OK, here goes...

[Spacenut]

Hi everyone - the more I read on this forum, the more certain I am that someone can help me get out of a geometric hole with my Nova. That is, a Nova kit car from the 1970's, not a Chevy, most certainly not a small Vauxhall!

 

The suspension I am using is from a Lee Noble designed Lotus 23 replica. The wishbones look very similar in design to the Ultima which Lee designed at the same time. The geometry is a classical SLA, with a front roll centre around 50mm off the ground. All the wishbones are Rose jointed, and in the absence of any specific alignment data, I have set a caster angle of +5°, and zero camber with parallel toe. Wheels are 7 x 15" Weller Racing steels with a 16mm offset and 185/65HR15 tyres (its a car from the 1970s after all ).

 

The front uprights are Mk3 Ford Cortina, with a standard KPI of 4.8°, and the steering rack is also Ford.

 

The major problem I have is steering kickback. When I first got the car on the road the springs were waaaaay too stiff, but a proper analysis of the suspension natural resonant frequencies sorted that. Still the steering kicked like a mule. I had the rack modified to accept a telescopic steering damper, which helped a bit, and confirmed that the bump-steer geometry was OK (shimmed the steering rack up a bit, that was all).

 

Still the kick-back.

 

Finally I come around to the idea that the scrub radius might be the problem. I have drawn out the wheel and tyre combination with the mounting plane etc., and plotted where the upper and lower balljoints are located in relation to it. The KPI line intersects the ground 90mm from the centre of the tyre contact patch.

 

This, I presume, is rather a lot???

 

From what I am reading on this excellent forum, my oversized scrub dimension might also explain why the inside shoulders of both front tyres have worn down to the limit, while the rest of the tyre has virtually no wear!

 

This being the case, I need to reduce the scrub dimension to something more sensible. For various reasons I need to change my front uprights. This gives me an opportunity to increase the KPI (from 4.8°) and mitigate some of the scrub. I have sourced some Alfa Giulia brakes and using Alfasud hub bearings in a suitable fabricated carrier I can increase the KPI to 12°. Is this too much?

 

Obviously I will have to redesign the wishbones as they will be too short for the new uprights (which will tuck further inside the wheel).

 

...And I will have to get new front wheels with more offset to restore the front track. This is a headache, as my 15" steelies (ET16) are no longer available from Weller Racing, and the only alloy wheel design that looks suitable (I'm sorry, but modern wheels on a car like mine just look naff), is the 15" TH-line from Schmidt, which in spite of being a 3 piece split rim can only achieve a maximum offset of 25mm with a 7" rim.

 

When I put this all into my drawing the KP axis intersects the ground at 30mm positive. This is less than half the scrub I started with, so I assume that the kick-back force will be halved as well, but is it enough?

 

When I add up the cost of fabricating uprights, extending wishbones, track rod extensions (fortunately available for Caterham and Westfield "wide track" conversions) and new wheels and tyres, I am in pretty deep, so I would like this to work. If the benefit is not going to be that great I may as well just repair the existing damage and live with the kick-back and the tyre wear (its taken 8,000 miles to wear the inside edges down, some FWD cars do that anyway ), but I would really like to sort this problem; in this respect my car is many times worse than the original Beetle-based Nova, and given the amount of time and money lavished on it over 16 years, that's a bit galling

 

Lots of questions I know, sorry!

 

Lauren

[Tony]

Difficult one without more information.... Kingpin off-set, castor off-set and roll force lever. As the car stands now what is directly adjustable?

[Spacenut]

Difficult one without more information.... Kingpin off-set, castor off-set and roll force lever. As the car stands now what is directly adjustable?

 

Well, directly adjustable right now, camber, caster (by winding in and out the various Rose joints), toe and ride height (on the SPAX spring plates - about 2" of adjustment available here). With the new upright, all of the above plus KPI, and wheel offset can be changed. I can't tuck the wheels any further into the arches, as the tyres foul the chassis rails and limit the turning circle.

 

I'm afraid I haven't come across kingpin offset, caster offset and roll force lever terms before. If the latter refers to roll couple around the roll axis, I haven't determined the C of G, but the car as a whole stands 38" high, and the engine is a boxer unit, so I am guessing a C of G height of about 12-15"? The roll axis will be lower at the front, but I haven't worked out exactly how low...

 

What should I be measuring/doing to provide a better picture of what's going on?

 

Thanks for your help - this is really interesting... in a brain-exploding kind of way

 

Lauren

[Tony]

Knowing the wheel width, actual camber and KPI i could calculate how much tyre is in the SR, this is all well and good but theory is one thing actual results another. The only real downside at the moment is the inability to move the camber negative, but being able to project the KPI should belay this inability.

 

I would like 9 degrees of KPI, 5 of castor and zero toe, obviously you have zero camber but i assume on bump the migration is negative?

[hms]

Hi Lauren,

were you at 'Soft Top Sunday' at Goodwood? think I saw it there plus it also got a photo in iirc complete kit car or Total Kit Car.

 

It may be easier for you to get to WIM and have Tony get it up on the ramps so his system can pick up all the measurements.

 

h

[Spacenut]

Knowing the wheel width, actual camber and KPI i could calculate how much tyre is in the SR, this is all well and good but theory is one thing actual results another. The only real downside at the moment is the inability to move the camber negative, but being able to project the KPI should belay this inability.

 

I would like 9 degrees of KPI, 5 of castor and zero toe, obviously you have zero camber but i assume on bump the migration is negative?

 

Thanks Tony - just to be clear, camber is adjustable as well, I have just used zero as a starting point. You are correct, migration on bump is negative.

 

So you would recommend a KPI of no more than 9° for this application then? Given the brake and wheel clearance options that would give me a scrub dimension of about 70mm. Is that acceptable? The Nova is fairly light (about 900 kg with fuel and driver on board), will the steering kickback be reduced to an acceptable level?

 

Thanks for your advice - its greatly appreciated!

 

Lauren

[CIH]

what's "SLA" by the way ?

[Spacenut]

Hi Lauren,

were you at 'Soft Top Sunday' at Goodwood? think I saw it there plus it also got a photo in iirc complete kit car or Total Kit Car.

 

It may be easier for you to get to WIM and have Tony get it up on the ramps so his system can pick up all the measurements.

 

h

 

Hi, H - yes, that was me, hiding forlornly under the tin roof. I thought the people at Goodwood would see the funny side if I drove in with the canopy open - after all, it is an "open top", but they didn't see the joke and gave me a right dressing down. Its rather ironic that CKC should have snapped me there (I haven't seen the magazine, but I've been told I made an appearance), I've been to several breakfast clubs where I have been "on-theme"... Oh well, they do say any publicity is good publicity!

 

I would be happy to go up to WIM, but the problem is that my existing uprights need to be replaced due to one of them having a bent spindle, and with the poor scrub geometry they impart (KPI = 4.8°) I thought I would re-design to optimise scrub and improve my Ackermann geometry at the same time - hence my endless questions...

 

Do you think that a session on the ramps prior to any modification would be useful?

 

Lauren

[Spacenut]

what's "SLA" by the way ?

 

It stands for "Short-Long Arm" - the upper wishbone is much shorter than the lower. I believe some designers prefer this arrangement, others prefer equal length wishbones. I guess it has a lot to do with how well the body roll is constrained, as I believe the SLA can cause some large excursions in the instantaneous roll centre

 

Lauren

[CIH]

ah, MX5 has un-equal length wishbones too (or rather, did, pre multilink).

[Tony]

Dealing with the SR won't impact the Ackermann, this would come from the TRE or hub design.

[hms]

Hi Lauren,

were you at 'Soft Top Sunday' at Goodwood? think I saw it there plus it also got a photo in iirc complete kit car or Total Kit Car.

 

It may be easier for you to get to WIM and have Tony get it up on the ramps so his system can pick up all the measurements.

 

h

 

Hi, H - yes, that was me, hiding forlornly under the tin roof. I thought the people at Goodwood would see the funny side if I drove in with the canopy open - after all, it is an "open top", but they didn't see the joke and gave me a right dressing down. Its rather ironic that CKC should have snapped me there (I haven't seen the magazine, but I've been told I made an appearance), I've been to several breakfast clubs where I have been "on-theme"... Oh well, they do say any publicity is good publicity!

 

 

Do you think that a session on the ramps prior to any modification would be useful?

 

Lauren

 

I'll see if I can scan the mag, if I can find it for you.

Regarding going on the ramp at Tony's, he's the one to ask, although you might get useful base information from the machine. I know the machine can account for bent wheels, don't know if it would account for bent spindles.

By the way, he will charge for this, but it's a minimum fee, and you get to know exactly where you stand geo wise. (Tony correct me if I'm wrong on this!)

 

h

[Tony]

Your spot on hs..... A full chassis image costs £30+ and we can go way deep into the analysis for that.

[hms]

If you're going on a journey, it's always worth knowing where you are starting from!

h

[Spacenut]

Of course you need to charge for such work, I wouldn't expect anything else!

 

But 30-odd notes for a starting point sounds very reasonable.

 

Like I mentioned before, I love my car and I just wanted everything to be right - but there is a limit to what you can do with 40-year old bits (I mean the Cortina uprights in this case), and it may be that I am just being picky - after all, Cortina uprights were good enough for Lee Noble to use on the track, why shouldn't they be OK on the road?

 

(Apart from the minimal KPI)

 

The more I look into fixing the geometry problems, the more issues get thrown up - to reduce the scrub radius I will have to:

 

(a) Redesign the hub carrier, or use a production item that has a better geometry to start with (Sierra, Wilwood etc.)

 

(b ) Redesign the wishbones to suit

 

(c ) Buy at least two new wheels with offsets in the ET49 range (which won't be available in "classic" styles)

 

(d) Spend lots of money

 

I've become fixated on scrub radius as the main problem with the car's driveability - but am I being too harsh? I know the scrub isn't usually 90mm, but should I be that worried?

 

My alternative approach would be to have both spindles machined off and bolt-in spindles fitted. I can get this done by a grasstrack specialist who produces modified uprights for oval racing. A set of heavy duty aluminium hub carriers to replace my cast iron jobs and the repair is complete. I keep the Ford KPI of 4.8°, and my steel wheels, and the scrub radius remains at 90mm.

 

I was going to change the angle of the steering arms if I redesigned the upright because the projected line from the arms intersect the vehicle centreline about 12" behind the rear axle line. As it is the steering is rather heavy for a mid-engined car, but that would also be retained in my "no frills" solution.

 

Once everything is fixed, I drive over to WIM for a full geometry check and target parameters such as camber, caster and toe (front and rear) can then be established. I then only need to purchase a new pair of front tyres and I am back in business!

 

Should I be taking this approach instead? On the basis that optimising the geometry will significantly improve the driving experience, how awful would it be?

 

Sorry for the about-face on the re-design, I've been grovelling around under the car in the rain today and I'm feeling a bit downhearted. I've had to increase my ride height by 15mm to get the trackrods parallel to the ground (the springs have settled over the last year of use), and my damper top mounting bolt has cold-welded into the sleeve of the bush and I can't get the bloomin' thing out

 

As always, thanks for all your helpful advice!

 

Lauren

 

PS - don't worry about the scan HMS, I went and bought a copy of CKC yesterday. What a waste of £4.25 - a new Tiger 7even that really stands out from the crowd because the back end resembles a Series 3 Lotus. I'm sooo impressed

 

It wouldn't be so bad if it was dynamically OK, but the road test reveals a rock hard ride and poor brakes - all of which will be "fixed before the car goes into production", that well-worn chestnut

[Spacenut]

Sorry, I meant a Lotus 7 S4 - the one that was only available as a fully built car...

[CIH]

Look at this thread, if you haven't already. Proxy diagnosis is all well and good but you can't beat having a good, 1st hand look.

Not to sound like a coroprate shill but I'm sure a trip to WIM will be of immense benefit.

[Tony]

You know what i think you are over analyzing things..... I fail to see why the "off-the-shelf" components can't be configured to include a decent scrub, ever more so since the chassis is adjustable..... It really would save time to book the car in and let us expose the entire chassis positions, at least then you have a real time clue toward your conclusions.

[hms]

And for those who didn't see the mag.

 

 

h

[Tony]

Nice one h

[Spacenut]

You know what i think you are over analyzing things..... I fail to see why the "off-the-shelf" components can't be configured to include a decent scrub, ever more so since the chassis is adjustable..... It really would save time to book the car in and let us expose the entire chassis positions, at least then you have a real time clue toward your conclusions.

 

Thanks Tony, I think you may be right, although I can't imagine how you can achieve a sensible scrub radius with a 4.8° KPI unless you specify ludicrous amounts of wheel offset. I had a chance at Goodwood to look at any number of 7even clones (being soft-tops they were in a huge majority), and all of the Cortina-based suspensions have the same geometry issue - massive scrub radius. The modified Sierra upright improves matters a bit (KPI = 9°), but still places the wheel centreline a long way from the KP intersection with the tyre contact patch.

 

In fact, the only cars with half-decent scrub were the Caterhams, which in spite of marketing their own branded uprights are really using the old Triumph Herald geometry.

 

There again, my ultimate Supercar muse, the original Countach LP400 (no wings, no wheelarch extensions, no Pirellis) had a 9° kingpin angle and a scrub radius comparable to that of my Nova, if the section drawings in my book are to be believed... and I don't read many contemporary road test reports slamming the feedback through the wheel. So maybe I am over-analysing matters.

 

OK, here's what I will do - I will get my existing uprights repaired (with the bolt-in spindles), replace the wheel bearings and hub carriers for heavy-duty aluminium units and then book a sesh with you to get the full geometry picture.

 

I'll limit the use of the car from now on so I still have a legal depth of tread on the front tyres - I don't want to be replacing them while there is still a tracking issue (bent spindle = wobbly bearings).

 

Hooray! This way I get to keep my steel wheels and M16 calipers!

 

Lauren

 

PS - thanks for the scan HMS!

[Tony]

Reads like a deal

[hms]

Lauren,

you're welcome.

Keep us updated

h

[hms]

Lauren,

you're welcome.

Keep us updated

h

[CIH]

Echo!