98 replies to this topic

[alexb]

Tried to measure front bumpsteer. Started with the rear to prove the method against the known Lotus bumpsteer curve and got results more or less as expected. Front is more difficult, as you don't want the rack to move. Made a steering rack immobilizer and put a level on the steering wheel for good measure. Quite sure that it didn't move while taking measurements. But I get this:

 

 front bumpsteer curve, measured.jpg   30.31KB   38 downloads

 

This is about 8 times more than I expected. For comparison the results for the rear,

 

 rear bumpsteer curve.jpg   24.2KB   30 downloads

 

Method was using a laser pointer fixed to the hub. Bounce the laser off a mirror so that you automatically compensate for in-out movement of the hub and only measure rotation. With the mirror at 3 m, you get a total path of 6 m, which means you have a gain of 12 (my rims measure about 50 cm). I'm quite sure the steering wheel didn't move while measuring, I checked this for every measurement taken. I also measured both going up and down and 3 times in total and get the same results (within measurement error).

 

What am I doing wrong? I expected something like the rear or even a bit less toe change. What I measure is massive however and I have a hard time believing it's right.

 

Rack is already in the upper position btw. Set up:

 

 

 

[Exmantaa]

Goed bezig!!  

 

I spend ages looking for solid info on front bumpsteer (like the rear curve), but found nothing for our Loti stuff. 

The only info I got is that you want to have it toe-ing OUT in bump, as your measurements show. But almost 8mm...

(You have 60mm bump & 60mm droop travel with your front dampers? That is quite some...)

 

The rear is toe-ing IN on bump, right?

[alexb]

Yes, you're right, rear is toe in, front is toe out. I'm binary challenged, so always mistake left for right etc

 

About the bump and droop, I measured almost full travel and not only the part that I'm interested in at 115 mm ride height to see what the influence is of lowering from say 140 to 115. Actual travel will be a lot less. It does toe out in bump, just far too much. I was really wondering if I had the track rod connected to the upright correcty, but there is no other way of doing it. The track rod does get into a steep angle in bump, but that only registered when I put everything together again, otherwise I would have looked at the length of the trackrod compared to the upper wishbone.

 

And the car is on blocks, level in roll and 10mm in pitch (10mm higher in the rear as aimed for ride height is 10mm different front to rear). Shocks and ARB disconnected obviously. No detectable play in the wishbones/upright. The whole idea was to measure the suspension leverage correctly (1.30 rear and 1.51 front and no rising rate that I could measure in the front) plus measure camber change and geo the car while on blocks. So I had to be pretty thorough about things. Really bothers me that I see this. Was already thinking about measuring both front wheels at the same time. Would be nice if someone with some experience would comment

[2-20]

Not sure about the sign for the ride height.

Negative value corresponds to compression ?  Am i right ?

 

[Exmantaa]

Not sure about the sign for the ride height.

Negative value corresponds to compression ?  Am i right ?

 

 

He uses the same notification as Lotus, so upper part of the graph (+) is wheel bump travel.

 

As discussed here with the Lotus rear bumpsteer graph: http://www.vx220.org...o/#entry1548345

 

Scuffers explained some of it here, but no hard numbers for the front settings: http://www.vx220.org...-2#entry1548430

 

Now if he could join in here to comment on your measured values (and gave some hints where to aim for), that would enlighten us al...

Edited by Exmantaa, 20 January 2014 - 07:45 PM.

[Exmantaa]

Some info about this subject here: http://www.lotustalk...774-post13.html

(Further down that thread are also some numbers mentioned for the front bumpsteer.)

[alexb]

Thanks for pointing to that thread. Interesting. Quote: Sweeping +1” to -1” for bump steer on the front with ride height at 118 you will see approximately .260” of tow out compressed with the OE arms. With the V2 Arms that is reduced to .032”.  0.26" is 6.6 mm while I measure about 5.5 mm. So that points in the direction of these numbers being correct. TBH, I still can't believe it. Especially with everybody saying these cars are so sensitive to geo and the rear bumpsteer well under control.

 

Without having had a closer look, I think the issue is the tracking rod. Length and position. At the back the toe link is the same length as the lower wishbone arm and has the same pivot point. So it follows an arc that is at least close to the lower swing arm, depending on the pivot point on the upright. At the front I don't think that's true. I'l measure it and post. If it's really significantly shorter than either swing arm, the measured numbers make sense. But better not draw conclusions without looking first

 

[Arno]

Just curious.. At 115mm do you already have the Lotus steering rack raiser plates fitted?

 

https://www.verboom....0618/index.html

 

Might at least move the graph up some, so the change around '0' is less extreme.

 

Bye, Arno.

[alexb]

Yep, raiser plates fitted. Looking at that measured graph again, it seems that at 140 mm ride height and say +- 30 mm travel, you're ok. It's the left part of the curve and toe changes by just 1 mm. Lowering the height gets you into an area with much bigger changes, even with the raiser plates fitted. Assuming the curve is more or less correct that is. 

[alexb]

Had a closer look at the front geometry. Actually getting quite good at jacking up and removing wheels, so at least it's good for something Can't really measure, but it looks like the tracking rod is shorter than the upper arm by a decent amount. So it moves through an arc with smaller radius. The more in bump, the more the difference. Given that a 1 mm of difference at that location gives about a 4 mm toe change at the rim, I'm not that surprised anymore.

 

Raising the rack helps, but if you want to keep the angles remotely the same, it should be a lot more than 8mm. Going from 140 to 115mm ride height I mean. Obvious best solution is one of these uprights where the hub is lowered. All angles stay the same and bump steer wise, you're in the sweet spot.

 

Anyhow, almost convinced myself that the measured curve is at least remotely right. Next step is a geo with the car on blocks again. Then I can check against a reference line and both wheels at the same time just to be sure.

[Exmantaa]

It says somewhere in that Lotus thread that the Elise front bumpsteer is designed to be about 3x the roll understeer amount of a "normal" car, due to it's rear engined layout...

 

Alex, as it's winter and you obviously have nothing else to do  , can you repeat the front bump with the rack in the lower position? Just to see the difference a few mm makes there?

And if possible, loosen the steering-endlink on the hub arm and lower that (just temporarely as its a cone) with a shim stack to see the influence??

 

 

 

[alexb]

Nothing to do ...I got several tons of stones and curbs waiting for me to put the first spade into the ground ... It's ahobby, OK

 

I'll see what I can do, but don't hold your breath. First thing I really need to check is if that method of measuring yields correct results, so check it with a reference line. I don't have fun memories of undoing the steering rack bolds, seem to remember I got stuck at some point with my feet pointing upwards.

 

What do you mean by: ...And if possible, loosen the steering-endlink on the hub arm and lower that (just temporarely as its a cone) with a shim stack...? I just replaced the track rod ends, so they should be relatively easy to un-do. But, as you say, they taper, you can't get them any lower. Ideally that's what you want.

[Scuffers]

 

Scuffers explained some of it here, but no hard numbers for the front settings: http://www.vx220.org...-2#entry1548430

 

Now if he could join in here to comment on your measured values (and gave some hints where to aim for), that would enlighten us al...

 

Hello!

 

OK, looking at the graph, I would suggest he has measured it about right, (as in that looks about the right shape)

 

No, this does not mean his bump curve is 'right' just that I think he has measured it correctly.

 

without looking at it (the car that is!) this looks like a classic case of trying to run the car too low without the corresponding modifications to correct for bump etc...

 

 

as I said in the post quoted, you want it set so that on bump, it toes out, and on rebound, it either does nothing, or toes in a smiggen, ie. if you go to his measured graph, ride hight should be at ~+60.

 

moving the rack will only get you so far, and (depending on front wheel/tyres sizes) I very much doubt you can get close to right at 115mm ride heights, you need adjustable steering arms. 

[alexb]

Welcome!

 

When I was measuring around that +60, the changes in toe were very drastic, as you can see in the curve. I'd be looking for a nice gradual change in the interval that I'm interested in, which is about 1" on travel in bump and droop I assume. That's another one of my hobby horses: what's a normal interval for shock travel? I do realize that depends on the spring rate and damper setting (actually it doesn't unless you damp over critical, which I don't think anybody does), but it is important information that I can't find anywhere. Back to the subject, to me the -25 to -30 region, which happens to be the normal driving height of 140 mm, looks far more promising: no sweeping changes, everything within +-1 mm change.

 

I'm not the only one aiming for 115 mm ride height with standard components, but looking at that curve I start to wonder about the consequences. On the one side, the total change from 1" bump to 1" droop is a little over 5 mm. That's about 0.6 degrees. On the other side, I don't have experience enough to say what I will feel and what not. The thing that I find strange is the rear is so well documented and the front not. But the rear is not a problem. The geometry even with standard uprights can handle lower ride heights. The changes at the front are much bigger and probably more noticeable, but much less documented. Care to comment?

 

(PS as you do seem to know a little about these things : why is the ride frequency at the back not higher than at the front? Any combination I've seen so far has higher frequency at the front, even with the standard springs. In general, with exception of use on very level tracks, that's not a good idea. I know it's just a guideline, just wondering if someone can explain the reason)

[Exmantaa]

I think Scuffers meant that your curve around a ride height of -40/-60mm (so basically around stock height) has the corect shape; being that it does nothing/slightly toe in on rebound and toes out in bump.

 

From your front curve it's fairly obvious to see that a pair of lowered hubs can give you the lower ride height while still retaining a decent bump curve... Shame they are quite expensive, but seeing this curve they make a lot more sense to get any decent handling! (And some have spent more on fancy carbon or a pair of seats.... )

 

Group buy discount anyone??

[alexb]

Hm, you mean more money in the black hole poorly disguised as a car. I had a closer look at the changes in camber and taking the 1" both way travel, around 115 mm ride height, there's a 1.7 degree change. Around 140mm it's less than 0.5 degree. So it seems that at 115mm and standard components you're really running at angles that you're not supposed to run at. Well, you can always go back to 140 mm .... oh wait, that means going back to standard shocks .... I'm screwed anyway

 

Had my Nitrons refurbished and was on standards for two weeks. Have to say it didn't feel that bad. Not that I ever want to go back, really don't like the pitch and roll. But with a decent geo, standard height may be difficult to beat. In case I want to go that high again, I'll just drive my wife's pick up truck

[Scuffers]

you can solve the bump issue with steering arms, however, your still going to have roll centre issues at those heights...

 

if you must run that low, look at smaller wheels/tyres and/or uprights, otherwise, don;t go there.

 

Being blunt, lowering the VX is not a cleaver thing to do unless your prepared to change a lot of stuff....

[Exmantaa]

What do you mean by: ...And if possible, loosen the steering-endlink on the hub arm and lower that (just temporarely as its a cone) with a shim stack...? I just replaced the track rod ends, so they should be relatively easy to un-do. But, as you say, they taper, you can't get them any lower. Ideally that's what you want.

 

I mean here that instead of buying a pair of special steering arms, for test purpose only, you could loosen the conical rod end and maybe re-mount it with some thick washers underneat so it sits lower. Yes, it's a conical sleeve and it's a bodge, but it might give you some info with the unloaded testing...

[Hammy of hamhouse]

Does bouncing it off a mirror double the angle ? not sure ?  also the length of the laser level?? a small movement of the hub gives a larger movement of the end of the level  ?? - need someone good at maffs or do a scale drawing?? I suspect something  is amiss there somewhere in those two

 

 

http://www.physicscl...efln/u13l1c.cfm

Edited by Hammy of hamhouse, 21 January 2014 - 07:15 PM.

[Arno]

Umm.. With shims you'd be raising the rod ends as they are mounted from the top..

 

 

Perhaps someone should do 'inverted taper' steering arms and mount the ends from the bottom up...

 

Bye, Arno.