Controlling the roll center at the top of the axle would make the center pretty high in the vehicle. I am having trouble visualizing this ( a sketch maybe) but if the relationship between the rack and gears move at all in any plane I would think it would bind?
Use a Jacob's Ladder instead of a Watts Linkage. Takes up a lot less room.
In reply to ShawnG :
its roughly a similar amount of room from my perspective, unless obscenely small parts are used, as far as I can tell. I would like to keep as much of the car above the floorpan/axle level as possible, which isn't necessarily possible in all cases, but for most of it, its definitely possible. my design measures up at less than 30 cm wide (it just needs to fit between the frame rails, along with the link bars to connect it to them), 6 cm wide ( any spinning/moving parts would be covered and protected, and it would be fitted as close to the same level as the frame rails as possible), and 2.5 cm tall. From what I can see, the jacobs ladder also has the same problem as a panhard bar, which is almost completely unacceptable for my purposes-any amount of sideways travel is undesirable, as I'm not building for a drag race.
What purpose are you building for? A properly designed panhard bar would allow lateral movement in the neighborhood of .1" in a road car length suspension travel, is lighter, simpler, and can have roll center adjustment built into it.
Can you use a simple roller in a track like the diDion tube is located on a Taraschi or a Lister? The vertical track could be as simple as 2 square tubes. Roller bolts via a tripod to the back of the final drive housing, and its vertical position, and thus the roll center, might even be made adjustable. Vertical track on the chassis. Very small, and only a stout wheel bearing assembly for the roller. Weight not much heavier than the rocker and 1/2 the mass of the arms of a Watts linkage. It is heavier than a panhard rod, though.
In reply to TurnerX19 :
I'm not using an IRS setup to begin with, modifying a rear end into a di dion tube would be both less effective and far more costly.
In reply to Racebrick :
I'm building for road racing, I don't want to use a panhard bar.
In reply to CornerAddict98 :
Road racing is a perfect application for a panhard bar. Also, I understand that the watts linkage isn't geometrically perfect, but you'll never notice. I had a P71 with a factory watts linkage and it was incredibly stable and predictable. 10/10, would buy something with that style of suspension again.
I went with an underslung watts setup. I don't think it takes up much more room than you propose, and it puts the roll center very low.
There are some details towards the bottom of this page - https://grassrootsmotorsports.com/forum/build-projects-and-project-cars/1981-camaro-c-prepared-build/59501/page7/
white_fly said:In reply to CornerAddict98 :
Road racing is a perfect application for a panhard bar. Also, I understand that the watts linkage isn't geometrically perfect, but you'll never notice. I had a P71 with a factory watts linkage and it was incredibly stable and predictable. 10/10, would buy something with that style of suspension again.
Yes, the tire will have more movement in squirm than the panhard bar.
In reply to CornerAddict98 :
I did not imply that you should convert your live axle to a diDion, only that you copy the lateral location technique with the roller located to the live axle and a track on the chassis.
CornerAddict98 said:In reply to TurboFource :
essentially, any movement in one rack must be mirrored by the other, just as the pivoting link on a watt's link forces both ends to move in opposite directions. So, the same general motion as a watt's link but linear instead of elliptical. it wouldn't bind up, assuming its fitted properly and all the teeth ar seated, unless outside debris gets into the mechanism.
I struggle to understand from this drawing how you intend to constrain the rack for the suspension to articulate.
Additionally, for a road racing setup a panhard bar will have negligible lateral displacement and watts link non-linearity at the extremes of travel is a non issue when you're talking something that has to travel maybe 3" max for road course duty.
Fundementally the idea of a watts link but using continuous gear teeth in place of discrete levers works to accomplish the same goal over a wide range of travel, but I think you would find that what you're proposing, if even feasible to execute from a constraint standpoint, would be exceedingly difficult when it comes to appropriately sizing it, supporting it, and managing debris ingress.
Edit: fundamentally this is a non starter because the axle needs to be able to move fore-aft and there is frankly no way, even if you were to figure out how to pin the racks to pivot on the chassis and constrain them to the spur gear, that you could keep them meshed and aligned from the side-view.
Considering this, just use a watts link, they fit on SN95s and have been done plenty of times before. Put it on the back of the axle housing so that the roll center isn't way too high and it will do everything it needs to.
Idk if this is worth it or not and I don't really care. Sounds cool, have fun with it! Curious to see how it comes out.
In reply to Gimp (Forum Supporter) :
I hadn't thought about mounting it on the bottom. From the link you posted it does look like it has most everything I'm looking for, I'm curious tho, at a certain point, do to the arcs each link follows, wouldn't the width of the rear end interfere with the actuation of the centre link? It seems like this is only accentuated by the angle the link bars would need to be at in relation to the axle, unless you could find a way to mount them nearly even with it. It seems like a good idea, but it appears to have tho opposite problem that mounting the link on top of the rear end would have.
To clarify, I don't want to add a lot more weight to the vehicle (i.e., a cage to hold the rear end so Ican bolt the controll arms to a point vertically close enough to the pivot just to make it work), and it seems like that would be the only option with this method. While its a good fall back if I can't find anything else, it doesn't seem ideal. Theoretically, I'd like to keep the weight under 3000lbs for the entire car when its finished, the lower the better.
In reply to spandak :
My question is, have you ever heard of anything similar? I'd rather not need to design the whole thing from scratch, altho its not terribly expensive or labourious to do so, I'm not 100% sure how much force it needs to hold, so my biggest concerns are extra weight from being overbuilt, and lack of strength from being under built. If you have any info on how to calculate that it'd come in handy.
In reply to TurnerX19 :
Hm... I'm not entirely sure what you mean. In all of the drawings and pictures I can find of a DiDion tube, it shows the differential mounted to the body and a "lateral locating arm" that appears to work similarly to a panhard bar. Is there a different method?
In reply to Racebrick :
So, you understand how not having any movement from a panhard bar at all will help to limit the overall movement? Perfect! So what do you suggest instead of a panhard bar?
In reply to CornerAddict98 :
That's not what he said. The flex of the tire is an order of magnitude larger than the lateral movement of a properly designed lateral link in a road race application. The tiny lateral movement of the axle due to the arc of the Panhard is negligible compared to the deflection of the tire.
I not saying the gear rack is a bad idea, but it's probably not going to show up in lap times. As for parts, it seems like some steering racks might be a good place to start.
Can you diagram how the racks would attach to the other parts of the car? I'm not getting this. But I'm fascinated.
CornerAddict98 said:In reply to Gimp (Forum Supporter) :
I hadn't thought about mounting it on the bottom. From the link you posted it does look like it has most everything I'm looking for, I'm curious tho, at a certain point, do to the arcs each link follows, wouldn't the width of the rear end interfere with the actuation of the centre link? It seems like this is only accentuated by the angle the link bars would need to be at in relation to the axle, unless you could find a way to mount them nearly even with it. It seems like a good idea, but it appears to have tho opposite problem that mounting the link on top of the rear end would have.
To clarify, I don't want to add a lot more weight to the vehicle (i.e., a cage to hold the rear end so Ican bolt the controll arms to a point vertically close enough to the pivot just to make it work), and it seems like that would be the only option with this method. While its a good fall back if I can't find anything else, it doesn't seem ideal. Theoretically, I'd like to keep the weight under 3000lbs for the entire car when its finished, the lower the better.
I run out of shock travel well before I run into any limitations due to the watts link. This is an autocross car, but I still probably have 6-8 inches of total travel available (I don't use nearly that much). You can see some pictures of the axle movement here - https://grassrootsmotorsports.com/forum/build-projects-and-project-cars/1981-camaro-c-prepared-build/59501/page12/
CornerAddict98 said:In reply to TurnerX19 :
Hm... I'm not entirely sure what you mean. In all of the drawings and pictures I can find of a DiDion tube, it shows the differential mounted to the body and a "lateral locating arm" that appears to work similarly to a panhard bar. Is there a different method?
The design Turner is referring to is also often called a "sliding block". A strong pin on the back of the axle and a track on the chassis that only allows the pin to move up/down, with some clearance for fore/aft/rotation.
Autodelta(Alfa Racing dept) had a sliding block set up. They tend to be maintenance nightmares and the roll center moving with the differential causes more handling issues than the side to side movement from a panhard swinging through its couple inch arc.
jgrewe said:Autodelta(Alfa Racing dept) had a sliding block set up. They tend to be maintenance nightmares and the roll center moving with the differential causes more handling issues than the side to side movement from a panhard swinging through its couple inch arc.
The roll center change is annoying, but watts link is the same and panhard is still half the change. I think the larger detriment is that as the body rolls, lateral loads produce a potentially large jacking force due to the angle that those forces act on the sliding track once it is canted over. Mounting the track on the axle could mitigate that issue and keep the roll center more or less static relative to the CG.
It'd also be tricky to design a slide block which accommodates things such as roll steer without lots of play (and noise and therefore wear). The beauty of linkages is that they allow for displacements out of plane which lets your suspension do the things it needs to do.
Another note on the gear rack, if you draw this out with some suspension travel, you will find that lateral displacement of the axle does actually occur due to the tangency of the racks off the pitch circle of the spur gear, which means that it will be potentially worse than some panhard bar setups.
An unrelated but related question; Why are you putting a 9" in a SN95 that already had a 8.8?
Patientzero said:An unrelated but related question; Why are you putting a 9" in a SN95 that already had a 8.8?
I am imagining ease of ratio swaps and better floater setup availability may play a factor.
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