Is a chain tensioner really necessary?

I am rebuilding a 390 using a 1980 XC frame and swingarm and want to know whether it is really necessary to use a chain tensioner as well as the chain guide.

Here is a pic of a bike with a 1980 XC frame and I can't see a tensioner on this one.

View attachment 5906

What do you guys think? Has anyone run without the tensioner with success or problems?

Thanks

one keeps the chain tight with long travel. the other keeps the chain from jumping off the rear sprocket...yes...you need both.

tried to run without the guide once,,,,kept thowing the chain.

The spring loaded one tends to be high maintance and can wear so that it starts riding on an angle. When I rode these things exclusively the dealer told me not to bother with it. I think I rode farther after not bothering with it than before. The modern x ring or even o ring chains are much better about staying straight than their predecessors. You will notice when they went to the aluminum swing arm some models had one lower roller in that general position and some models had a lower and higher roller, both fixed in location. If there is modern bike with the roller on a spring like that perhaps someone could state the make and model. Yes these things will accommodate a 16 tooth front sprocket which my more modern two stroke bikes wont because the sprocket is marginally farther away from the swing arm pivot. I try and keep the guide close to the rear sprocket. That spring loaded thing on the bike I had/have fluttered on road sections. Generally the only times I jump the sprocket is jumping in the air and landing which I don't recall happening since Using didvmxring chain. At least for the stock swing arm length.

If you don't know what to look for on the swing arm for that part's evidence it just will look like a horizontal small pipe perhaps an inch and a half long and half an inch inside diameter.

Now about protecting the magnesium pocket the bearing near the coutnershaft which holds that sealed or half sealed bearing from heading towards needing a bronze sleeve you can ponder that yourself. If that is an issue for the 390.

Are you going to be hopping over logs? Tensioner kind of gets in the way at times if I recall correctly.
Fran

I ride and race the following bikes with only a good chain guide and no tensioner setup. 100S of hard miles and not a single problem.

83 500CR, 84 500XC, 83 510TX, 84 510TE, 86 510TX.

I found the tensioner a nice touch, even have a Uptite version of it, but it remains an encumberance in many ways.

you can use a non spring loaded type..they actually work well...
i use a high dollar skate board wheel,,they dont wear out as fast.

l belive the mid 80s 500cc just had guides, lve run a uptite spring loaded tensioner aswell,briefly , however with the 120/100-18 tyre it was to close (so its for sale

Thanks for all the advice, it is very useful indeed. I am planning to fabricate a much more effective guide which is much closer to the rear wheel to reduce the risk of unshipping the chain, then perhaps some kind of fixed roller up towards the front of the swingarm, just to keep things under a bit more control, but from what you have said above, I am not sure if that will be necessary.

Interesting comments regarding the modern O ring chains being more rigid. That makes a lot of sense. I could invest the cost of a new tensioner assembly (they are not exactly cheap) into a better quality chain and end up with a more reliable solution. Food for thought.

Thanks again gents, great info as always!

the single shock huskys had a straight arm with wheel, and a guide that flopped in the breeze as well.
a KTM style guide works well, and looks very nice.