Basket Cases

[Kurt]

Other than calculating the change in compression I didn't measure the exact shape of the chamber Greg.
It would be interesting if there would be significant gains to be made by the shape of the squish/combustion chamber on a Trials bike given the low revs at which they operate.

I don't have a split head yet Greg although some might say so.
I am interested as to why you have lobotomised your head though for that silver tray.

This has also got me thinking, could we do this to the TY250 also?
Bore & Stroke are both 70,00 x 64,00mm
Compression Ratios:
TY250 6.0:1
DT250 6.8:1

[David Lahey]

This has also got me thinking, could we do this to the TY250 also?
Bore & Stroke are both 70,00 x 64,00mm
Compression Ratios:
TY250 6.0:1
DT250 6.8:1

Yes Kurt it's definitely a thing to do that, especially with the head from the radial fin DT250 models B, C, D, E, F not just for the greater compression, but also because they look so different.

[Greg Harding]

Hi Everyone,

Other than calculating the change in compression I didn't measure the exact shape of the chamber Greg.
It would be interesting if there would be significant gains to be made by the shape of the squish/combustion chamber on a Trials bike given the low revs at which they operate.

I don't have a split head yet Greg although some might say so.
I am interested as to why you have lobotomised your head though for that silver tray.

There would be a reason why the Two heads are different, my guess is the DT with much higher gearing needed more torque to get off the mark? TY 175s need more bottom end when labouring up hill especially after a really tight turn that limits run up speed. My question in part was: Can the same improvement be made using the original head and that is why I was curious about to volume and shape?

The other part was: Do you have one of these?

IMG_20230301_174635.jpg (274.7 KiB) Viewed 14496 times

And finally as I know you are curious as well, I was testing another theory:

Is it actually better to have a frontal lobotomy
Or simply have a bottle in front of me?

[David Lahey]

Hi Everyone,

Other than calculating the change in compression I didn't measure the exact shape of the chamber Greg.
It would be interesting if there would be significant gains to be made by the shape of the squish/combustion chamber on a Trials bike given the low revs at which they operate.

I don't have a split head yet Greg although some might say so.
I am interested as to why you have lobotomised your head though for that silver tray.

There would be a reason why the Two heads are different, my guess is the DT with much higher gearing needed more torque to get off the mark? TY 175s need more bottom end when labouring up hill especially after a really tight turn that limits run up speed. My question in part was: Can the same improvement be made using the original head and that is why I was curious about to volume and shape?

Well I don't have a fancy profile gauge like yours Greg, but I do have something else to add to the story.
Here are photos of the two heads in question. A DT175A ( model 443) head and TY175 ( model 1N4) head.
Measured with my eyeballs, the combustion chamber appears to be the same shape on both and the squish band has the same angle and width on both. Where they differ is in the step up between the squish band and the gasket surface. The TY175 head has a 1.5 mm step and the DT175 head has a 1.2 mm step. So to answer your question, yes you could replicate the compression benefit of the DT175 head by machining the gasket surface on the TY175 head, as many people have done. An even more common trick with the TY175 is to use a thinner-than-standard head gasket or no head gasket to increase the compression ratio.

20230304_111655.jpg (1.69 MiB) Viewed 14492 times

20230304_112200_HDR.jpg (1.54 MiB) Viewed 14492 times

The reason they decided on a lower compression ratio for the TY175 compared with the DT175 is because the TY175 was designed to be ridden by complete novice trials riders and they certainly achieved that with the heavy flywheel, small carby and reduced compression. It is ridiculously easy to ride a standard TY175 and because of this, many thousands of people continued on riding trials after their first attempt. I can remember my experience as one of those novice people as a teenager. I had been riding trailbike class trials (very poorly) on a TS185 L and then a Model 99 Alpina (325cc). The Alpina was not easy to ride in sections due to a worn-out carby, poorly set ignition timing, clogged exhaust, trailbike suspension settings and non-functional brakes. When I brought home a fairly clapped-out TY175B in 1976 and rode it in my practice sections, I could barely even feel the bike at all because it felt so light. The motor was perfect for me at that stage, super predictable, impossible to stall and with a power curve as flat as the Nullabor. It didn't matter that it was so lowly powered because that meant I didn't get into trouble.
Nowadays a standard TY175 is still a great bike to learn to ride on but for people who have become experienced at riding trials, they do feel slow in the engine department and short in the wheelbase.
Many years ago at a trials school, I remember hearing Kale Reed talking about why the TY175 is such a good learner bike for trials and it was something like "because they have so little power"

[Greg Harding]

Hi Everyone,

Just sharing what I have been up to with a comparison from then and now. The first photo by David Lahey back at Twinshock Masters in November 2015:

Twinshock Masters 2015

Screenshot_20230518-140447_Chrome.jpg (488.25 KiB) Viewed 14393 times

The second photo was taken today by me. Excluding the Brick house of gas, gas being the by-product and everything else in the background.
Can you spot the differences? Please post your answers here and will confirm or deny.

Garlic patch today!

IMG_20230521_100413.jpg (1.85 MiB) Viewed 14393 times

[tat ty]

Greg ... You seem to have painted that carb stealth black and for very obscure reasons have undrilled the holes in the shark fin

[tat ty]

And what has happened to that fork brace ... Must have been beefed up to better support that flash front guard.

[tat ty]

Footpeg relocation

[eagle]

Is that the standard size rear sprocket ?

[David Lahey]

It still looks great, Greg

Footpeg is lower.
Exhaust header is bare metal finish.
Carby is black and is a different type.
Chain is not adjusted as tight.
The bar pad is different.
Name plate is a different colour.
Lanyard elastic is a different colour.
Clutch lever is different colour.
Brake lever is different colour.
The petrol in it is newer than 2015.