Morning All
My car has been on the road now for a few weeks and one of the issues I have is the car is twitchy above say 40mph. As in it doesnt feel stable and I
struggle to keep it in a straight line.
My current suspension settings are:
Front Camber -1.5 degrees
Rear Camber -1 degree
Front Toe in 0.5 degrees
Rear Toe in 10 mins? (handwritten sheet struggling to read it.)
Over the weekend I altered the toe in on the front to 0 degrees and dropped the tyre pressures to 16psi (from 20) and it did improve slightly. I do
have a 2.5 quick rack fitted but i have used quick racks in the past so I dont think its that.
Do any of my settings look that far out? I do need to clarify the rear toe setting to see what exactly Ive got.
Cheers
Steve
When my car was twitchy it was because of too much toe in. 10 minutes is roughly 0.15 degrees so that's sounds about right on the rear.
Is your rack secure ? Have you tried to measure bump steer ?
Hth Dan
Is your ride height correct? It may be a good idea to check the front and rear ride heights against the manufacturer's recommendations...
it's a good starting point for setting up the suspension, anyway!
If the front is higher than the back then it will get 'interesting' the faster you go - mine used to wander about, until I set it
correctly. I used the standard Westfield settings for my book Locost, which puts the rear a little bit higher than the front (I guess the
driver/passenger weight levels it off).
Best only change one thing at a time.. Tire pressures make a large difference..
do you have any droop in your suspension travel, can you jack the car up more than a few mm before the wheels leave the ground?
quote:
Originally posted by David Jenkins
Is your ride height correct? It may be a good idea to check the front and rear ride heights against the manufacturer's recommendations... it's a good starting point for setting up the suspension, anyway!
If the front is higher than the back then it will get 'interesting' the faster you go - mine used to wander about, until I set it correctly. I used the standard Westfield settings for my book Locost, which puts the rear a little bit higher than the front (I guess the driver/passenger weight levels it off).
Also, it's worth doing a spanner check now that everything has had a chance to bed in a bit - mine started to feel really wayward until I found
some things at the back getting loose (not due to my spannering, but due to design).
M
Ride height should be about 10-25 mm higher at the back - I'm at the lower end of this range at the moment and finding it very stable.
Check the bolts securing the rear toe links, mine came loose resulting in a very loose backend (but not exactly twitchy, just happy to break away
smoothly when cornering under power).
If you're using the standard GKD spring rates on your car, I would expect they are overkill at the front for your bike engine - that would be the
first thing I would change if so.
Ride height is set.
Tyres are all at 16psi. I initially had 20 then 18. Handling improved slightly at 16psi.
Rear is 15mm higher than the front.
I've done a spanner check after each shake down run out.
Suspension droops more than a few mm before leaving the ground.
I will have a look at bump steer, I've not heard of any other Gkds suffering with it but I'm the only one with a BEC.
Hi Dave I was hoping you might see this post.
No I'm not using the gkd springs I'm using 275lb front 170lb rears these were recommended by procomp as a starting point.
What sort of castor do GKDs run? You might find you need a bit more with a bike engine (lighter front end).
castor angle / tyre pressure / and tyre brand can and will effect this.
Hmm not too sure on the castor angle It isn't adjustable anyway without modifying the wishbone.
Tyres are toyo r1r's
quote:
Originally posted by sdh2903
Hmm not too sure on the castor angle It isn't adjustable anyway without modifying the wishbone.
Tyres are toyo r1r's
Castor angle (excessive or lack of) has been the ruin of many a
seven, on such a lightweight car it is fundamental to steering feel.
quote:
Originally posted by phelpsa
quote:
Originally posted by sdh2903
Hmm not too sure on the castor angle It isn't adjustable anyway without modifying the wishbone.
Tyres are toyo r1r's
Not being adjustable doesn't mean that it isn't your problem however
Well I've been in touch with gkd, castor angle is 3 degrees. Is this about right? He also said there is no issue with bump steer with the legend. He did however reccomend a full alignment and corner weighed. Standard answer I guess.
Bump steer is always an issue, you need to check and shim as appropriate... No matter how good the chassis design you don't need to move the rack up or down by much to change bump steer..
Bump steer ?
Not sure if this affects GKD's but jack up the front, lock the steering wheels, remove the shock springs and move the wheel from full droop to
closed, you should have minimal deflection on the wheels when pointing straight forward.
This will make it very twitchy !!!
Mine did this first time out.
It turned out, that the ride height was too high front and back, making the shocks top out in the dips of the road.
Brad.
Hmm, I thought castor was much higher, around 7 degrees IIRC. I'd check with Dave at Fibreform who did the chassis design. I agree with Pete
about bump steer though, I've never found any issues there - the height of the rack is fixed at what appears to be the right position.
Can you be more specific about the twitchiness? Under what circumstances does it occur? Is the problem more at the front or the back?
quote:
Originally posted by sdh2903
Well I've been in touch with gkd, castor angle is 3 degrees. Is this about right? He also said there is no issue with bump steer with the legend. He did however reccomend a full alignment and corner weighed. Standard answer I guess.
Hi.
As above 3 deg is low for your application. It would be OK "Just" with a big BMW lump in the front but for a BEC you really want 5 Min - 7
deg. As also mentioned though check rake / ride heights. The other thing that greatly affects the finer points of handling is the dampers. GKD as do
most of the manufacturers just supply STD valved dampers which are not really suitable for lighter weight cars. With the compression being set to high
this can make it feel twitchy as though the spring poundage is too high even with light springs fitted.
Suggestions would be get the rake sorted then the bump steer mapped out and have the dampers Dyno tested to see A. if they are valved correctly for
the application. And B. matched and in balance.
Cheers Matt
quote:
Originally posted by procomp
Hi.
As above 3 deg is low for your application. It would be OK "Just" with a big BMW lump in the front but for a BEC you really want 5 Min - 7 deg. As also mentioned though check rake / ride heights. The other thing that greatly affects the finer points of handling is the dampers. GKD as do most of the manufacturers just supply STD valved dampers which are not really suitable for lighter weight cars. With the compression being set to high this can make it feel twitchy as though the spring poundage is too high even with light springs fitted.
Suggestions would be get the rake sorted then the bump steer mapped out and have the dampers Dyno tested to see A. if they are valved correctly for the application. And B. matched and in balance.
Cheers Matt
sounds like bump steer to me. take the front dampers off and jack the car up and down in the middle and measure for changes in toe. if the toe is changing as you jack it up and down adjust the hight of the rack until you minimise it. if it still does it after you have tried the rack in loads of different positions then the rack may be too long or short at the inner pivot points.
Make sure you have a decent amount of travel before the bumpstop at your normal ride height. if your hitting the bumpstop too soon then the effect is
like having a massive increase in spring rate. This will make it twitchy though bumpsteer does sound favourite. Maybe a touch too much front camber.
I would have gone with that ammount on the rear and half that on the front, Rear toe in (it is toeing in not out?) sounds good, and again halve it for
the front. As for Castor anywhere between 3 and 7 degrees will be fine. Theres no substitute for a full 4 wheel laser alignment as its the only way
you can properly check thrust angle.
I will check for bump steer when I get some time later in the week just to rule it out.
I'm going to take it back to where I got the geo set and have him corner weight and generally cast his eye over it.
Cheers for all the input
Steve
quote:
Originally posted by chillis
Make sure you have a decent amount of travel before the bumpstop at your normal ride height. if your hitting the bumpstop too soon then the effect is like having a massive increase in spring rate. This will make it twitchy though bumpsteer does sound favourite. Maybe a touch too much front camber.
I would have gone with that ammount on the rear and half that on the front, Rear toe in (it is toeing in not out?) sounds good, and again halve it for the front. As for Castor anywhere between 3 and 7 degrees will be fine. Theres no substitute for a full 4 wheel laser alignment as its the only way you can properly check thrust angle.
Front and rear axle alignment might also be an issue.
Hi.
Bump steer may be present in the geometry. BUT you need to map it out in both vertical movement and in a roll situation.
Some build bump steer in to the geometry to give a gain in grip levels.
Don't believe everything that's been written in in books a long time ago.
Cheers Matt
All of the above but going back to simple things damper settings too hard ?
Well a bit of an update.
I did a quick check for bump steer and there is very little movement at all.
However I did find what i think may be the culprit. My car uses a spitfire rack with half bmw half spitfire column and the lower triumph universal
joint, which has rubber washer type 'donuts' in it. when moving the steering side to side there is a lot of slop in these donuts which
I'm hoping is the cause of my issues. Ive ordered an uprated donut-less version and will give that a go.
Thanks for all the suggestions and will report back when i get the new bit fitted.
i used to run only 12 psi in the front and 16 to the rears with an r1 lump and it was nigh on perfect.