59 replies to this topic

[Scuffers]

Shame that good bumpsteer settings seem one of the most guarded secrets on the Lotus forums.    

 

 

it's not a secret, it's just that it takes some time and patience to get it right.

 

 

Interesting stuff, thanks Simon. I have a full blown race car now so no issues associated with not being setup correctly

 

in that case, stop messing about with soft springs, if it's track only, you should be looking at 650/800 minimums.

 

also, spend more time on getting the basics right, front/rear bump is not a 'nice to get right' it's intrinsic to how the cars behave.

 

Same with brakes, if it was me, it's Padgig/Padgid or Padgid for pads, no cheap nasty disks, and don't be in a hurry to bolt on 4 pots.

[SteveA]

My springs are much, much softer than that as my racecar is a 500kg Caterham (as per my profile picture). Shocks, springs, calipers, pads, disks, ride height, camber etc etc are tightly regulated so I can't change anything. The only thing I can change is the front toe and then only by 1 degree. I've not had enough testing to alter it and see what difference it makes to the way the car drives so that will happen next season.

Edited by SteveA, 21 January 2016 - 09:25 AM.

[Exmantaa]

 

Shame that good bumpsteer settings seem one of the most guarded secrets on the Lotus forums.    

 

 

it's not a secret, it's just that it takes some time and patience to get it right.

 

 

 

 

It may not be a "secret", but good information regarding the right amount of bump-steer for a certain purpose/set-up is hard to come by. At least not if you want check this yourself. (I'm in NL and not a lot of Lotus chassis set-up specialists over here...)

There is this Lotus rear bumpsteer curve floating in the net, but nothing written for the front set-up.

Sometimes subtle clues are given in postings, but I can imagine that for these "movement dependant" toe corrections, the curve settings for hard track ARB/springs.shockss + sticky tires are different than for relative soft road suspension on normal tires?

 

Alex did a lot of bumpsteer measurements on different height settings, but in the end he also got stuck without the right knowledge where to aim for in his car...

[alexb]

Measuring something like bumpsteer is one thing, understanding why it was designed that way quite another. And it was designed that way, as it's quite easy to do it any other way you like. One thing about bumpsteer is that it's only interesting in the bump-droop interval that you really use. If you decrease that interval by mounting higher rate springs, you still have the same bumpsteer curve, but you're using a smaller interval in that curve. From that perspective it might even make sense to increase bumpsteer, bringing it back to the absolute values of the original interval, to stay within original design specs.

 

TBH, the more I look at it, the more I like my bicycle.

[Scuffers]

Measuring something like bumpsteer is one thing, understanding why it was designed that way quite another. And it was designed that way, as it's quite easy to do it any other way you like. One thing about bumpsteer is that it's only interesting in the bump-droop interval that you really use. If you decrease that interval by mounting higher rate springs, you still have the same bumpsteer curve, but you're using a smaller interval in that curve. From that perspective it might even make sense to increase bumpsteer, bringing it back to the absolute values of the original interval, to stay within original design specs.

 

TBH, the more I look at it, the more I like my bicycle.

yes and no.

 

going stiffer does reduce the effects of bumpsteer, however, that's the sticking plaster approach as opposed to fixing the problem.

 

without a lot of changes, you cannot modify the bumpsteer curve, only the area in it that you are operating and the rate of change.

 

unless you're doing something out of the norm, the book setting for the rear is the way forward.

 

Front is a bit harder as it is (A) more subjective and ( dependant on a lot of other things,

 

ie. different construction tyres respond differently, so an optimum setup for one will be less than good on another.

 

I actually end up going by eye setup for the front then fine tune by what it feels like more than anything else, although this can be quite hard work to get spot on.

[alexb]

Two of the three pivot points that determine the bumpsteer curve are rather hard to change, as they're determined by the uprights. The third however is quite easy to change, both front and back. And that does change the curve. We might be talking the same thing btw, as for me changing the curve also means changing the slope, or rate of change as you call it. Shape essentially remains the same. If you change the other pivot points I guess you will get more adventurous shapes.

 

You hit the nail on the head as far as dependencies go. Far too many of them around (ask a mathematician to calculate the stability of a table and he will come back with the right answers for zero and infinite amount of legs). Most of the calculations I have seen are gross simplifications. E.g. non-compressing tires. That works in some cases, but when you increase your spring rate, there will be a point where things you have simplified away will become important.

 

I can't even figure out if I like driving without bumpsteer better than with. Front I mean. Tried both the original set up and no bumpsteer. Issue was that you should do that in a controlled environment for it to have any meaning while I was changing things left and right. Would still like to try it in that controlled environment, but have to create that first.

[D-DAWG83]

What is the 3rd element that you can change then?

[Spitfire Engineering]

What is the 3rd element that you can change then?

 

Toe link vertical plane fixing

 

[D-DAWG83]

???, can you buy these?

[Exmantaa]

3rd element is the height of the outer steering arm/toe link pivot point... (and the height of the inner point with raising the steering rack)

(edit; balljoint spacers on aftermarket toe links, or on aftermarket steering arms.)

 

@ Scuffers;

I'm still struggling to to what a "decent" front bumpsteer curve should look like. Not sure if I know more when I measure it from a OEM standard VX height/settings. (due to 17"wheels , the uprights compared to an S1 etc.)  

Edited by Exmantaa, 22 January 2016 - 10:44 AM.

[SteveA]

Project Binky episode 11 has a good bit on bumpsteer and Ackermann angles if anyone is interested.

Edited by SteveA, 22 January 2016 - 02:19 PM.

[Spitfire Engineering]

???, can you buy these?

 

Well you cannot buy height as such even though it is considered to be a concrete noun  

But you can get adjustable toe fittings, on the earlier recessed upright cars this can be achived with the OE setup or an aftermarket option.

For the later tapered uprights you need an aftermarket kit with an adjustable capability.

 

[alexb]

 

@ Scuffers;

I'm still struggling to to what a "decent" front bumpsteer curve should look like. Not sure if I know more when I measure it from a OEM standard VX height/settings. (due to 17"wheels , the uprights compared to an S1 etc.)  

 

The curve is fixed by the three pivot points. Ride height only changes where the origin is and spring rate ultimately determines what interval around that origin you use. If you look at the bumpsteer curve that Lotus published for the back of the Elise, the interesting part is that it curves back. And the interval that is used actually has the curving back part in it. That means that when you corner the car and one side is in bump and the other is in droop, both sides change their toe in the same direction. Built in rear wheel steering I guess. Lotus is not the only one doing this. If you change the ride height, the curve stays the same, but you're in a more linear part and you don't get that effect. If you just change the spring rate with ride height staying the same, you still have the same effect, just less so. 

 

For the front, it doesn't make a lot of sense to do the same. It would make some sense to dynamically change Ackerman angles and perhaps that is what they were aiming for. We discussed that a while ago on the Dutch forum. That's just guessing however. Pity that Lotus didn't publish anything about the front.

[Exmantaa]

Yeah, I need to dig that info up again...

 

For the rear, you want the wheel to toe-in with bump, so in a corner the ouside loaded wheel gets some extra toe-in and counteracts possible oversteer moment. This is pretty well documented with that Lotus curve, only the toe-in gain @ max droop is a bit strange. (I think that droop toe-in part should be kept minimal.)

 

Opposite for the front; you want some toe-out in bump for the same reason. Only just how much is right and how should the wheels behave @ droop? Probably something we should test-out and see how different settings feel...  

[ufods]

@ Scuffers

 

from your experience, does this sound well balanced?

 

 

AP 2 front, RS14 pads, 308 Hispec discs

AP 2 rear, RS42 Pads, 288 Hispec discs

no bias valve!

 

 

[Scuffers]

@ Scuffers

 

from your experience, does this sound well balanced?

 

 

AP 2 front, RS14 pads, 308 Hispec discs

AP 2 rear, RS42 Pads, 288 Hispec discs

no bias valve!

 

 

no idea about the disks, but assuming they are similar castings, and the AP 2-pots are the same all round, I would be looking to fit a bias valve,

 

that said, you may well be OK, it's a fine line, and having not tried that I can't really say either way.

 

end of the day, if you brake really hard (and progressively as opposed to stamping on the middle pedal) and the rears are not locking, then it's clearly not a disaster.

 

it will be at is riskiest in heavy braking with a lot of weight transfer, initial braking will probably be terrific!

[ufods]

Thanks. But if i start with ds2500 in the back i will never reach the optimum. Installing the ap bias valve means fitting two?

[Scuffers]

Thanks. But if i start with ds2500 in the back i will never reach the optimum. Installing the ap bias valve means fitting two?

no comment on pads...

 

bias valve, no, fit it between the M/C and ABS, (single line)

 

never fit anything between the ABS and the calipers, do this and the ABS is flying blind.

[ufods]

Thanks for your advice Can you post a picture of your installation?

[bazzio]

How would rc5+ on front and rc8 on the rear fair? Anyone tried this set up?