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Dyno roller question

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I've been researching dynos now for a fair while and apart from manufacturer specific claims I can't seem to find a consensus about dyno rollers.

My gut feeling is the larger the diameter of the roller(s) the better because 1, it lowers the revs the retarder has to deal with, &, 2. it simulates the "flat earth" design better. If anyone has any thoughts on this I'm all ears.

What I am having difficulty coming to an opinion on is a 1 roller dyno (wheel runs on top of a roller) better or worse than a 2 roller dyno (wheel runs between 2 rollers)? Is 1 safer than 2 or vice-versa? Is 1 more accurate than 2 or vice-versa? or is it al about personal opinion and/or strapping down?

Cheers.

Michael.

Hi Michael

I chip in only partially on this since your post questions are quite broad.

-->My gut feeling is the larger the diameter of the roller(s) the better because 1, it lowers the revs the retarder has to deal with, &, 2. it simulates the "flat earth" design better.

Larger diameter provides more surface yes, but!..roller knurling and strapping technique are more important

--> What I am having difficulty coming to an opinion on is a 1 roller dyno (wheel runs on top of a roller)

1 rollers are (to my knowledge) inertia type, meaning a rather large and heavy drum to be spun. Usually these are lower cost and often not able to measure steady state, limited to WOT runs and less sensitive.

--> better or worse than a 2 roller dyno (wheel runs between 2 rollers)? Is 1 safer than 2 or vice-versa?

Safety is a matter of proper strapping. Both types are as safe if done right by the operator. The rear roller on a 2 roller are called "idle rollers" and are only there to keep the vehicle aligned somewhat. Under load after a certain point, the wheel will have to leave the idle roller.

--> Is 1 more accurate than 2 or vice-versa? or is it al about personal opinion and/or strapping down?

Sensitive load bearing types use 2 lightweight rollers.

Hi Dynodom, thanks for your reply.

>>I chip in only partially on this since your post questions are quite broad.

Actually its not broad at all it is rather specific. With all else being equal I'm interested in rollers.

>>1 rollers are (to my knowledge) inertia type, meaning a rather large and heavy drum to be spun. Usually these are lower cost and often not >>able to measure steady state, limited to WOT runs and less sensitive.

There are eddy current 1 roller dynos around.

>>Safety is a matter of proper strapping. Both types are as safe if done right by the operator.

So in other words with all else being equal a 2 roller dyno is not more or less safe than a 1 roller dyno. Is that correct? This is what I am trying to find out.

>>The rear roller on a 2 roller are called "idle rollers" and are only there to keep the vehicle aligned somewhat. Under load after a certain >>point, the wheel will have to leave the idle roller.

There are videos on youtube (search dyno fail and you'll find them) of front wheel drive cars coming off 2 roller dynos because of torque steer. All else being equal I am yet to find one of this happening on a 1 roller dyno. That's not to say it hasn't but rather that I haven't seen one.

>>Sensitive load bearing types use 2 lightweight rollers.

Not all do and thus my questions.

I'm in the process of planning a dyno build. I am not in the position to manufacture rollers myself so I will get a very experienced engineering company to do it for me. If I only need 2 large rollers compared to 4 medium sized rollers, with all else being equal, I will opt for the 2 unless there is some good reason to go for the 4 and the extra associated costs.

Thanks again for your reply, I'll take what you have said into account.

Cheers.

>>Safety is a matter of proper strapping. Both types are as safe if done right by the operator.

So in other words with all else being equal a 2 roller dyno is not more or less safe than a 1 roller dyno. Is that correct?

--> Yes, correct.

_________

>>The rear roller on a 2 roller are called "idle rollers" and are only there to keep the vehicle aligned somewhat. Under load after a certain >>point, the wheel will have to leave the idle roller.

There are videos on youtube (search dyno fail and you'll find them) of front wheel drive cars coming off 2 roller dynos because of torque steer. All else being equal I am yet to find one of this happening on a 1 roller dyno. That's not to say it hasn't but rather that I haven't seen one.

---> FWD must additionally be cross strapped. ie, add one strap on the front left subframe then run it to a anchor point on the right hand side of the bay and vise versa ( X fashion ). Make sure forward travel and climbing up the rollers is controlled properly. Once the vehicle is on the rollers,.. engage a low gear and roll for a little bit without holding on to the steering wheel and no brakes (also no handbrake) applied in order to let it align itself, then tighten the straps equally in two steps and occasionally re-check. That's all.

_________

>>Sensitive load bearing types use 2 lightweight rollers.

Not all do and thus my questions.

---> The heavier the roller, the more dampening effect.