it will wear.

Too low and the side walls will flex. This will make the sides of the tire hot and wear will take place at the side of the tire.

So if you compare the side rubber temp versus the center rubber temp they should be about the same, given that there is enough flex in the tire to distribute the heat evenly. With practice using your hand to feel the temp is generally enough to tell "low, high or about there".

Disclaimer: The physics behind this seems sound enough, and i have used it with a fair amount of accuracy myself I havent scientifically proven this method of testing. Although if anyone has access to accurate temperature recording devices I would be interested in proving it...

Good idea - and really that was the question - what bad things could happen by dropping the pressure a bit. If the worse is it starts to get a bit sticky to steer (I'd imagine we'd have to go under 20psi for that to happen) at some point then we can experiment a bit and see how it feels.

Bonez - you're dead right about the size of the tyre, or rather width of the tyres - bloody ridiculous but made to look cool. They make the ones on my Transalp look pretty skinny and it has to carry a lot more weight and deal with shit this bike will never see - and its a lot easier to flick around.

[Ruralman]

Heh heh,another dirt biker who can't cope with the idea of running the high pressures the road bikes use these days.I've never used more than 30 psi on any bike,and I'm really reluctant to go over 25 psi,it just seems so wrong.

Theres some truth to that but I actually run 32-34psi on the front of the Transalp - but then look at the suspension travel

I've run as low as 25psi in x-plys with no ill effects.

Thanks for that - looks worth experimenting - might have to do a trip and take a hand pump with us, although we'd then have to make some sort of allowance for the pressure rising as the tyre warms up - unless we have plenty of pub stops to let it cool down a while??? Just to ensure accurate testing of course