How to keep drive chain tensioned

[Q]

Hello all, I'm currently doing some brainstorming about how to tension our drivetrain this year, and was looking for some tips on the different options.

1) tension through pillow blocks
We wouldn't be using a big box at the back of the chassis, but two separate pillow blocks on the differential setup (4 mounting points total, 2 top, 2 bottom). Sadly I'm not sure how I would make sure both pillow blocks are straight with the chassis, and set at the same distance from the chassis. Any idea? Also, if they aren't straight to the chassis, what's the worst case scenario?

2) idler gear

Tried it last year, it messed up quite a bit, but i think that was due to poor heat treating on the gear, and tight/constrained packaging. The bearing was crushed when we pushed it in, and a weld on the chassis actually broke. Total chain tension should be under 2000lbs this year. Couple questions: should i use a gear or just a roller? What's a good material for the roller (so far i've heard nylon or the stuff they use for deflection plates on motorcycle chains)?

Finally, anyone have any experience with the idler gear setup? Opinions? Comments?

Thanks a lot guys, i searched the forums and only found bits and pieces, so any help is appreciated.

[mtg]

1) Precisely machine a jig that attaches to the output sprocket of the engine to locate the pillow block mounts on the frame. Make it stout.

The worst case is that you have some chain mis-alignment, which isn't a huge deal, but will probably gobble up some horsepower and wear things out quicker. How much? Don't know.

[Grant Mahler]

Originally posted by mtg:
The worst case is that you have some chain mis-alignment, which isn't a huge deal, but will probably gobble up some horsepower and wear things out quicker. How much? Don't know.

I've seen and heard of people going hundreds/thousands of miles on motorcycles with untensioned and/or misaligned chains. I'm pretty sure you will notice excessive chain noise long before the play gets out of hand.

FWIW bicycle chains are almost always "highly" misaligned...

[mtg]

Yeah, the mis-alignment probably isn't a big deal if you're chain driven.

Bicycle chains seem to be designed to handle much more mis-alignment than motorcycle chains. The lateral gaps between the links are much larger.

If you go belt drive, alignment does become very important.

[Drew Price]

But how many stock bikes run Aluminum rear sprockets?

Delrin would probably be a good material for the idler, or slider blocks. It's the stuff the idler gears on the rear derailleur on your bike are made of, among other things, and it's (I think) tougher than nylon.

At least on our car, if the mounting blocks are severely misaligned the bearings that support the diff case will bind. With the half shafts out and chain off so you can spin the diff, hold a block in each hand and have someone spin the diff for you, and then tweak the blocks relative to each other. You will feel and see it start to bind at a few degrees misalignment, so just get it set up perfectly straight with the shafts and chain off, then just tighten or loosen both sides the same amount when you re-assemble everything.

Best,
Drew

[Conor]

Replace your idler sprocket with a puck of ultra high molecular weight polyethylene. It's cheaper, easier to work with, and all around much simpler. It will wear, but very slowly. If your bearings are failing from chain tension, I think your chain is probably a little too tight. From what I've heard, a good rule of thumb to use is, once the chain has been tensioned, you sould be able to wiggle the chain up and down about .5 inches each way. Somebody correct me if I'm wrong on this one.

[Rob Comer]

We've tried sprung chain tensioning, with plastic and aluminum guide wheels and they always wore out. Now we stick to shims and eccentric mounting to adjust the chain. Its not a big deal to adjust it a little after every few hours of driving. I believe the proper tension is about .25" for every foot spacing between the sprockets (center to center).

[HenningO]

With an unaligned chain you might get periodical loading in your sprocket and other components which might lead to pre-mature failure due to fatigue.

[Adam L]

As others have said its not the worst thing in the world. You will however create more friction with a misaligned chain and that results in decreased efficiency. One of the things I come across when putting a new chain on my kart is having to move the motor about 1-1.5" rearward in order for a new chain to fit with proper tension(yeah its stretches that much).

What I would do is design a roller system that is easily adjustable (turn of a bolt) when you need to. Check for play after all of your test, log that data, repeat, etc. etc. It's a simple problem with a simple solution.

If you want me to elaborate a bit more on the roller setup let me know.

[Adam L]

Oh yeah, and Conor is dead on with his chain play figure.

How is Kalamazoo these days? Its been about 8 years since I was last out there, humid as heck I assume eh?

[Conor]

Originally posted by Adam L:
Oh yeah, and Conor is dead on with his chain play figure.

How is Kalamazoo these days? Its been about 8 years since I was last out there, humid as heck I assume eh?

I'm glad you agree about the chain tension, it's good to know I haven't been screwing things up too bad. Kalamazoo is doing great, not so much humid right now - more so really cold and snowing. Did you attend WMU? I'm sure you can imagine how pysched we are to have the competition in Brooklyn this year, we're probably the closest team to it now.

Sorry for hi-jacking the thread.

[Adam L]

Originally posted by Conor:

I'm glad you agree about the chain tension, it's good to know I haven't been screwing things up too bad. Kalamazoo is doing great, not so much humid right now - more so really cold and snowing. Did you attend WMU? I'm sure you can imagine how pysched we are to have the competition in Brooklyn this year, we're probably the closest team to it now.

Sorry for hi-jacking the thread.

Haha no worries. I did not attend WMU, transferring to UCSD actually but thats a whole 'nother story. I bet your stoked about the competition being close though; that was one thing I couldn't get over about the freeways back east. They are either really packed, or they are long stretches with stop lights every mile or so.

Good luck this year, the website looks pretty sharp and I'm glad to see another team come to their senses and use a proven powerplant. http://fsae.com/groupee_common/emoticons/icon_smile.gif How far along are you guys on the chassis? (there was no link to the chassis page)

[Conor]

Chassis is complete. Drivetrain including diff, carrier, axles, and hubs are all complete. Still working on the engine intake and exhaust as well as tuning. The suspension control arms are being made and we're waiting on the springs and dampers to arrive. The steering is being put together and the rockers are almost ready to be machined. Uprights and ball joint housings will be sent out to be machined tonight. We're hoping to have the car together and running by January 15th.

[Q]

Hey Adam, could you elaborate on the roller setup a bit?

Drew, how often do you have to readjust your blocks like that? I'm assuming you have individual tensioners on each of your pillow blocks, and are tensioning through eccentric movement then?

thanks for all your help guys

[Adam L]

Originally posted by Q:
Hey Adam, could you elaborate on the roller setup a bit?


I wish I had an illustration for you; one of my old Suzuki's had a similar setup except that it was an automatic adjustment.

Basically you have a urethane wheel (almost looks like a caster setup) which is mounted via threaded rod. This assembly mounts to the frame so that is aligned with the sprockets/chain.

When you need to adjust the chain you thread out the assembly and this adds tension. Yes there is a bit of friction, and I'm sure that there are better setups out there, but its a simple solution. I got the idea from my CRG kart motor mount plates, you move the motor fore/aft to adjust the chain, this is just a spin on that design.

If I don't make any sense, let me know, I'll try and make an illustration if you need me to elaborate further.

[Dennis Seichter]

I wouldn't use any tensioning mechanism that introduces more friction than necessary into the assembly. This especially includes slide shoes of any kind..

In our team, we have made very positive experience with excentric bearing carriers - minimum design space, easy adjustement.

[Christopher Catto]

those cars which have a bulkhead (be it dural or steel tubes) mounted on the back of the engine can have two carriers for the diff and in each carrier is an eccentric cam. So, provided you're not foolish and turn one cam more than the other, this is how you move the diff fore and aft (and admittedly a bit up or down but not that that makes much difference to the driveshafts).

For cars that run a diffcage I dunno if this is a great solution but I think it can work ok, just the meaty carriers add to the weight of the diffcage. On the other hand, I'd rather have a bulkhead with sturdy diff carriers than a large diffcage (great for wishbone loadpath) with diffs mounted/adjusted via threaded studs.

University of Hertfordshire hasn't had rave reviews on idler gears. We found them rather noisy if steel. Never tried "plastics" but we just got rid of it because nobody liked the chain being bent and any loss of efficiency. we thought it's best to keep things easy to understand and tune and the guy who decided to use the cams had experience from hillclimb cars. it's my guess but I think that if the delrin pulley has a force on it then some horsepower might be lost. Am I totally wrong? Maybe yes.