Geometry check - Oversteer in braking zone.
Discussion
Morning,
Would appreciate some advice on areas to check regarding my cars oversteer in braking.
Sure there is not an easy answer to this one (online) but would appreciate some directions to investigate if possible.
Balance of my Caterham is very good / neutral on the whole but seems to be very hard to handle when braking with any amount of turn. Pretty expected to a degree but its a little more than I think should be reasonable and looking to dial out some of it.
Noticed mostly this week at Snetterton 300 which has a couple of corners where you have to scrub speed whilst moving the car, notably Nelson (after Bentley straight) and Murrays (after Corram).
Given in general it seems fine could it be the brake balance, front / rear arb, rake, other? Basically where to look to correct a certain behavior as above without screwing up the rest too much
I'm not in to 'winning' track days so run treaded tryes (no semi slicks) and pretty tame road focused pads. Did bugger about with tyre pressures but that didn't really do much.
Brakes are AP fronts with standard rears, orange front arb, second softest rear arb, spring rates need to check.
Geo'd and corner weighted as below.
Would appreciate some advice on areas to check regarding my cars oversteer in braking.
Sure there is not an easy answer to this one (online) but would appreciate some directions to investigate if possible.
Balance of my Caterham is very good / neutral on the whole but seems to be very hard to handle when braking with any amount of turn. Pretty expected to a degree but its a little more than I think should be reasonable and looking to dial out some of it.
Noticed mostly this week at Snetterton 300 which has a couple of corners where you have to scrub speed whilst moving the car, notably Nelson (after Bentley straight) and Murrays (after Corram).
Given in general it seems fine could it be the brake balance, front / rear arb, rake, other? Basically where to look to correct a certain behavior as above without screwing up the rest too much
I'm not in to 'winning' track days so run treaded tryes (no semi slicks) and pretty tame road focused pads. Did bugger about with tyre pressures but that didn't really do much.
Brakes are AP fronts with standard rears, orange front arb, second softest rear arb, spring rates need to check.
Geo'd and corner weighted as below.
That all looks good!
Try the softest ARB at the rear (furthest out hole) and if that induces under-steer at other places, drop the front a little. Rear tyre pressure for track perhaps a little lower at 16-18 psi. Caterham get the balance of the brakes by choosing pads and the Delphi brand they sell (or used to) does work well, although they wear out very quickly on track. Their standard AP front pad RS14 is well match to these too. If the problem persist then a Tyre change may help.
Try the softest ARB at the rear (furthest out hole) and if that induces under-steer at other places, drop the front a little. Rear tyre pressure for track perhaps a little lower at 16-18 psi. Caterham get the balance of the brakes by choosing pads and the Delphi brand they sell (or used to) does work well, although they wear out very quickly on track. Their standard AP front pad RS14 is well match to these too. If the problem persist then a Tyre change may help.
If these corners are at a decent speed, yet you also have to kill a lot of it in a trail braking style to not slide off the line, then there are an awful lot of dynamic things going on, and hard to analyse.
Just to be a twit, you could brake a bit earlier and feather off before any great turn in and settle the suspension, and then not worry as much... :-)
To get more into classic suspension stuff;
more front sway bar to make it less tail happy and keep the car flatter under braking
less rear sway bar if it feels like the inside wheel is lifting too much and tail happy everywhere
less rake/stiffer front springs or even just damper settings if this is a short time of weight transfer to keep flatter front/rear
A bit more toe in may help - hard to judge. Usually makes the car less snappy.
If rear brakes seem to lock too early, then change brake pad Mu at one end.... um, if it seems the rear brakes get suddenly sensitive in the high speed brake, but not in very short applications, the rear brakes may have a friction curve that gets them suddenly stickier just as you hit the turn in. A bit odd, but I have brakes that are great cold but almost tear the tread off the tire in a couple seconds of higher force braking as they hit the temperature sweet spot. A really odd friction/heat curve, but cool to make use of.
A limited slip differential can have some interesting drop throttle braking effects. Depends on LSD type and setup. Some setups can really lock up the LSD under lifted throttle, which can make things odd in fast corners, especially if one side gets quite light.
Tire age/compound/width front to rear.
You could make shims to give a little more rear toe-in. *shrug*
Oh... worn rear bushings in the a-frame, or perhaps even a weak/cracked/bent de dion tube can do some really odd things. Off spec/loose ear bolts for rear ears/de dion interface
rear dampers no longer damping well.
That's about all I can think of.
If you can give more specific info, feel, for what is going on, it would help.
Just to be a twit, you could brake a bit earlier and feather off before any great turn in and settle the suspension, and then not worry as much... :-)
To get more into classic suspension stuff;
more front sway bar to make it less tail happy and keep the car flatter under braking
less rear sway bar if it feels like the inside wheel is lifting too much and tail happy everywhere
less rake/stiffer front springs or even just damper settings if this is a short time of weight transfer to keep flatter front/rear
A bit more toe in may help - hard to judge. Usually makes the car less snappy.
If rear brakes seem to lock too early, then change brake pad Mu at one end.... um, if it seems the rear brakes get suddenly sensitive in the high speed brake, but not in very short applications, the rear brakes may have a friction curve that gets them suddenly stickier just as you hit the turn in. A bit odd, but I have brakes that are great cold but almost tear the tread off the tire in a couple seconds of higher force braking as they hit the temperature sweet spot. A really odd friction/heat curve, but cool to make use of.
A limited slip differential can have some interesting drop throttle braking effects. Depends on LSD type and setup. Some setups can really lock up the LSD under lifted throttle, which can make things odd in fast corners, especially if one side gets quite light.
Tire age/compound/width front to rear.
You could make shims to give a little more rear toe-in. *shrug*
Oh... worn rear bushings in the a-frame, or perhaps even a weak/cracked/bent de dion tube can do some really odd things. Off spec/loose ear bolts for rear ears/de dion interface
rear dampers no longer damping well.
That's about all I can think of.
If you can give more specific info, feel, for what is going on, it would help.
Thanks all!
Tyres are AR500s, started at 22psi hot and worked my way down to 17-19psi which was the best I could get. ARB was on softest prior and I moved to one notch stiffer which seemed to help everywhere else but little difference in the braking zone.
Below is a link to an example. I'm provoking in a couple of areas so ignore those, bust see 1min 50:
https://www.youtube.com/watch?v=P-hfI5AF14Q
I do have a weird knocking noise when rocking the rear of the car that nobody can trace, we have lubed the obvious with no change so only thing left is the springs. When rolling along it knocks and creaks, almost like a binding bush sound, without the binding bush
Tyres are AR500s, started at 22psi hot and worked my way down to 17-19psi which was the best I could get. ARB was on softest prior and I moved to one notch stiffer which seemed to help everywhere else but little difference in the braking zone.
Below is a link to an example. I'm provoking in a couple of areas so ignore those, bust see 1min 50:
https://www.youtube.com/watch?v=P-hfI5AF14Q
I do have a weird knocking noise when rocking the rear of the car that nobody can trace, we have lubed the obvious with no change so only thing left is the springs. When rolling along it knocks and creaks, almost like a binding bush sound, without the binding bush
DCL said:
That all looks good!
Try the softest ARB at the rear (furthest out hole) and if that induces under-steer at other places, drop the front a little. Rear tyre pressure for track perhaps a little lower at 16-18 psi. Caterham get the balance of the brakes by choosing pads and the Delphi brand they sell (or used to) does work well, although they wear out very quickly on track. Their standard AP front pad RS14 is well match to these too. If the problem persist then a Tyre change may help.
They are Delphi rear, cant see the fronts but understand (from paperwork) they are Mintex 1144s. Replacing all soon so will check when I pull them out.Try the softest ARB at the rear (furthest out hole) and if that induces under-steer at other places, drop the front a little. Rear tyre pressure for track perhaps a little lower at 16-18 psi. Caterham get the balance of the brakes by choosing pads and the Delphi brand they sell (or used to) does work well, although they wear out very quickly on track. Their standard AP front pad RS14 is well match to these too. If the problem persist then a Tyre change may help.
analog_me said:
Rear wheel bearing play ?
Nothing obvious, need to jack it up to have a good look but that's the sort of feeling it gives.Another corner example it wanted to swap ends is at Brands Indy, the transition when braking between Surtees and McLaren. Would often find myself backing in to McLaren with little warning.
griggsy2 said:
ARB was on softest prior and I moved to one notch stiffer which seemed to help everywhere else but little difference in the braking zone.
I think there is clue in there - you've stiffened the rear to give a better balance. In effect you are reducing rear grip to counter under-steer. But it may be better to increase front grip by lowering it at the front. That would give more rear grip in the transition phase of the corner and may make the difference you are looking for. DCL said:
I think there is clue in there - you've stiffened the rear to give a better balance. In effect you are reducing rear grip to counter under-steer. But it may be better to increase front grip by lowering it at the front. That would give more rear grip in the transition phase of the corner and may make the difference you are looking for.
I softened the rear as I had too much over-steer, especially under power, it was originally on the third setting (one from stiffest). I disconnected completely and had too much roll and understeer in all areas, it still wanted to switch ends under braking though. Put to softest, nice in fast corners but still a little understeery under power and then went to second which seemed to get me the best balance overall. Weirdly the braking issue I'm struggling with was there throughout and hardly changed which I'm completely confused by which is making me feel its a brake issue, mechanical or geometry.. The toe comments above make sense but I don;t realy have much to compare my settings above with.I need to check my spring rates and rake as I don't know those. For the latter, is it front of chassis behind the front wheel and chasis in front of the rear wheel that's typically used on Caterhams?
It depends who's instructions you are following but I use the same a Simon at Meteor, which is where the tube turns square (measure on the square) and just in front of the rear wing, 10-15 mm . Caterham do it from just behind the lower wishbone which gives slightly higher figures.
Edited by DCL on Sunday 20th August 11:49
Front and rear spring shots - Not obvious what these translate to in lbs.
Caterham Part number suggests: 170lb
No such Caterham part number..
Google suggest potentially progressive springs:
Standard Rear Length 345 mm From 0 - 95 mm: 110 lb Next 87.5 mm: 130 lb Remainder: 200 lb
Caterham Part number suggests: 170lb
No such Caterham part number..
Google suggest potentially progressive springs:
Standard Rear Length 345 mm From 0 - 95 mm: 110 lb Next 87.5 mm: 130 lb Remainder: 200 lb
Edited by griggsy2 on Sunday 20th August 13:11
Edited by griggsy2 on Sunday 20th August 13:21
DCL said:
They're standard R400/R500 and I doubt they are the problem. Interestingly the front are fitted upside down - not necessarily wrong, but can cause clearance issues. What date is the car?
It was registered 2013, not an official R500, starter kit (Arch imperial chasis) with 2.0d NMS engine. Coooper/McArthur's old car:http://caterham.mcarthur.org.uk/
Without me in the car (struggling to find weights suitable) the heights are:
115mm front and 150mm rear on passenger side.
120mm front and 160mm rear on drivers side.
Used just behind front wishbone for front measure, just in front of rear arch for rear.
Can't see my fat arse dropping the rear 30mm so this seems overly aggressive to me and supporting what DaveJ is saying above?
115mm front and 150mm rear on passenger side.
120mm front and 160mm rear on drivers side.
Used just behind front wishbone for front measure, just in front of rear arch for rear.
Can't see my fat arse dropping the rear 30mm so this seems overly aggressive to me and supporting what DaveJ is saying above?
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