Front Suspension Geometry

[Jim c]

This is something I have been toying with for sometime in my head and on paper.
It is the front suspension on my car RF105.
In general I thought the look let it down it has a degree of bump steer which is not a large amount and can be sorted easy with the original set up.
I have been told the Ackerman is out so with layout I can bolt a steering arm onto the front face which will fix both these.
The bearing is a modular bearing with ABS senser for traction control or speedo.

One of the other reasons for the change I feel that a wheel alignment should be a simple job as the service side of the car should not be a hassle.
I have put external adjustment for camber and a caster adjustment in the top arm with a 5/8 clevis.
The top arms can be spaced up or down from the upright to adjust roll the centre.

I have prototype arms top and bottom I am happy with them in general but will source some suitable ball joints instead of the spherical bearings.
I have found the housings and spherical bearings are restrictive in droop in a race car it would not be a issue but more street than track it will be.
I could move a few things here and there to get them to work but you seem to run a fine line but with the ball joints I feel I will have more scope for changes.

One thing I have done with the upright is offset the top and bottom holes to make it easy to make this gives me the inclination of 7.5 deg which is as per the STD stub axle.
I am thinking I may half this as it may not require this much.This I would like feed back on.
In turn I would have to lengthen the top arm in turn reducing camber gain but with the small amount of travel on these cars I don’t think I will see much anyway.
The longer arm may work in my favour as far as ride and feel. This I would like feed back on.

I worked out the angle through the ball joint centres but the bolts run parallel to each other with the top set back in toward the chassis
So in effect the ball joints are not on the same axis but it does not seem to have any ill effect if anything I think it is pulling more neg on turn in than the STD set up,This I would like feed back on.
I have notice manufacturers doing some odd stuff with front ends (2 upper ball joints 2 link set ups and seem to get the same effect as I think it may induce caster on turn in hence neg also.
I still need to finish the top link to take some readings as it goes through its motions so I need to confirm that yet.
(yes the upright is wood its cheap and easy to make changes)

The main reason for this post is that when I posted my rear end I learnt a lot from members on this site.
I realized how much there is to learn.

So knowing suspension is all about compromise I would like to get some feed back on what I am doing to iron out any major issues that I may not see.
I can give more specific figures when I finish the top caster link I can then strap it all down and get accurate readings and post them over the next day or so.

The car will see some track work on road tyres Im hopping to get 16" rims over the brakes.
Some road rallies ect
I don’t know spring rates yet as would like to scale it first but I don’t want it to stiff.

Jim

A thought on the 2 upper links with 2 ball joints on the top off the upright.
If one link ran forward as per top arm in pic then another ran at 90 deg to the wheel back to the chassis I think this would allow you to run less neg camber but on turn in it would increase caster and camber allowing you to run less static neg.
The offset being the steering may get heavier on tighter turns ?.

[Frank Catt]

Hi Jim, its good to see that others are taking a very radical " blank sheet of paper" approach to front suspension design, its easy to just bolt on what someone else has made, but it invariably do's not work as well as a custom design for your specific application. I have been on this for a little while, and found that each chassis has unique characteristics that will affect the way it works. My approach is to set the tyre footprint / wheel station postion first, from this work out the steering knuckle position to get the ackerman right, then consider and set the king pin inclination so that the intersection point is correctly set under the tyre. These various decisions will give you the wheel offsets, and indicate the brake and disc positions. Its no good hanging the hub onto the chassis then hoping everything else will fall into place, or compromising the ultimate handling and braking set up just because youve already done the suspension arms. Once the swivel positions are set, then at last you can design the upright, then the suspension arms themselves. Seems backwards I know, but it works. Hope this helps. Frank

[Frank Catt]

Trying to add picture ( one thing I cant seem to do )

[Frank Catt]

try again !

[Jim c]

Thanks Frank looking at your pics has answered one of my questions about the ball joint centre line being more an offset is ok.
And the approch you use I will remember.
If I leave the KPI I can leave the arms but If I change it I will have to redesign them.

Thanks

Jim

[Frank Catt]

Jim, KPI is one of the critical elements, dont ignore it because it is convenient. Let us know how you get on.

[Big-Foot]

I was just about to ask about the optimum KPIA and how it was derived. Also - How did you determine the optimum Ackerman? It seems that some schools of thought are now saying that cars such as these actually function better with anti-ackerman.. Something I read in Racecar Engineering a number of months ago..

Great work and keep the updates coming!!!

[Jim c]

Randy

Being a street car I will run ackerman.
The only info I found was from the centre of the rear axle string a line forward to the front axle and this is the path the steering arm follows.
It is also effected by forward or rearward location of the rack.
(Carrol Smith book)
If someone has more info on this it would help.

Franks quote
Jim, KPI is one of the critical elements, dont ignore it because it is convenient. Let us know how you get on.

Is the kpi not there to selfcentre the steering wheel and also with caster will give you camber change on turn in.
A few have made the comment that rule of thumb kpi is similar to caster figures (I would think around the 4 deg).
But having said that the camber gain helps due to the minimal travel expected in these cars.
Is this the convenience you are talking about.

Jim

[Frank Catt]

Jim, Forget conventional, forget the "experts", you have to work this out yourself to make your car work, and that may not be "conventional" at all. My cars are set up with 14.7 degrees KPI, and I have been told time and again they cannot possibly work like that. Problem is we have just ( last week ) had three days testing by some of the best qualified test drivers on two race tracks in the UK, and all three were more than delighted with the way the car handles and turns, saying that by a long margin this was the best of any GT40 replica they had all driven. So you can be a free thinker and get results, stick at it and it will come to you. Jim, my word "convenient" in my earlier post meant "easier ! ", Frank

[Jim c]

Got you Frank

Jim

[Russ Noble]

So Frank,
Can you outline for us the rationale that was behind the decision to run nearly 15* KPI? Also how much caster are you running?

Cheers

[Frank Catt]

sEE pAUL tHOMPSONS VIDEO ON GT40S at Silverstone

[Paul Thompson 'Hooligan']

http://www.gtd40club.co.uk/videos/Si...rch%202007.wmv

If you 'right-click' on the above link and then 'save-as' - it will copy straight to your PC (30MB) - Avoiding return/repeated downloads will save my bandwidth - thanks.

[Russ Noble]

Nice footage Paul.

Frank, can I assume the relevance of this video is that 6GTD has your front suspension on it?

I was wondering what led to the decision to use 15* KPI. Was this an initial chosen design parameter, if so why, or did it result from from having to compensate for some other feature? Just interested in the reasons behind the decisions on this "radical blank sheet of paper" setup.

Cheers

[Jim c]

I am no expert in this feild ( dont flame me if im wrong) but I did some numbers I know it gets the scrub radius in a much better position (lighter steering and less kick back).
It helps with camber gain as the top arm will be shorter.
I would think you could run less caster as the camber gain is increased through the arm length not in the caster and give lighter steering.

Jim

[ChrisL]

Another non-expert here Actually the whole concept of KPI has never even occurred to me so it’s really got me fascinated. At a glance I would conclude that increasing the KPI would have the effect of increasing the roll resistance on the loaded end (working on a similar principle as anti-squat except rotated by 90deg). Would this be correct? If so then it would be a great way of keeping the car level without the use of a heavy sway bar or the complexities of reducing the roll couple… or maybe I know just enough to be a danger to myself

[Martin Weigold]

I have to say that the new suspension setup that Frank has done for me works a treat. The car is extremely enjoyable to drive, and you can really attack the corners with it - a comment both Mike Wilds and the Evo test drivers commented on.

[Jim c]

As a point of interest and a datum was there a car with new and one with old suspension to back to back.
Or lap times pre new suspension same driver for comparison on the improvements.

Jim