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Thread: Fitting a steering rack to a spaceframe chassis

  1. #1
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    Fitting a steering rack to a spaceframe chassis

    So I'm fitting a power steering rack to a spaceframe chassis.
    Ill be using either an R8 rack or Gallardo rack as they're almost identical in build and length.
    I've heard that there is a process to follow in order for the rack and wheels to sit correctly (to position the rack correctly).

    This process involves jacking the chassis up so the front wheels are off the floor (after loosening the suspension?) then lower it back down with the wheels dead straight, then position the rack to suit from here?

    Or something like that....

    Anyone know what this is or exactly how it should be done correctly.
    There is no position for the rack to be mounted marked on the chassis. This has to be done from scratch.

    Ideas anyone?

  2. #2
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    Hi.
    Ideally it is best to get a rack first & then to setup the chassis & suspension to best fit the steering rack.
    If your uprights have adjustable steering pickup points/arms, you can get away with using a rack that's close to the right fit.
    To get the hight of the rack, pivot point 3 must line up with a arm pivot points 1&2 & 6 must line up with 4&5.
    If all lines up, the hight is set. Make sure everything is square.
    I use temporary brackets that I can clamp the rack to, then without the springs mounted, the wheels can lift up & down.
    With the rack now in place, lift the wheels one at a time, seeing if the wheel turns in or out when going up & down.
    If it does, then you will have bump steer. Now you can move the rack back or forth, or the outer pick up on the wheel up or down to eliminate the bump steer.
    Hope this helps you.
    Cheers Jose'

    Attachment 31674

  3. #3
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    Hi Jose, That's a pretty good example of a rack fitment, I have read a lot of info on bump steer, but have not found an answer to the following, would you also agree that if point 6 was say two inches outside the kingpin line that point 3 should also be 2 inches outside points 1 and 2, and conversely if point 6 is 2 inches inside the KPI that point 3 should also be 2 inches inside the line of points 1 and 2 my understanding is that the arms should follow the same arc, and don't forget mr ackermans view also, that the arms if a rear steer rack should ideally point at the centre of the diff or be at least within close to that point, iam using a front steer rack, and also the steering arms and hubs need to be from a donor with a similar wheelbase. I believe it is also possible to move the rack forward and aft to adjust the Ackerman. I am using the Toyota Soarer or sc400 hubs and they appear to have the steering point on the KPI so NO Ackerman so what to do then, because when I mocked up the donor suspension in my project the wheels definitely turn more on one side than the other so there is Ackerman present , but I'm not sure how if the steering point is on the KPI
    in NZ its illegal to heat or bend the steering arms to achieve this Ackerman angle. We have a very strict 500 page build manual to follow, and I have attached a pic of their view on Ackerman, and bump steer. But I am very interested in your opinion on the above question on the position of the steering point.

    Best Regards
    Graeme
    Attached Images Attached Images   
    Last edited by Lambolex; 06-18-2015 at 06:19 PM.

  4. #4
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    Hi G.
    You gonna give me headaches with questions like this
    1, 2, & 3 must always line up. 6 Will effect your acma, being a few inches out on a front mount rack, & in on a rear mount rack.
    To get the bump steer out, the hight of 6 can be adjusted.
    When I start designing a chassis, I always get the rack first, then build the front frame & a arms to match, this saves a lot of headaches when doing the final setting up on the car.
    The diagram shows the ideal setup, but you will still have fine tuning to do, even if all the measurements are spot on.

    I have added a pic of the uprights I like to use. The steering arm bolts on to the spindle, so it gives you a lot of room for adjustments.
    By fitting the arm to the spindle with just the back bolt will leave the pickup point of the steering free to be moved up & down. Tighten the bolt so the arm needs to be tapt up or down with a hammer & check the movement of the wheel by lifting it up from where you would have maximum droop to maximum lift.
    Once the bump steer has been eliminated, you will know exactly where the front bolt needs to be secured, then just ad a backing plate to the spindle with this mounting point drilled on it & secure the arm.

    If your upright has no acma built in, it can still show the effects if the rack is not exactly where its should be, it normally happens when the rack is moved to far back or forward by trying to eliminate bsteer.
    So basically one arm will be straightening while pushing thus getting longer, while the other will be pulling in at a angle getting shorter, so one wheel will turn in sharper than the other.

    I the rack has bolt on pick up points where 3 will be, you can always strip the rack, shorten the inner shaft, & have engineering cut new treads for the pick up joints to go on to.
    This will give you some adjustment to the hight you can go if your uprights are fixt.
    Steering can be a real b... to get right & can spoil a very nice car, but its like building a motor, just follow all the right steps, & hours of fine tuning, & it will work right.
    Ps. & listen to as much advice from lotus you can find
    Cheers Jose'
    Attachment 31699Attachment 31700

  5. #5
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    Rack and hub position

    Thanks Jose, that makes it clearer can you tell me the brand and model of car the hub shown comes from, as a adjustable steering arm makes a lot of sense , given I cant modify what I have, we used to have something similar here 30 years ago in a GM Holden HQ product but you cant get them any more, do you have any dimensions, and are they front steer.
    I had this info salted away I think from Locostusa but its gone from their site, but here is a screen shot as the word doc is two big to post, have a look at this comment by Sportscardesigner this what I was getting at about the outer steering point being either inside or outside the KPI and as you can see if the arm attaches outside the KPI then he seems to draw the inner pivot outside the 1 and 2 points shown above, I suspect my steer point must be slightly outside the KPI, because the current setup does have some ackerman, I have mocked up the original Soarer sub frame and complete suspension on my build board but the inner points are all in line as in 1-2-3, my concern at present is also the Ackerman ,as the soarer wheelbase at 2690 is much longer than the Countach at 2540
    Best regards
    Graeme
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  6. #6
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    Hi Graeme.
    The pic is of a mustang spindle, not sure which model. The ones i'm using are of a chev. Most of the old cars like the chevs, & mustangs use these type of spindles with loose steering arms. I normally use them on heavy cars, & on smaller light cars I use alfa gt uprights, also with the loose arms.
    If you get one with quite a flat mounting surface, you can of course just flip the arm to use as front mount & vv to use as rear mount. Otherwise just have new ones laser cut to the shape & size you need.
    On the brakes side, with the spindles having flat mount surfaces, the backing plate that I cut for them allows me to fit just about any caliper to it. I tend to use the ap racing ones with Toyota landcruiser, bmw x5, or ford focus st discs.

    Most of the info you seem to have on the acma will be correct on your setting up, but I have found, especially with setting up the atoms, that the computer setups & the real life driving setups can be quite different from each other. Unless you know exactly all the specs of the car, eg weight, size of tyres, spring rates ect. you are only going to have a starting point off the suspension programs.
    The atom must be the car were the smallest settings can make the car handle completely different.
    I battled quite a bit getting it to have a happy all round setting, then eventually got hold of the lotus exige setup specs, which was just about spot on as the cars have a similar centre of gravity.
    That is the nice thing about having the adjustable bits, when you drive the car, there is a lot of room for setting changes.

    Regarding the 123, not being in line, I cant really comment on, if one can get away with it, as I've never gone out of my standard setup, which has always worked for me.
    Maybe sometime we can put together a basic setup thread for those who need to do setups like this, "if there isn't one already" sure it would be helpful, as I know how long it took me to learn all this cr*.';,.

    @ stevewushu , I hope all of this is helping you.
    Cheers Jose'

  7. #7
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    I kinda get what you guys are talking about but it's a bit over my head. think I'm gonna get a specialist to set this part up for me.

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