GTD quick rack.

[Robert May Eng.]

Did anyone ever find a quick rack set up for the gtd40 .Its been a few years since I last played with the 40 and if I recollect the cortina rack was not ideal in the first place ( bumpsteer) . I just wondered if any progress was made on the rack front before I do some research or have a one off made.

Bob

 

[Howard Jones]

I cured my bump steer problem mostly, I would say it was better by 90%, by raising the steering rack with shims by 7/16". The middle 70% of the normal suspension travel is pretty good now, especally on track where the road is fairly smooth.

Now the front isn't trying to jerk the steering wheel out of your hands if you curb the car at the apex of a corner.This is a cheep fix (good enough for me) for bump steer.

I am happy with the ratio. Above 125MPH I wouldn't want it any quicker anyway. 125 is about the spot where the brakes begin to really get worked at the end the two long straights I run the car on most offen.

Stopes nice and straight but I think it might get twitchly if the steering ratio was a lot quicker.

Thats my 2 cents worth.

 

[Howard Jones]

I have a GTD also. I cured my bump steer problem mostly, I would say it was better by 90%, by raising the steering rack with shims by 7/16". The middle 70% of the normal suspension travel is pretty good now, especally on track where the road is fairly smooth.

Now the front isn't trying to jerk the steering wheel out of your hands if you curb the car at the apex of a corner.

This is a cheep fix (good enough for me) for bump steer. I am happy with the ratio. Above 125MPH I wouldn't want it any quicker anyway. 125 is about the spot where the brakes begin to really get worked at the end the two long straights I run the car on most offen.

Stopes nice and straight but I think it might get twitchly if the steering ratio was a lot quicker.

Thats my 2 cents worth.

 

[Robert May Eng.]

Thanks for the reply, I vaguely remember the bit about shimming the rack but it sort of goes against the grain to compromise on the bump steer, I think I would like to see it correct from the off if at all possible. A lot of the problems like a kerb wrenching the wheel from your hand come from the massive scrub radius of the granada upright. I have had southern gt replace the whole of the backend to sort out the lift off oversteer and I am now going to attack the front. The granada hubs are going so there wont be any scrub radius, the top wishbone will be re made to facilitate a bit more caster so it just leaves the rack. I would like to fit a quick rack at the same time I fit the corsa steering column.

Bob

Edit. is there an alternative to the cortina rack??????

 

[Malcolm]

Howards suggestion to raise the rack cures the bump steer and was a factory fix for the issue. A quick rack doesn't change bump steer situation. Changing the geometry of the steering joints without raising the rack will change things if you are happy to work out the correct geometry position. Raising the rack moved the joints into the correct plane so sorted the problem.

Can't remember who but someone did get hold of a fast rack and fitted it but swapped back as they didn't like it. Probably too fast and so spoiled the driving.

Are you trying to solve bump steer or the speed of the rack?

I have the same Southern GT mods you have. Haven't felt the need for a quick rack though. Not heard of the Corsa steering column being used before. If you are going to use Southern GT front arms and uprights on your GTD (I think that is what you are saying?), whilst there is a difference to the Southern GT in a GTD I would be careful going away from Micks geometry. Did all that years ago but then went to Mick and have been happy since.

 

[Robert May Eng.]

Howards suggestion to raise the rack cures the bump steer and was a factory fix for the issue. A quick rack doesn't change bump steer situation. Changing the geometry of the steering joints without raising the rack will change things if you are happy to work out the correct geometry position. Raising the rack moved the joints into the correct plane so sorted the problem.

Can't remember who but someone did get hold of a fast rack and fitted it but swapped back as they didn't like it. Probably too fast and so spoiled the driving.

Are you trying to solve bump steer or the speed of the rack?

I have the same Southern GT mods you have. Haven't felt the need for a quick rack though. Not heard of the Corsa steering column being used before. If you are going to use Southern GT front arms and uprights on your GTD (I think that is what you are saying?), whilst there is a difference to the Southern GT in a GTD I would be careful going away from Micks geometry. Did all that years ago but then went to Mick and have been happy since.

The car is being built up with track days in mind Malcolm. The bumpsteer, scrub radius, uprights ,front brakes and steering geometry are on the hit list. I have a few options as far as the uprights and brakes go, the lack of caster is also to being dealt with at the same time. But first on the list is the rack, the corsa column will be in and the shaft to the rack will be a one off so I dont want to have to do this twice, the rack is also a pain to get at once installed so it wants to be a fit and forget job. I will get the measuring stick out and see how far out all of this is.

Bob

 

[Malcolm]

Be interested to see what you do.

 

[Glenn M]

Hi Bob.
Easy cure for front geometry - use a Southern Gt chassis! A bit of a smart arse comment I know, but just trying to point out that it is possible. The front end is superb, no bump steer at all from factory and the front end grip is phenomenal - I've never had a hint of understeer (always a problem with these cars) and in the wet, which always shows up faults, it is sublime.

Re quick rack; I too desired a quicker rack when pushing on hard, but drew a blank, so I had a different ratio pinion and rack bar made for the Cortina rack. I investigated many different custom racks, for race cars normally, but couldn't find one with the correct width and with the pinion in the right place. It wasn't a cheap process as you can imagine, but then that's why we build them ourselves isn't it, so that we get exactly what we want. I am really pleased with the rack and it's speed and wouldn't want to go back to the standard one for anything. I think it's one of those things that you don't know you're missing until you try one! - especially on a track.

Glenn

 

[Robert May Eng.]

Hi Bob.
Easy cure for front geometry - use a Southern Gt chassis! A bit of a smart arse comment I know, but just trying to point out that it is possible. The front end is superb, no bump steer at all from factory and the front end grip is phenomenal - I've never had a hint of understeer (always a problem with these cars) and in the wet, which always shows up faults, it is sublime.

Re quick rack; I too desired a quicker rack when pushing on hard, but drew a blank, so I had a different ratio pinion and rack bar made for the Cortina rack. I investigated many different custom racks, for race cars normally, but couldn't find one with the correct width and with the pinion in the right place. It wasn't a cheap process as you can imagine, but then that's why we build them ourselves isn't it, so that we get exactly what we want. I am really pleased with the rack and it's speed and wouldn't want to go back to the standard one for anything. I think it's one of those things that you don't know you're missing until you try one! - especially on a track.

Glenn

I appreciate the input Glen. I think I will right one wrong at a time here:idea:.
Irrespective of the rest of the set up the true steering rack length is set in stone by the intersecting top and bottom wishbone pivot points. This cannot be changed so first job will be to determine that length, I have made contact with a guy that makes the escort quick racks and he can cut a new rack and pinion for around £150. Ratios are 2.2,2.4 and 2.5 to 1 lock to lock.The rack housing will remain unaltered which will be a plus. The only problem I will have is there are minimum quantities of ten of each type so could you let me know what ratio you did go for in the end and I will follow your lead. I only need two of these so the next problem would be to off load the remaining 8. Any takers on here????????????????

Bob:laugh:

 

[Mick Ridley]

I appreciate the input Glen. I think I will right one wrong at a time here:idea:.
Irrespective of the rest of the set up the true steering rack length is set in stone by the intersecting top and bottom wishbone pivot points.

Bob:laugh:

Bob,

If you do that, will it give you zero bumpsteer ?

Mick

.

 

[Robert May Eng.]

Bob,

If you do that, will it give you zero bumpsteer ?

Mick

.

There are a few other factors that impact on this but the basics are in the diagram below.

 

[Glenn M]

Bob, I'll have to dig out my papers to remember the ratio I used. I'll look at the weekend.

Won't the Escort rack have the pinion in the wrong place and be the wrong length?


Glenn

 

[Mick Ridley]

Bob,

Thanks for the diagram, I can see Near zero bump steer is possible..

I was at Thruxton, about 25 yrs ago, when I started my GT40 (Thats another story.) and was chatting to a guy in the Pit-Lane, who was racing a Le-Mans style car, Im not sure what class of cars they were, but they were all "one off" cars, and asked him what would be a good steering rack to use for my GT40, and he adviced me to use a Triumph Herald steering rack.

The length is 27.5 in. which is within half an inch of my ideal length, so I should be able to keep the bump steer down to a minimum. My wishbones layout is very similar to the J / MK IVs cars

Just as a matter of intrest, what is the ideal length of steering rack for your car

Mick

 

[Robert May Eng.]

Bob,

Thanks for the diagram, I can see Near zero bump steer is possible..

I was at Thruxton, about 25 yrs ago, when I started my GT40 (Thats another story.) and was chatting to a guy in the Pit-Lane, who was racing a Le-Mans style car, Im not sure what class of cars they were, but they were all "one off" cars, and asked him what would be a good steering rack to use for my GT40, and he adviced me to use a Triumph Herald steering rack.

The length is 27.5 in. which is within half an inch of my ideal length, so I should be able to keep the bump steer down to a minimum. My wishbones layout is very similar to the J / MK IVs cars

Just as a matter of intrest, what is the ideal length of steering rack for your car

Mick

Mick a rack that is short can be made exactly the right length in situ with rack extensions, too long and its a machining job.

Bob

 

[Malcolm]

Interesting, you imply that adding extensions is acceptable but raising the rack so the joints align was a compromise too far. Surely if both give zero bump steer then both are acceptable or both are a compromise too far?

 

[Robert May Eng.]

Interesting, you imply that adding extensions is acceptable but raising the rack so the joints align was a compromise too far. Surely if both give zero bump steer then both are acceptable or both are a compromise too far?

Malcolm I dont think raising the rack or adding extensions too it are ideal, both are a partial fix for a system that was poorly designed in the first place. I have already paid for all these components once and most of car two`s suspension is in Micks scrap bin :furious: Raising the rack only improves a situation whereas the extensions could eliminate it. This is car two I am building at the moment, all I am trying to do is make the thing feel a bit more planted at speed and over less than ideal road suface`s. As is it feels very twitchy and does not inspire me with much confidence, car one was on a test track a few weeks ago and was up to 167mph,the driver backed out of it because of the "feel" of it. Just should not be imho.


Bob

I would like to add that for normal road use virtually non of this applies and driving the finished car displays fine road manners but 70mph is not what these cars are about.

 

[Malcolm]

No problems with anything you say Bob, but from that description of the problem experienced, I would not blame bump steer but general suspension set up. Probably castor was too low if high speed instability occurs. GTDs have got closer than that to 200 mph with the rack raised method and bump steer wasn't a problem. I've done nearly 30,000 miles in my GTD and I would say my car doesn't have bump steer. But then again it doesn't do 200 mph either! Hope you find what you are looking for.

 

[Paul V]

Re: Bump Steer Elimination - GTD quick rack.

Hi from new member,

I joined partly to post some information to your discussion.
To eliminate bump steer the inner pivot point for the steering arm when using rack and pinion steering only has to align in the plane connecting the inner pivot points of the control arms.The extension of the imaginary line from the steering arm does not need to coincide with the roll center as the imaginary line connecting the control arms and meeting at the longitudinal center. Aligning the inner point is more crucial than aligning the outer point. If you do not want excessive tire scrub when turning sharply at low speeds then the outer connection should fall on the same plane as the plane intersecting the upper outer and lower outer pivot points. a general rule of thumb for more stable feel at high speed is that the line drawn through the pivot point should intersect the tire about 2% or so off the centerline of the tire, most usually to the inside. Maybe I can post a chapter from a book I have that covers steering systems. In it anyone can find information on how to correct or redesign a steering system so it works properly all the time. Not sure why this wasn't done back when the GT40 was first designed and built.
The other angle that is of concern can only be viewed from above the car and this is the Ackermann angle. This can be found through a relatively simple calculation and depending on where the rack centerline is located in relation to the vertical line connecting the outer pivots, the calculation will tell you what the angle of the steering arm needs to be to get the correct Ackermann angle. There is an interesting story about the Akermann effect.

 

[Paul V]

One off steering racks can be obtained from Maval Steering or they can modify one, especially if it is too wide (long). Check their website as they may have one that is perfect for your application ready to ship. They are an OEM supplier but they do custom work too.

 

[Dylan]

When some of you are saying, "making bump steer minimal" or "eliminating bump steer", what are the actual measurements at the wheel, over what travel?