smart51
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| posted on 9/3/06 at 11:53 AM |
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handling balance
If I drive round a nice wide road island and gradually increase my road speed, the car feels like the rear slip angles are increasing and that if I
continue, the rear is going to step out. There is no trace of understeer. I'd rather have a more neutrally balanced car where the car remains
in the right attitude to the road, determined by the steering rather than the throttle. I'd rather the radius of turn increase with speed
rather than the back end go out.
How do I achieve this?
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DIY Si
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| posted on 9/3/06 at 11:55 AM |
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Less front camber, more rear?
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britishtrident
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| posted on 9/3/06 at 11:59 AM |
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Starting with the easiest ---
Reduce front tyre pressure increase rear
Soften the rear springs and/or stiffen the front springs.
Fit a front anti roll bar.
Fit wider tyres to rear than front.
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NS Dev
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| posted on 9/3/06 at 11:59 AM |
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I'd start with the spring rates, softer on the rear, stiffer on the front, for a kick off.
(edit, post overlapped britishtrident!)
[Edited on 9/3/06 by NS Dev]
Retro RWD is the way forward...........automotive fabrication, car restoration, sheetmetal work, engine conversion
retro car restoration and tuning
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smart51
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| posted on 9/3/06 at 12:38 PM |
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Thanks for the tips.
Reduce the front tyre pressure / Increase the rear? How does that work? I would have thought that lower pressure would increase the contact patch,
increasing grip.
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nre
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| posted on 9/3/06 at 02:23 PM |
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*Generally* it is normal for a tyre to give a higher cornering stiffness (more lateral force per degree of slip angle) at a higher pressure (within
sensible bounds). That is why manufacturers alter recommended tyre pressures for different vehicle loading conditions, ie fully laden most tin tops
will have a more rearwards weight distribution, and therefore the tyre pressures are recommended to increase quite a lot at the rear, and a little at
the front, to try and maintain a similar(ish)handling balance.
These rules don't always work though. I use tyre test data in my work, and have seen some tyres that don't change much with pressure, and
others that do the opposite to what you expect...
Hope that at least explains the theory!
Cheers,
Neil
Edited to say: I have recently been playing with spring rates on the Mojo, partly to improve the ride (was very firm to start with) but also to try
and make it a bit more docile (rear end tends to dominate on the track!). I've dropped the rear rates quite a lot, have yet to try it in anger
on a track but feels a bit more friendly on the road... See HERE if you want
more info...
[Edited on 9/3/06 by nre]
[Edited on 9/3/06 by nre]
Sylva Mojo Zetec on GSXR750 throttle bodies...
www.mymojo.co.uk - Facebook feed
JPSC Forum - A dedicated forum for discussion on all things relating to Jeremy Phillips designed cars
Including the Star, Leader, Striker, Phoenix, Fury, Jester, Stylus, Mojo, Riot, J15 & Vectis
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britishtrident
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| posted on 9/3/06 at 02:34 PM |
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Yeah altering the tyre pressures worked better with crossplies but still good for the first port of call.
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smart51
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| posted on 9/3/06 at 03:49 PM |
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OK. A good first attempt then is tyre pressures. I'd just guessed at 18 psi all round (cold). The effect is not bad and I don't get the
back end coming round. If I can improve the feel of the car then I'll be happy.
What should I do first, increase the pressure in the rears, decrease the fronts or a little of both?
[Edited on 9-3-2006 by smart51]
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JB
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| posted on 9/3/06 at 07:42 PM |
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Balance
Tyre pressures may well alter the balance but if your pressures are correct then you are doing this by sacrificing grip, which is not the way to do
it.
You should always try to increase the grip on the end that is not working.
There is generally an optimum tyre pressure. Basically if you are too hard the centre of the tyre touches the road, if you are too low then the
outdside edges do the work (at extremes of pressure). Therefore you should be somewhere in the middle. The average of the outer temperatures should
equal the middle.
Next spring rates. basically you want the softest springs that keep the car off the road and the suspension off the bupstops on anything but te
largest bump.
Then balance the car with a front ARB.
In your case my first fix would be to soften the rear springs, unless of course it is hitting the road or bump stops.
Always try to increase grip tobalance a car and not kill the grip.
John
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smart51
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| posted on 9/3/06 at 07:58 PM |
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That makes sense. There is no recommended tyre pressure for sucha light car and so I just took advice from here. The springs are the ones supplied
by MNR for the kit. For now I'll assume they chose the right ones. They were labled front and rear so I know I haven't got them the
wrong way round . I'll adjust the rear pressure and see what effect that has.
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Dave Ashurst
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| posted on 9/3/06 at 09:56 PM |
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I found an infrared thermometer good for optimising tyre pressure. Someone on here told me about it. Sorry can't remember who, but it seemed
like good advice.
I went for uniform temp across the width of the tyre after a run. Sadly my i.r. thermometer got nicked (with my rucksac) from a works pool car on an
overnight site visit in Hackney 18 months ago. Otherwise I'd have lent it to you, honest!
Maplin still have them in their online shop I think.
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britishtrident
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| posted on 10/3/06 at 10:12 AM |
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Maplin have infra red thermomoters on special at 29 quid but the old fashioned thermistor probe type work fine also - but you have to be quick to
take the measurements.
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JB
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| posted on 10/3/06 at 05:03 PM |
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Tyre Temps
For a starting point I did a long run down the A1 then pulled into a layby or service station and as quick as possible jumped out and measured the
temps.
I started hard then let a bit out until I had uniform temperatures.
John
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smart51
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| posted on 10/3/06 at 07:45 PM |
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I measured the temperature of the tyres after a 5 mile run, just to get a feel for the starting point of my tests. The tyres were 18 psi cold and 19
psi warm. The temperature of three of the tyres was 2°C warmer in the centre than the edges and on the other was 4°C warmer on the inside edge than
the outside edge.
What does this mean and do I need more pressure or less?
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JB
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| posted on 10/3/06 at 07:55 PM |
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If it is warmer in the centre than the edges this indicates too much pressure. however 2 deg is pretty close to being OK.
If it is warmer on the inside edge than the outside edge this indicates negative camber (assuming all the wheels are aligned correctly). This is quite
normal and is why you should take the average of the outside edges when comparing to the middles.
Just one tyre different from the rest indicates something odd with the set up.
John
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smart51
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| posted on 10/3/06 at 08:30 PM |
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the one tyre that is different was actually at 17 psi cold and 18 psi warm. Rather than inflate it, I just wanted to see what difference it would
make incase I had to let a bit of pressure out.
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smart51
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| posted on 11/3/06 at 10:46 AM |
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I reduced the pressure to 17 psi all round and did the same trip as yesterday. nice even gradient of temperatures accross all 4 tryes. the fronts
bulge a bit more at the bottom than before. As for the roundabout test, I'll leave that to a dry day.
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smart51
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| posted on 15/3/06 at 09:08 PM |
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The roads were dry again today. That 1 psi drop in tyre pressure changed the balance so that the front tyres started to slip first. Amazing what a
small change does. the car felt slightly less settled so more experiments needed. I should be able to get it sorted nicely. Thanks all.
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Mark Allanson
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| posted on 15/3/06 at 09:59 PM |
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I run 18psi at the front, 20psi left rear and 22right rear. Sounds odd, but if the right rear is any less, I get the left hand front locking up
prematurely.
If you can keep you head, whilst all others around you are losing theirs, you are not fully aware of the situation
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NS Dev
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| posted on 15/3/06 at 11:53 PM |
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sounds like you need the tweak the right rear spring platform up a touch Mark.
Retro RWD is the way forward...........automotive fabrication, car restoration, sheetmetal work, engine conversion
retro car restoration and tuning
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Mark Allanson
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| posted on 16/3/06 at 06:27 PM |
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I have the rear left and right platforms set the same, but I weigh 17st just out of the shower!
Is it better to have the platforms asymetrical or the tyre pressures?
If you can keep you head, whilst all others around you are losing theirs, you are not fully aware of the situation
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britishtrident
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| posted on 16/3/06 at 06:59 PM |
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Really got to tweak the corner weight to suit the cars normal loaded condition ie. with the driver onboard
Weigh each corner put it into a spread sheet to work out (1) the total, (2) front total (3) rear total, (4) left total, (5) right total from that
then work out the correct weight for each cornerbased on precentages.
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