Alfa power build.

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Midlana1
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Re: Alfa power build.

Postby Midlana1 » 11 Oct 2017, 06:56

I checked the second edition and confirmed the 15 degree comment is incorrect - should be 8-ish (that's on page 175).

The comment about where the rocker arms go is on page 205, and is correctly stated as 10.3".

I started a list in the book thread for known errors in the second edition.

ChrisS
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Re: Alfa power build.

Postby ChrisS » 11 Oct 2017, 08:42

I should say, I’m working from the 1st edition.

ShadowCat38
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Re: Alfa power build.

Postby ShadowCat38 » 11 Oct 2017, 10:53

As am I. The second edition released literally days after I placed my order in Lulu.

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Re: Alfa power build.

Postby Midlana1 » 11 Oct 2017, 11:07

Understood. I'll edit the post appropriately so that it's of equal worth to both 1st and 2nd edition owners.

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Re: Alfa power build.

Postby ChrisS » 13 Oct 2017, 13:56

Had a couple of days with no shed related activity but did a bit today. After much messing I fixed the position of the rocker mounts and tried it out. I was a bit unsure about just how far the rocker went at full bump TBH, but after doing some careful measurements of movement at upright vs. movement at damper, I'm happy its pretty close to what Kurt intended. The ratio is within a few percent of 1:1 right up until it gets to the last 15mm of travel at full bump, when it starts to go a bit away from ideal...but it'll not get there without hitting the bump stop anyway, so for all practical purposes, it has neutral or slightly rising rate at all points of normal travel. Result. Nice work Kurt.

Looking at how high the end of the rocker goes, its probably going to need a bit of a bulge in the hood to clear that I think. I did look at trying to get the rocker at more of an angle at full droop to help reduce it, but it all starts to get a bit fraught if you try pulling it too far. I did have to trim the underside of the rocker on the damper side of the pivot and I also had to notch that short linking tube as well to get clearance for the rocker towards full bump. Now I know where it needs to be, I'll just cut out a corner and plate it over so its tidy. I'll do the same for the other side pre-emptively I think as its bound to be similar.

At full droop. Pushrod is just clear of the chassis tube. About a coat of paint in it.
Full droop.jpeg


At nominal ride height. All looks comfy here.
Ride height.jpeg


At full bump. It looks quite extreme at this point and it would start to go very wrong it it had to go much further...but it doesn't, so it's all good.
Full bump.jpeg


Damper bottom mount. I've rotated it a bit from book design to ease bolt access, and I initially tried getting fancy by keeping the box section tube complete with the top on it, but there wasn't clearance for the adjuster so gave up on that idea and reverted to a slight mod of the book design. Not visible in this shot but there is a 1-1/4" hole directly underneath the bottom damper fixing so anything finding its way into the U of the bracket can fall out the bottom.
Damper mount.jpeg



Just the other side to do now then finish-weld all the brackets and add a few extra bracing pieces. It is quite a bit of extra work to do the inboard version, but I'm happy I've done it this way.

On the hardware front, I did buy a bunch of ordinary S grade UNF stuff but the grip lengths were all over the place. I had initially thought I would just tweak them but decided it was just too much hassle and cutting a thread extension on a rolled thread isn't a great idea either. Need to return most of that now. Order placed with Aircraft Spruce for proper AN stuff now. Ouch.
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Re: Alfa power build.

Postby Midlana1 » 13 Oct 2017, 15:57

Regarding the full-bump picture above, will the tire even be on the ground where the upright is?

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Re: Alfa power build.

Postby ChrisS » 13 Oct 2017, 23:52

That position does look suspect doesn’t it. I’m working on a total of 5” of travel before adding the bumpstop, so more like 4” of free movement. Does that sound about right? It’s entirely possible I have misunderstood the damper travel figures and am trying to achieve more wheel travel than you intended or is even usable. For clarity, I have a 1.5” spacer under the balljoint at full droop and a 6.5” spacer under it at full bump. As always, comments welcomed.

I’ll have a measure today and see if that position would be using the bottom of the chassis on the road as the bump stop.....I confess to total target fascination and just looking at the limits set by my wooden spacers while totally not seeing where the wheel might be. Blindingly obvious now you mention it :oops:

I did look at the spec of the QA1 dampers but couldn’t determine if the quoted travel was without a bump stop or not. I’m not using QA1 though, the Protech 17” dampers have more travel and I’m not using it all so will need to fit a solid spacer to the shaft to shift the bump rubber down a bit so they emulate the QA1. I could have gone with a shorter damper of course, but I wanted to keep the spring as long as possible so decided to stick with a 17” open length.

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Re: Alfa power build.

Postby Midlana1 » 14 Oct 2017, 06:56

Shock travel never includes bumpstops.

We don't ever want the chassis to contact the ground. There's the obvious reason that it'll remove rivets, but more importantly, it could well cause the car to go straight off in a turn due to losing traction.

The shock hard stop is supposed to keep it just clear of the ground, while the bump rubber slows it down before it gets there.

Making this more interesting is tire deflection; hitting a hard dip can cause the tires to squish down more than an inch, which has to be figured into bumpstop height. Then there's shock valving...

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Re: Alfa power build.

Postby ChrisS » 14 Oct 2017, 13:41

Indeed, I don't have any desire to emulate the F1 types with sparks spraying off the bottom. The suspension needs to constrain the travel BEFORE the chassis can hit the deck. I don't think even I thought that having the chassis rub along the road was a good thing.

Shocks specced minus bump stops - understood, thank you. That helps me to clarify some questions.

OK, some measurements.

At that crazy looking full bump point, there is still about 5/8" of tyre below table top, so assuming no tyre compression (unlikely at that much bump), 5/8" clearance from chassis to ground. Not enough.

My nominal ride height gives bang on 3.75" under the chassis.

Pushrod is 20.5" between centres.

The above makes me think the basics are there more or less, but could benefit from some tweaking.

Reduced pushrod to 20.25", notched the rail to let it sit lower and now the rocker sits with the bottom edge closer to horizontal at full droop of -2" on ride height. Damper mount now 0.25" too low of course. Oddly(!), it should now be precisely where Kurt has it in the book. Funny, that....

Full bump still looks a bit out there but once I shift the bump stop down it'll limit that.

So, as a final sanity check, what are my targets for bump & droop? I got +3 and -2 from my reading of the book but am happy to accept I got that round my neck.

Anyway, fast forward a few hours and after typing the above, but before seeing Kurt's reply, I had a fiddle. Notched the outer rail a bit and shortened the pushrod by an enormous 1/4". Makes a difference at the angles involved though so worth doing. Also moved the lower damper mount up to accommodate the change. I intend adding some extra bracing underneath the lower damper bracket as well to compensate for the fact it's not got all that much contact with the tubes now.

It now looks like this:

At full droop of -2" from nominal ride height.

new full droop position.jpeg



How the pushrod looks close to the tube now its notched.

notched.jpeg



At +2 from nominal ride height. This is where I'll start hitting the bump stop once I put a spacer in.

new bumpstop height.jpeg


4.5 inch travel.jpeg


I intend putting a 1" solid spacer on the damper shaft to shift the bump rubber down to the position shown in the picture above. The hard stop will then be at +3" of travel, although in practice I don't see the rubber compressing to zero so it'll come before that point. This then brings the effective travel of the dampers I'm using back in line with the QA1 units specified.

It'll do I think.
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Re: Alfa power build.

Postby Midlana1 » 14 Oct 2017, 14:47

Nicely done relief, looks nicer than mine.

Even though the car is light, so little tire pressure is used that the tires deflect roughly 1/2" just sitting there, so that has to be subtracted from the ground clearance.

About hitting a dip, I did just that, seeing it in time to slow somewhat, but not enough, between using up all the normal suspension travel, squashing the bumpstops flat, and compressing the tires, causing about a dozen rivet heads to be ground off in an instant. Fortunately it was on a straight; had I hit it mid-turn (at speed) I'd have likely gone straight off. Since then I've adjusted shock valving to handling such transient cases, in addition to upping spring rates.


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