In reply to TVR Scott (Forum Supporter) :
Spitfire racks are available in a couple of ratios (nominally "standard" and "quick" at 3.5 vs 2.6 turns lock to lock, depending on which brochure, and no idea whether "lock to lock" is the same throw), though I don't know off the top of my head whether it's just a 7 vs 6 tooth thing (a quick DuckDuckGo* finds at least one conversation suggesting it isn't, though at this point I'm acutely aware that LBC info on the Internet is... fraught)
Chop, chop:
That's 1/2" off the front mounts. I got everything sandblasted, and will weld it up once I unbury the TIG welder.
Frustrating, but that is the middle name of a fabricator. Every little change affects everything else. But you got this. Definitely agree you are better off raising the diff slightly as you are doing and then lowering the tail of the differential to align the driveshaft angles. That Good Parts kit is really nice but agree that he probably has not sold many of these to TVR owners and the few that have purchased the kit probably just make similar changes to accommodate it.
Looky what was on my porch today!
I guess they were on a wild tear to ship to the US last week!
In reply to TVR Scott (Forum Supporter) :
My, but those do look familiar!
Got the mounts tacked back in:
I'm letting it cool off and then I'll do a test fit.
More diff work today. I test fit the version shown in the previous post. It cleared but just barely (really hard to see in this view):
So I pulled it back out and cut the tacks, and did a bit more grinding. I just made the plates mount flush rather than sitting on the top of the brackets.
Better:
Then I tore it all out again to do final welding and painting.
Shiny...
Gratuitous close-up shot:
The diff casting needed a little more clean-up and then touch-up paint. I sprayed a little paint in a plastic cup and used an acid brush. We'll see how it dries, but I didn't feel like masking the whole thing again for spraying.
So far I'm pretty happy with this spray paint. It's proved to stand up pretty well to a beating and seems resistant to solvents too.
Very special guest tomorrow - watch this space!
Had a really great visit with Joe TR6 this morning. Joe brought me his cache of steering rack parts - thanks!
True to screen name, he drove up his absolutely gorgeous blue/green TR6. I was super impressed with the whole car, even though he was saying something about the numbers not matching. I was too busy admiring it to worry about such unimportant facts.
He says it's for sale, and it looks about as nice as I could imagine for a TR6 that can just be driven and enjoyed. My 16 y/o daughter was ready to break her piggy bank for it!
Ok, time for Adventures in Bump-Steer!
I'm not totally sure what I'm doing here, but I'll wing it.
My concern is that a shorter TR6 rack will create a bunch of bump steer, where the longer 2000/2500 Saloon rack will be fine. Data is required.
First things first, I hard-mounted the bare rack in the clamps with 3d printed bushings. This will let me hold everything static and just look at the bump-steer movement. I centered everything up, and made an initial mark for this version. To simulate the wider rack, I just moved this rack out 7.5 mm. I'm only looking at one side, so that should be fine. All these pictures represent the "wide" version like the factory rack.
Here's the overall setup - hack-tastic!
A note: mounted here is one of the old inner tie rods. The TR6 parts are much shorter - like an inch - and would not even reach.
Here's full-droop - note the distance on the janky level / square combination:
Here is roughly full-compression:
Doing some careful measurement, there is about 1/2" of toe-out rotation between full-droop and full-bump. The pointer-arm length is 17" long, so that amounts to roughly 2 deg bump steer.
Interestingly, it is basically the same with the "wide" setup and the "narrow" setup.
What I don't know is how much the camber change is affecting this. If the hub naturally swings in 1/2" over it's travel, then it would seem I'm totally fine. I'll need to do a little more measuring and messing to get a better feel for what the geometry is doing.
General question to those who KNOW, is 2 deg bump steer OK? Bad? A disaster?
I have always gone with the lower control arm and the rack/tie rod parallel to the ground at normal ride hight. The theory being it splits the toe change equally at droop and compression, therefore giving you the least toe change.
That seemed like a huge amount to me, 10x what I've read as acceptable, and it only just dawned on me that you're measuring the path of the whole hub, not specifically the bump steer. Had to absorb what you were talking about with the hub swinging in...
My impression is that it's all but impossible to get a good measurement of bump steer with a setup like that, as the target is so much finer than what you're going to see. You need to set something up that allows you to isolate steer angle from camber/travel/etc...
If you have a hinged board that's able to follow the inward/outward travel of the hub while maintaining parallelism to the car centerline (or the zero steer at some reference travel), you can use that pointer to measure how far from parallel it gets, which will give you the actual steer while removing the camber (and the fact that the whole hub is describing an arc before camber is considered).
Er, an example: https://www.mr2oc.com/threads/diy-bumpsteer-gauge-for-under-60.452807/
Without the dial indicator, you could start getting a rough idea with just a hinged board and your existing pointer.
EDIT: More specifically, hopefully you find that one of your two theoretical rack lengths shows virtually no bump steer...
First things first. With this design where the steering arm and rack are midway between control arms in the vertical, you will never achieve zero bump steer across the full range of travel. So consider what is important and where the compromise is least damaging. Here are my parameters. The amount of bump steer from normal ride height to full droop has very little importance, as the drooping wheel is unloaded. The amount from static ride to full bump is what you feel driving, especially at corner entry. Therefore any motion towards bump must be toe out. The first inch is the most important. Any toe in towards bump makes incredible scary oversteer! It is not measured in degrees, it is measured in thousandths of an inch or hundredths of millimeters depending on your tools. Bump steer on these cars will change with any camber or caster change you make, so some method of adjustment needs to be provided. Generally shimming the rack up or down is within the original design. I use a pice of aluminum jig plate attached to the hub and a pair of dial gauges on a stand. .025" per inch of travel is achievable here.
In reply to TurnerX19 :
Thanks for all that info. Good stuff.
The good so far:
I'll work to rig up some sort of better bump steer gauge. I'm feeling daunted and bummed out at the moment, but I can see roughly how to put it all together.
I don't know exactly what my ride height is. Is there a rule of thumb like mountain bikes - ie 20% compression at basic ride height?
To the TVR old-pros who have autocrossed in the past - you do anything for bump-steer? Was it a problem or much ado about nothing?
In reply to TVR Scott (Forum Supporter) :
During race car set up bump steer is very nearly the last thing done before the car goes on track. I know that your situation makes it much easier to address now while the body is out of the way. Figure out your ride height for now and adjust the spring perches as required to achieve it after the car is fully assembled. Set your camber, caster and base toe in now with solid false shocks that represent desired static ride. Then remove them, measure and adjust bump steer. My bump steer gauge holder is an old heavy brake drum with a piece of 1/2' steel tube welded ove a lug stud to make a vertical support for a square tube cross bar that holds the dials. Dials are fixed horizontally at 12", vertical is adjustable 6" to 24" so you can work with a height variable. Random squarish piece of 1/2"X15" jig plate with a center hole and an assortment of other holes I have drilled as needed to fit everything from a Sprite to a McLaren M8F. Only new purchases were the gauges.
In reply to TurnerX19 :
Got a picture of that set-up?
I get what Jesse is saying. You are measuring several effects at once and need to separate them somehow. With your current setup, could you lock the steering angle by removing the tie rod from the rack and clamping it to the lower A-arm, then trace the pointer movement without steering effects? Repeat with the rack connected. The difference should represent bump steer, right?
Or replicate TurnerX19's setup.
TVR Scott (Forum Supporter) said:In reply to TurnerX19 :
Got a picture of that set-up?
Gosh no! I can probably take one tonight though.
Some photos of a bump steer stand. This is not the exact unit in my description, as it migrated to Florida a few years ago with a former co-worker. This one has only been used on Swift and VanDiemen F2000 cars, so the jig plate false wheel has only the center hole for small center lock hubs. Also a piece of 1" steel plate in lieu of an old brake drum. Over engineered by my former room mate....It does need to be heavy! The dial gauges were buried I know not where, so the stand and plate only in pics. One inch linear travel gauges bolt to the tabs on the cross bar, 1/4-20. Note also the marked tape on the plate. Those are 1" increments. Hub must be retained from any rotation while measuring.
second photo
In reply to TurnerX19 :
Good stuff. Thanks.
How do you account for any arc in the hub due to the natural movement of the A-Arms?
The example Jesse posted had a hinge that would follow that arc. Pegasus has similar hinged assemblies for purchase, though I have no desire to buy one.
In reply to TVR Scott (Forum Supporter) :
How do you account for any arc in the hub due to the natural movement of the A-Arms?
That is why you use two dial gauges with a full inch of travel. Watch both gauges. If perfect they will travel the exact same amount.
In reply to TurnerX19 :
Ok, sure. That makes sense.
(I'm trying to remember now if I already have two dial indicators, or just one...)
I know you're past it (but I'm not), I put out the question about whether anyone in the TVR community had experience with the Goodparts R200 conversion to the TVRCCNA forum. The results were mixed, with no real answers, but still some interesting stuff about other mounts and/or diffs. You may want to see what's up on that forum. Maybe even broach the subject of steering racks and bump steer there... might lead to some interesting input!
I wonder whether, IF you're just trying to answer which one of two racks is the correct item for this car, whether you could get by with something as crude as a board "hinged" with duct tape and your pointer bar across the hub. Zero it at normal ride height-ish, and check it 2" above and below.
That would be wildly inappropriate for trying to fully understand the car's bump steer, but if you have some faith that the car is sound when fitted with the correct rack and are trying to determine which of two options it is, it might well show one as looking about right, and one obviously wrong.
Just in case that's the situation and this is becoming a deeper rabbit-hole than you were planning to pursue.
I'll have a look. I've never really read that forum at all. Never seemed to be a lot of info.
I feel like I've got the whole R200 thing sorted out. If you want dimensions or anything, let me know. Or if you want to wait a few weeks, send me your frame!
As long as you're putting stuff together, can you measure your steering rack for me? Is it taken apart at all yet? I'm guessing you have the original Triumph 2000 rack in there, not a TR6 one like me.
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