GTD40 Rear Camber

[Andy]

Let me start some discussion on the GTD40 rear suspension geometry and camber (with the original chassis).
On assembly of the rear suspension, I noticed the top arm was quite inclinated.
This means on compression, the camber will become progressively stronger.
Something which is needed but too much to my opinion.

Better, to my feeling is to put the top bracket 60 mm (2.4" higher).

Therefore I started to investigate, calculate and measure.
The graph below is some DIY measurement using a small laser projecting on a graph.
Vertical axis is the displacement of the wheel in millimeters.
Horizontal axis is converted to camber degrees.

The Dark blue line is the initial camber measured when compressing the rear suspension. Almost 3.5 degree of camber change.
The blue dotted line is the calculated line on the change (60 mm higher position of bracket) and aiming for a 1 degree camber change.
The red line is the measured new camber envelope.

The measurements had some hysteresis due to some play between brackets and bolts (which will be resolved later on).

I am quite happy with the result.

Now the challenge is to put the bracket 60 mm higher while staying away from the reinforcement plates and it bolts. Decided to remove reinforcement plates and design a new bracket which
1) replaces both brackets
2) saving weight
3) being more stiff
4) a new position for the torsion bar
5) a new position for the top arm brackets
6) and still able to use the old location as well.

I know the idea is not new, Mike told me Roy Smart did it already in the past. Probably for a good reason :thumbsup:
If you want to have the drawing to make this bracket, let me know.
And please share your experience since this is all theory...still.

 

[Howard Jones]

Here's how I did mine. I have 100s or hours on track with this setup and it works very well. This mod is necessary for a GTD. Otherwise you have a completely uncontrollable tail happy monster on your hands.

 

[Andy Green]

Does anyone know if the Tornado chassis has the same problem? It's something I never thought to check on mine
Regards,
Andy

 

[Howard Jones]

Here's the thread from when I did mine.

http://www.gt40s.com/forum/gt40-tech-chassis-brakes-tires-wheels/16176-gtd-rear-suspention-mod.html

 

[Howard Jones]

I don't think that any other chassis had the same issue as the old GTD. I also tried to find the D Bell article on this but couldn't.

 

[Brett James-McCall]

Andy (skorpion), speak to Mick. I have his SGT rear end on my GTD that I have been building for years. He may have already resolved this and it's as easy as chopping off the rear end and welding a new one on. Also, removes the kick at the back dso you can get the engine/gearbox lower. See here: http://www.gt40s.com/forum/gt40-build-logs/15571-bretts-rs-gtd-5.html#post202729

 

[Andy]

Hi Jones,
Credits to you, initially my statement was wrong.
When I was investigating this topic and heading towards putting the bracket higher, Mike showed me your solution.
Good to hear you have loads of experience with it and it works better to your feeling. Grip and handling are also personal to the driver. So it is a difficult topic to handle since there are so many variables in play.
Weight balance car, weight car, friction of tyres (width), track conditions, all suspension wheel settings, weight of the rims..it's endless but there are big variables and too me aswell the camber on the original is too progressive.

Looking to your construction - as an engineer - I can say it looks good in terms of forces and fatigue.

In your post you mentioned 7.5 degree camber. Is this the total degree of camber of both wheels combined of only one side measured ?
Since I measured 3.5 degree on 1 side only and times two brings it to your measurements.

The downside of my bracket is that I can not go lower than 2.4" (60mm) since part of the chassis is in the way to allow the small standard bracket to be mounted onto the new main bracket.
I can solve this by integrated the lugs into the main bracket but I preferred to be able to bolt the standard bracket to the main bracket.
The benefit of your design allows to get settings lower as 2.4" as you did and took 1.5".

What is the distance between the inner faces of the 2 chassis flanges where the bracket is bolted to ? To see how consistent the GTD's are in this area...

 

[Malcolm]

In the UK the standard solution to this issue was to lower the suspension arm mounting point on the upright. Just drill a couple of new holes (make a little jig up to ensure consistency) and away you go. The new holes normally went as low as you could get say about 30 to 35 mm lower. You can do this with only needing to remove the top arm and all else staying on the car so should take about 30 minutes start to finish.

 

[Malcolm]

Just re read your initial post. Have you allowed for the car rolling in the corners or just worked on simple bump and droop of the wheel while the chassis stays flat? If you haven't allowed for car roll in your calculations above then you may find that if you are precise with simple bump or droop control only, the wheel goes from negative camber to positive camber in the corner and then you will have lots of fun you didn't budget on!

Having read your other posts as well as this one, please be aware that most issues you are trying to solve now have already been addressed by previous owners of GTDs so don't feel the need to reinvent the wheel each time. Although that can be part of the fun too!

Good luck with your restoration.

 

[Howard Jones]

Malcolm, I think I remember I took the spring off and ran the corner of the car through its complete travel. I tried to check camber every 1/2 inch or so with a straight edge, a level and some trig. I did it with the tire on the wheel and used the tire's side wall as one side of the angle.

At the time, I came up with -1 degree setup + about 6 more - degrees as the car rolled to complete shock compression.

Granted the shock does not "bump" that much in real use but the method did reveal the problem.

One of the things I wanted to do with this mod was NOT change anything on the chassis such as welding on new brackets, slotting mounting holes or drill new ones in anything. My little steel brackets did that.

Skorpion, the car is in a position that makes it hard to get into for now, New car in build process filling garage. I'll try and remember to make that measurement when I can. We did use a set of brackets on another car with similar results.

 

[Dr. Bob Woods]

I am in the process of a full analysis of the Tornado suspension and an original GT40 suspension. Should have results in a few weeks and will write a report on it. In the mean time, I will make some comments.

There are so many factors to consider in suspension design to make a car handle. When you focus on one, you will probably degrade others. There are two things to consider about camber: camber change in heave (degrees camber per inch of chassis movement up and down) and camber change in roll (degrees camber change per degree of body roll). As you make one small, the other gets large. If you lower camber change in roll, it will handle well in corners but might have difficulties in braking or accelerating in and out of corners. The opposite will happen if you minimize camber change in heave.

You have 1.25 degrees camber change per inch of suspension travel. Although that is a little high, your camber change in roll might be very good. As you make the camber change in heave go toward 0.25 or 0.10 as in your data, your camber change in roll will be much worse. It depends on the type of driving or racing you intend to do.

It is best if you use a suspension analysis software or 3D drawings to investigate all factors at the same time to make tradeoffs.

-Bob Woods

 

[Andy]

Attached you will find a calculated comparison of the rear GTD40 suspension

Left : standard setup
Right : new bracket placed higher at the top

Top: ride height
Bottom : about 2 degree body roll

Roll center calculations...

For your review :thumbsup:

If you want to play with it...(only the standard setup)
http://www.racingaspirations.com/mods/3hj2nldd

 

[Howard Jones]

Bob, I agree that a complete study should be done before changing anything under usual circumstances BUT the rear camber change problem on old GTD's is well known and has been addressed by several people in as many ways. These cars are horrible to drive as originally specked. The rear will go from seemingly fair grip levels to instant snap away oversteer until the car slows down to take the roll out and then the grip comes back just in time to put you straight into the fence.

The huge camber gain in the rear must be addressed if you want to push this car on track. On the road you are asking for a very bad outcome when you lose the car.

The front does have some issues including bump steer and lack of adjustment for camber/caster but it can be addressed with a new upper a arm and raising the steering rack. Then you have a very capable car that can run with a lot of cars that cost 4 times as much.

Ol Bettty is a pretty fun car to drive fast now. She is predicable, stops very good, and has quite a bit of grip, even on street rubber.

GTD's are good cars once you change all the parts.

 

[Cliff Beer]

Here's an idea: how about an adjustable inboard mounting point for the top link? In other words, a mount (probably fabricated heavy guage plate) with vertical slots for adjustment up and down the range so as to increase/decrease camber change through the range of motion.

Like most suspension things, it's one thing to model it in CAD, and a whole 'nother thing out on the track. An adjustable mount would provide the on-the-fly adjustment which might be necessary to get it to "feel right" Just a thought.

 

[Howard Jones]

Cliff, One of the things I did do when I changed (improved IMHO) my suspension on my GTD was remove the slots wherever I could and changed the adjustment to washers. Slots move around and things change without warning. I don't like slots.

However you could use them in such a jig to make measurements of different mount positions for testing. That would indeed be a good idea. Then once things are finalized make new brackets with permanent mounting holes in the right place.

As far as the GTD rear issue, I think that lowering the lower a arm mount points at the upright with new holes warrants investigation. It seams to work for the guys in England if I have this right. Easier than making new mounts like I did.

 

[Terry Oxandale]

Here's an idea: how about an adjustable inboard mounting point for the top link? In other words, a mount (probably fabricated heavy guage plate) with vertical slots for adjustment up and down the range so as to increase/decrease camber change through the range of motion.

A product which could help in finite steps of adjustment that has the benefits of a slot, but prevents slippage is this:

U-B Machine 55-0254 Serrated Block .5 Slot

 

[Andy]

The serrated block is indeed a good idea.
I will replace my top front brackets on the suspension with the following design. Since I am using rod ends, I will have enough options to play around.
You could drill the setting holes closer to each other to get more options and the holes may overlap eachother.

 

[Malcolm]

I say Howard old bean, that's a tough description on handling for a non upgraded GTD that you make above! Most compared it to the earlier 911 P cars in terms of breaking away at the rear. Certainly you had to treat the car with respect and pick your moments with care! However in their hey day GTDs were pretty much the only replica on track in the UK and of the others, they couldn't match the GTD pace. That's all moved on now with new replicas coming to the market which is all good news for everyone.

On the slotted holes discussion in the 30,000 miles I have done in my GTD, I have never had a slippage and that is having run about half that mileage on track and a chunk on very sticky slick tyres. Suggest torque setting for these bolts is plain FT.

I did mention it in your other thread and acknowledge the choice is yours, but from this discussion here, you would seriously gain a lot by putting on the Southern GT rear end. If the original standard GTD handles like an early 911 ie tail happy, then I can assure you that by swapping to the Southern GT rear end the handling moves up close to 997 GT3! The lower engine position is great and the revised suspension geometry takes away all the concerns you are discussing above. I took my car to Spa after having the conversion done and I was just amazed at how well the back end stuck to the track by comparison. I only managed to get the back end loose once coming through the chicane and was lapping only a few seconds per lap slower than the 997 GT3 mentioned above. Evidence can be seen via this little video I made

https://www.youtube.com/watch?v=tRRo5Jc7hiM

Between 9 mins 40 secs and 10 mins 00 secs I would suggest is one of the best 20 seconds of driving experience I have ever had!

 

[Pierre-Louis]

Malcom,

Just to get the idea, what kind of gearbox do you have with your new rear end SGT ? did you stay with a UN1 (stock orientation) ? or did you jump on something else (inverted UN1 or G50 ?)
Does that new rear end work with the stock Granada uprights, or do you have to swap to those (not cheap) new cast ali uprights ?

I'm thinking about that new rear end myself in the future, but I wonder if this involves a complete new other rear trans settings : gearbox/flywheel/clutch/CV/axles/uprights/job/wife/home ...:uneasy:

 

[Malcolm]

I run a Renault UN1 gearbox, normal orientation. I did make some custom gearbox mounts so that it was mounted on poly bushes and not the normal rubber doughnuts. The purpose here was to reduce gearbox movement which can wiggle the gear stick as I have the Southern GT solid linkage set up and not cables. That was just a small welding exercise.

My uprights are the steel GTD items I had before (standard GTD Escort Cosworth single pot rear brake calliper mounted on the side not the bottom of the disc). Drive shafts are unchanged. As is job, wife and home! Well actually I am extending the house quite a bit at the moment so does still that count as unchanged?