HTD pulleys vs trapezoid pulleys

[lotus-62]

does it make any sense to replace my trapezoid pulleys on my 912 engine with HTD pulleys?   I am in need for a belt change and I have the htd pulleys so if, then now is the best time for me to do it but only if it makes sense. 

thanks, Rick

[drdoom]

It is my belief that reliability is meaningfully improved with the switch to HTD. The only automotive applications of the superceded Gilmer type are oil pumps of various sorts on race cars, nothing on street cars at all.

[lotus-62]

will the HTD belt help in the jumping belt problem as well? or is this a tensioner only problem, I have seen an conversion kit to go from a spring loaded (auto adjuster)  to a fixed tensioner? 

 

[drdoom]

I'm doing both, confident there will be no chance of jumping teeth when done correctly. The OEM's universally acted on this for good reason.

[rgh0]

The OEMs moves to HTD belts were to achieve the lower noise and longer wear life of the round tooth belt and pulleys profile.  Unless you doing high miles and want a car where you cant hear that lovely belt whine at 7000 rpm behind your head stick with what you have got IMHO.

 

Provided installed correctly and maintained right the trapezoidal belt profile is no worse at jumping teeth than the HTD belt, Millions of cars and many millions of miles point to that again just IMHO

 

cheers

Rohan

Edited January 15, 2021 by rgh0
added more comments

[jonwat]

37 minutes ago, rgh0 said:

The OEMs moves to HTD belts were to achieve the lower noise and longer wear life of the round tooth belt and pulleys profile.  

Provided installed correctly and maintained right the trapezoidal belt profile is no worse at jumping teeth than the HTD belt,

Back when Lotus announced the move to the HTD belt the explanation I read was that on a cold, frosty morning with a fully charged battery if the belt wasn't perfectly tensioned then on turning the key the sudden spin of the crank could case it to jump a tooth or two.

I can't remember now but what's the recommended change interval of the two belts? 

[andydclements]

I think the interval remained the same. I've seen over-due trapezoidal belts where the teeth  have gone and simply become wafers of rubber, not ripped off, just warn thinner and thinner until it's hope and prayer keeping the sprockets correctly aligned. Not seen (even over-due) sinusoidal belts do that.

I would still recommend the move, even if just to get a set of relatively wear-free sprockets on and get rid of the semi-auto tensioner system.

[jonwat]

3 hours ago, andydclements said:

I think the interval remained the same. 

That negates Rohan's claim HTD belts reduce wear then. I'm not saying don't do it, just querying the reasons. 😃

[snowrx]

If you do keep your trapezoidal pullies, remember there's a stronger blue belt option, the Gates T104RB

[drdoom]

My former physician, a brilliant MD and accomplished amateur circuit racer, owned the first Jensen-Healy sold into Vancouver and that suffered the belt jumping teeth with consequent engine damage. This occurred on cold start-up whilst on a winter jaunt to ski at nearby Whistler Mountain. He knew his stuff in regard to mechanicals and had the car serviced by well established chaps throughout. FWIW.

[andydclements]

2 hours ago, jonwat said:

That negates Rohan's claim HTD belts reduce wear then. I'm not saying don't do it, just querying the reasons. 😃

No, it doesn't negate Rohan's claim, it just means that if Rohan's claim is correct then Lotus did not wish to alter the interval despite reduced wear.

[jonwat]

16 hours ago, andydclements said:

No, it doesn't negate Rohan's claim, it just means that if Rohan's claim is correct then Lotus did not wish to alter the interval despite reduced wear.

So, Lotus converted to a belt that lasted longer but thought that increasing belt change intervals, so reducing servicing costs, wasn't a useful marketing ploy? OK

[drdoom]

I rather suspect potential buyers would have had servicing details furthest from mind when in pursuit of a new Lotus. Those were never widely affordable and perhaps seen as more practical alternative to Ferrari, such cars having truly dire servicing schedule and costs.  

[Andyww]

The Jensen Healey engine had a fixed tensioner and suffered the issue with the start on very cold mornings causing belt to jump teeth. So they very quickly designed the spring tensioner and added belt snubbers to overcome the problem. This was an emergency change initially.

The issue was caused by the long belt run and short amount of wrap around the pulleys. The Ferrari v8 engine of the time used trapezoidal tooth belts and fixed tensioners and never suffered this problem so they never went to HTD.

When the HTD belts became available they did extensive testing to check whether they could go back to the fixed tensioner because the spring tensioner is not good practice, other engines dont use this method. The testing confirmed they could.

At some point Lotus dod a 100,000 mile test of a cam belt and found no measurable tension change nor wear. But I cant remember exactly when this was done, it could have been some years after they first went to HTD on a later belt spec.

[PrecisionMike]

I'm building a 912 currently and have intention to use the htd drive system but I cant decide if I should revert to the fixed eccentric tensioner. I've run a 907 with a trapezoidal set up and auto tensioner for 36000 miles without issue. With the fixed eccentric tensioner I love the thought of simplicity but I'm not happy with the thought that thermal expansion simply results in increased tension. But with the auto tensioner which clearly accommodates thermal expansion and contraction, I hate the thought of induced belt slack when the engine kick back in the wrong direction a few degrees as can happen during engine switch off.

These are two mutually exclusive requirements. I'd be grateful for all thoughts please.

[drdoom]

Mike, as I see it the belt is one tough customer and better to see it loaded up a touch than toy with jumped teeth. Can't say I've had the chance to examine what the manufacturers have done over the years since in regard to tensioner type. Simple eccentric and HTD for me. 

[PrecisionMike]

Thankyou for your response. I'll give the matter further thought. I'll try to get hold of Gates who make the belt !

[Escape]

I think most (if not all) engines now use an automatic tensioner. But that could be for ease of fitting more than reliability.

Interestingly, I've replaced the timing belt on a Land Rover earlier this week and found the keyway in the drivepulley to be about twice as wide as the key itself. This is intentional, to allow some freedom on kickback and avoid shock loads or slack on the belt. I know Gates has developped pulleys with varying diameter (only very slightly) to further smoothen the load on the belt.

Filip

[Andyww]

Note that if fitting a fixed tensioner to an engine which has the spring tensioner you need to precisely drill and tap a hole in the block. If the car has A/C the compressor mounting bracket needs to be changed. If it does not have A/C you may need to borrow a bracket because thats the best way of gauging where to drill the hole. It can be used as a jig.

[PrecisionMike]

Thankyou....very useful info!

[PrecisionMike]

21 hours ago, Escape said:

I think most (if not all) engines now use an automatic tensioner. But that could be for ease of fitting more than reliability.

Interestingly, I've replaced the timing belt on a Land Rover earlier this week and found the keyway in the drivepulley to be about twice as wide as the key itself. This is intentional, to allow some freedom on kickback and avoid shock loads or slack on the belt. I know Gates has developped pulleys with varying diameter (only very slightly) to further smoothen the load on the belt.

Filip

Hi. I had understood that after 1986 / 7  the 900 series standardised on htd with fixed eccentric tensioner and not the auto tensioner? 

[Andyww]

1 hour ago, PrecisionMike said:

Hi. I had understood that after 1986 / 7  the 900 series standardised on htd with fixed eccentric tensioner and not the auto tensioner? 

Yes thats correct.

[Escape]

1 hour ago, PrecisionMike said:

Hi. I had understood that after 1986 / 7  the 900 series standardised on htd with fixed eccentric tensioner and not the auto tensioner? 

My bad, I should have specified engines in general, not the Lotus 9xx. The 'now' refering to 21st century. 😉

My point was that often such changes are driven by costs and ease of assembly or maintenance, and are not necessarily a technical improvement. I'm sure Lotus had good reason to use a fixed tensioner whereas most other engines probably chose the easiest solution.

Filip

[andydclements]

I think Lotus's reason for a fixed tensioner was simple cost. They tried it originally with trapezoidal and it wasn't great (belt slips) so they instigated the semi-auto system, then once belt and sprocket design improved enough they decided they could ditch that more expensive semi-auto tensioner and revert to original. Just how they did with early turbo engines being dry sump until the block was stiffened enough to return to wet sump.

[PrecisionMike]

On 26/02/2021 at 07:29, Escape said:

My bad, I should have specified engines in general, not the Lotus 9xx. The 'now' refering to 21st century. 😉

My point was that often such changes are driven by costs and ease of assembly or maintenance, and are not necessarily a technical improvement. I'm sure Lotus had good reason to use a fixed tensioner whereas most other engines probably chose the easiest solution.

Filip

Please dont get me wrong, I love lotus and I've owned mine for thirty years....I'm nit sure lotus had a strategy for solving an early belt jump problem.....I think in the early days thet lurched through a field of iterations in hope that the "next mod" would fix it. I've worked on various high reving Ferrari, alfa and Landis all of whom use auto damped tensioners!! Who really knows?????? We need a direct line to CABC!