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Cut down steering rack to avoid bump steer, or not?
novicebuilder - 30/12/13 at 07:37 AM

I am fitting my new Cortina mark 2 steering rack from Rally design on my Haynes roadster. I have read most of the old threads on cutting it down to size, so that it pivots in line with wishbones to prevent bump steer. There seems to be some disagreement as to whether it should be cut down or if it doesn't really matter? The ideal length between rack ball joints would be 532mm on my frame and the rack is supplied at 615mm. I can get it cut down but it brings other challenges with it:
1. I would then have to re-make my rack mount and 2. I would have to work out how to reach the the uprights with a shortened rack (I already have the long rack extensions from Rally Design). I understand the concept of bump steer but would appreciate hearing what others did when they built their Haynes roadster and what happened in the finished car when driven. Grateful for advice from the guys who have done this before. Thanks.

Cut steering rack down to size or not?
If it is cut down how do I reach the uprights with a rack that is 83mm shorter?

ashg - 30/12/13 at 08:59 AM

if your using the mk2 escort steering rack it will be fine, you dont need to modify the rack on the roadster design. my roadster has the steering rack about half way on the sdjustable height slots and doesn't suffer from bump steer.

with the dampers off the car attach the steering to the hubs and raise and lower the front end of the car and see if the wheels toe in and out as you raise and lower the car. if they do adjust the rack height until the wheels don't steer themselves when raising and lowering the car.

i think when i did mine, once setup there was about 0.2mm change in toe between the high and low extreme positions. in normal use you wont hit those extremes even on big bumps and you cant feel 0.2mm toe change. A little bit of bump steer can be good in the correct application as it can improve stability with a bit of toe in as you land from a bump.

the older locost design had issues with bump steer as the rack was in the wrong place but i believe it has been well documented how to solve. i think you are reading that and getting confused. the roaster with a standard mk2 escort rack needs no modification.

[Edited on 30/12/2013 by ashg]

britishtrident - 30/12/13 at 09:11 AM

Cortina mk2 rack ?!

[Edited on 30/12/13 by britishtrident]

Ben_Copeland - 30/12/13 at 09:23 AM

quote:
Originally posted by britishtrident
Cortina mk2 rack ?!

[Edited on 30/12/13 by britishtrident]


I would imagine its escort not cortina as rally design only sell escort ones

mark chandler - 30/12/13 at 10:03 AM

I could not, and never would have been able to eradicate bump steer without cutting down my escort rack, original book.

You purchase "steering rack extensions" to make up the extra length insert in eBay and plenty of options pop up or get a tap and die then make some yourself from stock.

Regards Mark

[Edited on 30/12/13 by mark chandler]

froggy - 30/12/13 at 01:17 PM

quote:
Originally posted by britishtrident
Cortina mk2 rack ?!

[Edited on 30/12/13 by britishtrident]


Mk2 cortina had a steering box .

I had to cut 90mm out of my rack to get the inner pivots to line up . Hard to see small changes until I put my laser kit on which showed a couple of mm of movement which I couldn't see before . What threw me was the arc that the steering arm moved in as its offset from the outer top and bottom b/j to give some Ackerman on most uprights .

novicebuilder - 30/12/13 at 06:14 PM

Yes I meant Escort Mk2 not Cortina Mk2.

Thanks for all the replies, very helpful. Thanks Ash I will test it using your setup advice. Thanks also for publishing your build diary I really enjoyed reading it and it has proved invaluable additional guidance.