1976 Nitro Grass Drag Set Ups

[midniterupper]

Hi Mutt, I appreciate your input and thanks for doing the math! Kinda gives a good range of theoretical top speeds to consider with the gear ratios doesn't it. Anyway, the drive shaft over all length measures out at 19 1/2" Hopefully today I will be able to assemble the drivers onto the shaft. May have to drill new holes for the two outer drivers but I did notice two more holes on the shaft that are not used with the two driver set up, not sure if those will line up properly. I'll check this post for your reply throughout the day in case you may have some dimensions for me. Thanks again!

[mrrupp1]

Hi Paul I wish I could give all the dimensions on installing a late model track, but I wasn’t smart enough to write anything down. I took the swing arm bogies and brought them in to run in the center drive cogs. I have plenty of spare liquid tunnels. So I just flipped it over and centered the track then I match the drivers and then drilled the holes. I don’t think any of the original holes work. I just don’t remember. The four holes in the driveshaft are for the Magnum because they had a four band track. I do know a 5/16 drill bit is the perfect size for your pin hole. I did make some spacers to show where to put the inner drivers I will see if I can find it. I’m sure you won’t have any problem getting her to fit. I hope some of my input helps make your Rupp faster. Clint

[mutt]

Actually the measurement was for me. I bought a Cat parts sled was very rough but had a nice tunnel. Had a couple of driveshafts in the belly didnt think much other than the extra parts. Its looking promising though measured the one its very close 19 3/8 or 5/16 but dont know what it came out of. I will need to compare to one I know came out of a 15 track sled to see if they are all like that or one from a 16 track. Will take some time.

[midniterupper]

Cool! Thanks guys, all the info sure is helpful. Today I was able to get some work done on the sled (in between watching the I-500 on line, Great Race!). As you all know very well, everything takes longer than anticipated, especially as we gain seniority on this planet. I figured I would look at the suspension to ensure everything is sitting straight and square. It wasn't so I had to play and adjust the springs to make the front cross shaft was equal on both sides. Then I took measurements of the inside of the track for the drive lug center to center distances then measured the center of the idlers. I see why you had to relocate the swing arm idlers, they just don't line up so I'll have to do the same. Get them all in line while I'm at it. I am making sketches of the set up with dimensions in case we may need them for future reference.
I worked on the rear axle set up also, that really took up time today. I measure the optimal center to center distances for the 4 wheels to follow according to the track's drive lugs to get the best clearances. After a lot of consideration, I found it easier to just make a new axle shaft and modify the bushings to accommodate the new dimensions due to using new wheels. Still have a little machining to do to it yet but it's looking good so far. Never know until it's in place!
Clint, I sure appreciate your words of wisdom regarding your procedure of lining up the track drivers on the axle. It makes perfect sense, I'll follow the same. I'll slip the track in the tunnel then install the drive axle assembly and get that in place. Then I can center the track, get the drivers in the correct location then use a 5/16"transfer punch to mark the holes. Get them on the Bridgeport and we should be good to go. I do like your idea of using bushings between the drivers, may help reduce the stress on the spring pins too.
Still working on removing the chisels too, I haven't found a way to enjoy the job....
Until next time, -Paul

[midniterupper]

Hey Clint, How did you relocate the upper swing arm idlers? Mine have to go closer together by about 1" and the shoulder bolts are too short to extend out. Wasn't sure if I need to cut and weld the brackets to get the offset that I need. Also, I could use some Rupp replacement idler wheels as these are beginning to crack and they wobble pretty bad too, do you guys have a source for them? I saw some for sale last year at the Marion Vintage oval races and swap meet but I didn't know I needed them at the time. Hopefully they will still be holding their event as scheduled for February 15, I will look for some then but I'd like to get the sled together asap.
Today I finished the rear axle and got the 4 idlers attached to the slide rails in their correct location for the new track. Also got the two tiny front wheels in position on the forward axle too. Took way more time than I expected to get everything aligned (hopefully!) properly and in position for this new track. Not easy but should be well worth it!
Let me know how you did yours when you get a chance, I sure appreciate your help!
-Paul

[mrrupp1]

Hi Paul, I just cut the idler mounts and move them in and weld them back up,it works great. For the swing arm bogies wheels any 5” with a 5/8 bearing mount will work. I have used 5” 3/16 bogies as long as it will clear the swing arm. They just have to have a 5/8 mount. I think you can get them from Dennis Kirk they will be plastic body with a rubber outer, the size is 5” 1/8 or 5” 3/16. They will work. If you can’t find any close to home. Let me know and I will dig some up and get them to you asap. Hope this helps. Clint

[midniterupper]

Thanks Clint! Your expertise never fails to astonish me! I will work on it today to see how you did it, looking forward to getting it done. I went through the old archives in the shop and found a couple of old manuals that are proving to be helpful. One is a 2002 Skidoo Racing manual and the other is a 1995 Arctic Cat Performance manual, both have all kinds of tips regarding engine mods and clutching setups, track and suspension info, lots of theory and more. The books are old and are starting to fall apart but pretty valuable. Both (Duh!) have specs for the 4th wheel rear axle and also the measurements for several drive axles and drivers. They go by part numbers only but it's easy to cross reference them to see which machine they go into.
I can make copies if anyone could use the info. I must be getting old, I bought those manuals brand new back in the day and now they apply to Vintage sleds! I forgot I had them, got boxes full of books. I'll have to dig through them one of these days, won't have to strain my brain so much!
Also, We have a nice gentleman here on the forums that has been following our posts (Mrmotors) that is sending me the Rupp idlers that I need, sure appreciate his support and generosity. You Ruppers are the Best! Keep ya posted on the progress, Thanks again!

[upmagnum]

When relocating the upper idlers you can just use one and center it like Rupp did with the 75 free air Nitros. I have done this with no problems plus you get a minor weight savings. Every little bit helps in drag racing. Joe

[midniterupper]

Okay, finally have some time to get caught up with the progress on this sled. Worked on it yesterday, ended up cutting the existing upper idler wheel brackets using a die grinder and cut off wheel. Before I did though, I placed the entire upper bracket on a flat surface to ensure the idler brackets were level and flush with the upper bracket surface, it was. That made it easy for relocating the idler brackets when we go to weld them. I located center of the upper bracket and marked that, then transferred a measurement 2.45" on each side from center. That gave me a distance of 4.90" between the idlers (same center to center distance as the 2 middle wheels on the back axle). Of course, tacked everything in place and ran out of mig wire! After completing that part of the project, I finished removing all the chisels from the new track. Wasn't too bad actually, used a 5/32" allen wrench socket mounted on a 1/2" breaker bar on the stud and a 7/16" deep well impact socket (and gun) set at #3 and it blew them right off without rattling my hands. I'll have to remember that trick, worked great.
The 4 wheel rear axle measurements (if anybody needs them) to fit a newer style, 4 driver track (everything measured from center) is: 4.90" between the center 2 wheels, 4.07" from the inner wheels is 4.90". When assembled, the outer wheels measured at 13" between centers. Maybe that will help somebody someday.
Today I finished up the 4 driver front axle. It took some time because I wanted to ensure the drivers floated sideways about a quarter inch between each row of lugs on the track and then I wanted to make sure the drivers were also centered on the tunnel, so a lot or installing and removing but it worked. Sure is helpful having a Bridgeport with a digital readout to make sure all hole locations were dead on. Worked out very nicely. Here are the measurements I ended up using for my 4 driver axle, using the bearing end as the reference edge. The axle actually rested vertically on the bearing edge for all measurements, should be the same for all of them that way I figured. Anyways, all drivers are measured at the center. They measured out as follows: 2.125", 6.125", 11.125" and the last one is at 15.125".
I'm excited to reinstall everything but I'm getting tired and have to get ready to go downstate tomorrow, so I'll hold off until later this week to wrap it up. Hopefully everything will line up nicely when it's all back together. Thanks to all for the support and suggestions, maybe we'll write a book!

[midniterupper]

.... Okay, Couldn't wait. Put the axle shaft on the lathe, only took .040" off to pretty much clean up and true the drivers. I forgot I had to remove the outer lugs on the two outboard drivers so I did that, then I went and trued up the outside edge of the drivers to eliminate most of the "wobble". Only took off about .020" from each side, they look much better now. Looks even better installed in the sled! We have just over and inch of clearance on both sides between the track edge and the inside of the tunnel. The tunnel protectors look like they will work also. I think we are on our way!

[midniterupper]

Well, it's been a while since I gave an update on the progress on our Nitro here in Michigan. The track went in without any difficulty but the suspension needed some tweaking to get the upper swingarm wheels aligned so they wouldn't rub on the drive lugs. No big deal, just a little shimming and we are good there. The replacement track is a bit shorter, had to pick up 1" longer track adjustment bolts to set the track tension correctly.
Installed the 13 wide plate gears, ended up with a 20 tooth top gear and and the 39 tooth bottom. That all fit like it was made for the the sussled, even used the Cat roller chain tensioner set up. It looks very solid, I don't think we will have any ratcheting on these gears at all!
The sled started easily, sounds strong, rotates the track without any issues or weird noises. Engages (with the good tach) at 5400 rpm and seems to rev up to 7500 rpm pretty easy while on the stand. I didn't push it at all, still burping the cooling system. All in all, consistent with the way it used to be which is good, we can get back to dialing it in later.
Now, I have a new issue: With this track I'm using, there isn't much room for the studs clearance inside the tunnel. I was going to use the same studs I removed from the original Rupp track but they will hit the aluminum. I would like to lower the suspension on the tunnel to gain some more clearance. Would love to use longer studs if I had the room. Have any of you guys re-positioned the suspension to allow studs? If so, how did you go about it? Would love to hear some of your ideas, still have many good events to attend before Winter is over
Thanks again for your help!
-Paul

[mrrupp1]

Hi Paul, I have built plates, and dropped the the stock suspension out of the tunnel where the suspension bolt is right under the running board it's about a 2 inch drop.. It should give you some more room. I have the dimensions for the plates at home. I use 1/8 plate steel, aluminum probably would work. I used steel because I trail ride pretty hard and I not very little. Do you have enough room for the studs by the bulk head? My oval sled I think I'm using 7/8 studs and they clear with out the plates, but I am using a Rupp track. I do not drop the front springs just the rear swing arm. I run those plastic simon cleats on my trail sled and they clear pretty good. I think they measure 1"1/8 from the top of the track. You do have to trim the swing arm down because I put it in between the plates. I'm not sure this will give you the clearance you need, but I think worth a try. It works great on my trail sleds. I will round up all the spec and get them sent to ASAP. Hope they help. Clint

[mrrupp1]

Hi Paul, I just remember I used 1/8 inch thick plates because they are the same thickness as the plates Rupp used so yo don’t have to do anything to the swing. It will fit with no modifications. Have good night. Clint

[midniterupper]

Thanks Clint, I wasn't sure how to approach this one yet, I just looked down the tunnel after I installed the first stud and rolled the track to see if they will hit. No room between the track and tunnel but plenty of room around the drive axle area. These are only .875" studs but I would like taller ones if I can get them in there.
Your bracket sounds promising, I'll study the situation some more today and see what we can do. Thanks again Clint, you have been very helpful and encouraging! Chat soon, -Paul

[mutt]

On your track being too short. I swap tracks in and out on my other sled all the time. On those old Camos and even a soft Kimpex I have when not mounted in the sled they shrink up hard to get in when before they fit. Adjust cold once they warm up they go back to their length and be too loose and they would rachet. I think bottom line have to adjust tension when their warmed up good. A lot of the time a studs length effectively will be longer when mounted in an r track because the belting is thinner

[midniterupper]

Hi Mutt, Thanks for your reply. I agree, the Camo track did stiffen up while it rested in the loft for 20 years. It is beginning to relax now that it's on the sled, rotates by hand much easier overall too.
I had some time to look over the bracket situation on the rear suspension that's needed for the studs. Thanks to Clint for the idea! I ended up making a bracket for each side. I made them out of 3/16" stainless plate that measured approximately 2.75" wide by 4.00" long, a little overkill but I can whittle the size down to make them look original. I did as Clint mentioned and spaced the new hole just under the tunnel's floorboard, enough room to get a socket on the nut. I still have to finish things up on it but it looks great so far, plenty of room for accommodating studs and for some reason, the rear suspension seems much smoother and will now allow a much improved weight transfer on the holeshot. We will see! I need to do a little more machining yet to secure everything and then install the studs. Pretty much ready to get on with the testing and fine tuning, still looking for some MR-10 Poo arms to try out also. Got some ideas for modifying other arms laying around here to try out too.
Well, that's it for now. I gotta get busy!
Thanks again to all for your helpful input and ideas, I feel this sled is truly a community effort. When we start winning, we'll need a billboard to accommodate everyone's names!

[mrrupp1]

Hi Paul, sorry I didn't get the specs of my plates to you. I own an auto shop and we been putting in some crazy hours, but it sounds like you have it figured out. One thing I did do after I lowered the suspension I put in some blocks on the tunnel where the end of the spring hits, so I could get some more adjustment sense I took it away when I lower out of the tunnel. I hope that made some sense, sometime I tend to ramble. It does give it a better ride not great, but better. Can't wait to here how she runs. Clint

[midniterupper]

Hi Clint, no problem on the print, I will post a picture of the bracket that I made. It was actually pretty simple but it works great. You have a good idea with adding a extra block where the spring arm meets the tunnel support, I haven't added them yet but it'll be in the works for tomorrow. Hopefully I'll button it up and take it out, the road out back is icy and smooth, perfect for testing.
Until the next time.....

[midniterupper]

Okay, We have some good news from Northern Michigan! I got the sled together today, installed about half the studs in the track for now and we have plenty of clearance between the track and tunnel.
Added some fresh fuel and took her out for a little test ride, not a warm sled by any means for sure! Coldest day of the year so far this winter, certainly refreshing.
Anyway, the sled is awesome. It rides relatively well, no more ratcheting noises at all (I know, you've heard me say that before!) but this time it's like an entirely new sled. Clutch kicks in nicely (well, abruptly) at 5500ish revs quickly to 7800-7900, Hang On! The track breaks loose like it is directly attached to the throttle lever, instant response, so much fun. I did make one 500' pass just to keep it pegged and get a recording on the tach. As I approached the 500' mark, I thought I felt something suck the power down a few rpm so I instantly thought I must be running lean so I shut it down and brought it back to the shop to check it out. Nothing apparent, checked the tach reading and it looked good and consistent. Spark plugs looked nice. The max exhaust temp was recorded at 1151 and 1156 so it isn't running lean. I'll check everything over better hopefully tomorrow.
One thing I did notice is I have a chaincase fluid leak originating around the drive shaft bearing. I couldn't find any type of gasket or seal when I replaced the bearing so I installed a 1/16" thick o-ring about 2" in diameter between the outer bearing race and the retainer plate inside the chaincase. Do you know if there is a better set up to eliminate the leak? It has always leaked a little but now it's getting worse, I don't want to run without oil in there after all this work! Now would be a good time to seal it up.
Overall, the sled is running great. Still needs a little fine tuning of course but man, this thing is a blast! Runs soo nice. Thanks again for helping get this thing performing so strong, I can't believe it's 44 years old.
As always, We'll keep you posted on the progress! -Paul

[midniterupper]

.....Uh-oh......